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2 Science Impact ASSESSMENT CRITERIA AND CONSIDERATIONS What powered the big bang? What happens to spacetime near a black hole? What is the mysterious dark energy pulling the universe apart? These fundamental questions lie at the heart of NASAâs Beyond Einstein Program. Einsteinâs theory of general relativity predicted the expansion of the universe from a big bang and the phenomenon of black holes. Einsteinâs general relativity equation contains a term associated with a âcosmological constantâ that may describe dark energy. Investigating the nature of these phenomenaâgoing beyond Einsteinâwill take space missions that harness the ingenuity, creativity, and technical sophistication of current and future generations. The Beyond Einstein roadmap lays out specific research goals related to each of the three fundamental ques- tions above. Investigating what powered the big bang requires probing the period of inflation, an early era when the universe expanded by some 30 orders of magnitude in linear scale. According to theory, inflation produced gravitational radiation, and a specific goal is to detect the level of this radiation, either directly or through its residual imprint on matter. Progress on this question will also be made by determining the size, shape, age, and energy content of the universe, which will better constrain conditions during the big bang. To understand how black holes affect space, time, and matter in the universe, one must first determine how frequently they occur, what their properties are, and how they interact with matter in galaxies and other structures. Thus, two of the research goals associated with black holes are to perform a census of black holes and to determine how they are formed and evolve. A third objective is to probe what happens in the very strong gravitational field very near a black hole by observing distortions of spacetime near its event horizon. A final objective is to observe what happens to gas and stars as they are swallowed by black holes. Understanding the nature of dark energy is the most pressing question in cosmology today. The research goal that has greatest promise of elucidating the nature of dark energy is the determination of its cosmic evolution. Determining the size, shape, age, and energy content of the universe is also necessary in order to constrain the properties of dark energy. National Aeronautics and Space Administration, Beyond Einstein: From the Big Bang to Black Holes, Washington, D.C., January 2003, â p. 5. National Aeronautics and Space Administration, Beyond Einstein: From the Big Bang to Black Holes, Washington, D.C., January 2003. â This document was part of NASAâs 2003 roadmapping effort required under the Government Performance Results Act of 1993 (Public Law No. 103-62). 15
16 NASAâS BEYOND EINSTEIN PROGRAM The mission suite designed in the Beyond Einstein roadmap to carry out the programâs research goals consists of two flagship missions, the Laser Interferometer Space Antenna (LISA) and Constellation-X (Con-X), as well as three smaller missions known as the Einstein Probes. The flagship missions are well defined and mature in their scientific formulation. The Einstein Probesâthe Cosmic Inflation Probe (CIP), the Black Hole Finder Probe (BHFP), and the Joint Dark Energy Mission (JDEM)âare typically smaller in scale, and multiple technical or observational approaches are being considered for their implementation. A competitive review will determine which of the implementation approaches of a given probe concept will be selected. The committee considered the scientific questions for each class of probe, as well as all proposed observational approaches, in reaching its conclusions about each mission area. As one of its overall criteria for evaluating the Beyond Einstein missions, the committee formulated a set of five criteria for use in assessing the scientific content and quality of the mission candidates. These criteria characterize the scientific readiness, risk, and progress that each mission promises relative to the Beyond Einstein science goals. These science goals are well conceived and are traceable through numerous strategy and planning documents, and the committee has therefore chosen to adopt them as well. â¢ Advancement of Beyond Einstein research goals.â The primary assessment criterion is how directly and unambiguously a mission candidate addresses the Beyond Einstein research goals. â¢ Broader science contributions.â Many of the mission candidates in the Beyond Einstein portfolio can provide data that are central to other astrophysical investigations not identified as part of the Beyond Einstein research goals. â¢ Potential for revolutionary discovery.â Will the mission candidateâs measurements truly alter current paradigms, or discover new and unexpected phenomena? â¢ Science risk and readiness.â Considering the mission candidate as designed, how much risk is there that the measurements will not answer the questions posed? This risk could be due either to systematic effects associated with astronomical phenomena not easily addressed with theory, or to uncertainties in the levels of the signal to be measured or the number of accessible astronomical sources. Are the theoretical frameworks for understanding the measurements in place? Are there foundational measurements that need to be made first (e.g., characterization of astronomical backgrounds, wide-field surveys to find targets, and so on)? â¢ Uniqueness of the mission candidate for addressing the scientific questions.â Are there other projects, either space- or ground-based, that are likely to compete in addressing Beyond Einstein questions before the completion of the mission in question? How essential is the vantage point of space for the proposed science? This chapter describes the science goals, potential impact, and scientific readiness of each of the five mission candidates. Note that because the current state of development varies greatly among missions, the level of detail in the following mission discussions varies as well. The chapter concludes with a comparative assessment of progress to be made against each of the three Beyond Einstein questions. BLACK HOLE FINDER PROBE Introduction The Black Hole Finder Probe is one of the three Einstein Probes discussed in the Beyond Einstein roadmap. BHFP is designed to find black holes on all scales, from one to billions of solar masses. BHFP will address the question âHow did black holes form and grow?â by observing high-energy x-ray emissions from accreting black holes and explosive transients. With a very wide field of view, BHFP can detect variable sources and bursts of x- rays that herald the birth of new black holes and map high-energy x-ray sources over the entire sky. By operating in the hard x-ray band (a few to 600 keV), BHFP can detect accreting black holes that are surrounded by obscuring This criterion is focused on scientific challenges inherent in the investigation, assuming that the technology challenges are or can be met. â The technology challenges for each mission are addressed in Chapter 3.
SCIENCE IMPACT 17 TABLE 2.1â Black Hole Finder Probe: Mission Description Parameter Value Primary measurement Hard x-ray all-sky survey Observatory type Coded-aperture telescope, 10-600 keV or 3-600 keV Projected years in orbit 5-yr primary mission, 10-yr goal Type of orbit 500 km altitude, circular orbit Mission phases One-phase, full-time scanning survey Science operations Continuous survey, with gamma-ray burst/variable alerts Other mission characteristics Covers entire sky at sub-day intervals material and are therefore not visible in the traditional x-ray bands below 10 keV (if they lie at low redshift). With sensitivity that is 10 to 100 times that of previous hard x-ray wide-field telescopes, BHFP can make a census of the accreting black hole population in local galactic nuclei over a wide range in luminosity, as well as detect the brightest sources out to redshifts of approximately 2. The BHFP instrument would consist of multiple coded-aperture âsubtelescopes,â each covering fully coded fields of view (FOVs) roughly 20 degrees on a side; the combined FOV of these subtelescopes is a fan beam covering nearly 180 degrees in its long dimension. The spacecraft would fly in a circular low Earth orbit with an altitude of approximately 500 km and would cover the entire sky by zenith-pointing and undergoing a nodding motion so that the fan beams would cover a full 180 degrees during each orbit. Because of the multiple detector units, the BHFP has a rather high mass, in the vicinity of 10,000 kg. Table 2.1 lists the primary mission parameters for BHFP. Two concepts for a BHFP mission were presented to the committee: EXIST (Energetic X-ray Imaging Survey Telescope) and CASTER (Coded Aperture Survey Telescope for Energetic Radiation). Both concepts employ a wide-field coded-aperture hard x-ray survey telescope, differing primarily in their detector implementation. Each would divide its total energy coverage into a high-energy and low-energy band. EXIST would extend to a some- what lower energy, down to 3 keV, to provide more detailed spectral energy distributions and reach the iron-line complex near 6.4 keV. Both EXIST and CASTER cover the energy range up to 600 keV, primarily to ensure access to the 511 keV electron-positron annihilation line. The accuracy of source positions is determined by the properties of the coded apertures and the strengths of the individual sources; for EXIST, the best position accuracy cited is 11 arcseconds (arcsec) (for 5Ï sources) for the Low Energy Telescope (LET) and 56 arcsec for the High Energy Telescope (HET), while CASTER predicts position accuracies (for 10Ï sources) of 42 arcsec for the Low Energy Imager (LEI) and 70 arcsec for the High Energy Imager (HEI). Table 2.2 summarizes the observational parameters of the scientific instruments, with separate entries for EXIST and CASTER. Note that the BHFP, as embodied in EXIST, is the only Einstein Probe that was specifically recommended in the National Research Councilâs (NRCâs) decadal survey report Astronomy and Astrophysics in the New Millennium, published in 2001. Mission Science Goals Contribution to Beyond Einstein Science Goals Three specific Research Focus Areas of the Beyond Einstein roadmap may be addressed by the BHFP and are discussed briefly in this section. Table 2.3 summarizes some of the key science questions that will be investigated by BHFP as part of these Research Focus Areas. Perform a Census of Black Holes Throughout the Universe.â For Beyond Einstein, the most directly relevant science goal of the BHFP is to perform a census of black holes throughout the universe. The proposed realizations National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001. â
18 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.2â Black Hole Finder Probe: Mission Instrument Properties Spectral Range Angular Resolution Spectral Resolutiona Collecting Areab Field of View Instrument (keV) (arcmin) (ÎE/E) (m2) (degrees) EXIST-HET 10-600 6.9 2% at 100 keV 2.7 at 10 keV 65 Ã 154 1% at 511 keV 2.7 at 100 keV 0.7 at 511 keV EXIST-LET 3-30 1.4 5% at 10 keV 0.6 at 10 keV 64 Ã 160 CASTER-HEI 200-600 12 ~5% at 511 keV 1.4 at 511 keV 40 Ã 160 CASTER-LEI 10-200 7 ~35% at 10 keV 3.1 at 10 keV 40 Ã 160 ~10% at 100 keV 3.1 at 100 keV NOTE: See Appendix G in this report for definitions of acronyms. a The spectral resolution for CASTER has not yet been optimized; values cited are intermediate between those for the current prototype and the best published results. b Only a few of the detectors see a given point on the sky at one time. The areas given are the total detection areas that are exposed to any part of the sky at a given time. Effective areas exposed to a particular point on the sky at a given time are between 10 percent and 20 percent of the values tabulated here. of the BHFP would carry out this census from low Earth orbit, using coded-aperture telescopes to survey x-ray emission at energies ranging from a few kiloelectronvolts to 600 keV. Previous x-ray surveys at energies below 10 keV are not sensitive to low-redshift active galactic nuclei (AGNs) with highly obscured nuclei (at high redshift, the absorption cutoff shifts into the traditional 1-10 keV x-ray band). This sensitivity is important, as evidence suggests that a substantial fraction of the nearby accretion energy from massive black holes has been obscured from the view of lower-energy x-ray missions. At the higher energies, the sensitivities of the BHFP would be up to 100 times better than the present INTEGRAL and Swift missions, leading to the expected detection of as many as 30,000 to 100,000 extragalactic hard x-ray sources. The proposed missions will localize 10-100 keV sources with an angular location accuracy of tens of arcseconds, with the sensitivity to detect objects having x-ray lumi- nosities (Lx) ~1044 ergs sâ1 out to redshifts (z) of 0.25 and (Lx) ~1046 ergs sâ1 out to z ~2. Previous studies indicate that AGNs with x-ray luminosities ~1046 ergs sâ1 are fairly rare, so the sample detected at high redshift may be fairly small. Thus, a wide range of black holes with masses of a million to a billion solar masses will be detected at low redshift, but only the most luminous AGNs (and most massive black holes) will be seen from the first few billion years of the universe. Within our own Galaxy and its nearest neighbors, several thousand stellar-mass black holes, both isolated and in binary systems, will be detected as they accrete matter from their surroundings or their companions. Determine How Black Holes Are Formed and How They Evolve.â The formation and evolution of black holes can be studied by two means. The census of x-ray sources described in the preceding subsection will provide x-ray luminosities for massive black holes, which are related to the accretion rates and hence to the black hole growth rates. Thus, by a somewhat indirect chain of reasoning, the x-ray luminosity studies can tell us how massive black holes grow in mass as the universe evolves. The other primary means of studying black hole formation and evolution is by monitoring high-energy x-ray variability. A very extreme form of x-ray variability is displayed by gamma-ray bursts (GRBs), and the BHFP will be a GRB detector of unprecedented sensitivityâperhaps 10 times more sensitive than Swift. Thus, it will detect the formation of stellar-mass black holes throughout the universe by both core-collapse (âlongâ GRBs) and merging compact objects (likely associated with âshortâ GRBs). The BHFP will re-image large portions of the C.B. Markwardt, J. Tueller, G.K. Skinner, N. Gehrels, S.D. Barthelmy, and R.F. Mushotzky, 2005, The Swift/BAT High-Latitude Survey: â First results, Astrophys. J. 633:L77-L80.
SCIENCE IMPACT 19 TABLE 2.3â Black Hole Finder Probe (BHFP): Beyond Einstein Science Programs Science Program Program Characteristics Program Significance Science All-sky Science Perform a census of black The BHFP all-sky survey will detect tens of thousands definition hard question holes throughout the universe of hard x-ray sources, determining the population programs x-ray distribution of massive black holes in external galaxies survey Measurements All-sky survey in a 10-600 and their contribution to the x-ray background. The keV (CASTER) or a 3-600 x-ray luminosities also will help determine how keV (EXIST) range black holes evolve (see science question below) by providing a characterization of the accretion rates of Quantities X-ray flux at low and high massive black holes. In addition, the all-sky survey determined energies; source localization will detect and characterize the emission from several of tens of arcsec; location and thousand stellar-mass black holes in our Galaxy, widths of strong x-ray lines undoubtedly finding new rare objects. Hard Science Determine how black holes The study of variability of extragalactic hard x-ray x-ray question evolve; observe stars and gas sources will be used to assess the accretion rates, variability plunging into black holes and hence the rate of growth, of massive black holes. study In addition, BHFP will detect rare events in which Measurements Variability of hard x-ray massive black holes shred and capture the matter from sources stellar-mass objects that approach too closely. Quantities Flux vs. energy for hard x-ray determined sources around the sky, on time scales from milliseconds to days Gamma- Science Determine how black holes are The formation rate of stellar-mass black holes over ray bursts question formed cosmic time, including their possible formation earlier (GRBs) than the first galaxies that scientists have detected Measurements Detection and characterization to date, can be probed by detecting a significant of GRBs population of GRBs at high redshift. These distant GRBs may herald the formation of stellar-mass black Quantities Flux vs. time of over a holes that provide seeds for the eventual evolution to determined thousand GRBs, with telemetry the massive black holes seen at the centers of galaxies. to ground enabling rapid identification of host galaxies for follow-up sky on time scales of hours, thus studying the evolution of the brightest x-ray sources on these short time scales. Ultraluminous x-ray sources, perhaps due to black holes with âintermediateâ masses between tens and thousands of solar masses, will be detected in many nearby galaxies. Their total density and duty cycles will allow inferences to be drawn regarding both their overall formation rates and their importance as possible seeds for the growth of more massive black holes. Observe Stars and Gas Plunging into Black Holes.â The unique BHFP capability of studying short time-scale variability of hard x-rays will result in the unprecedented detection of the tidal disruption and âswallowingâ of stars by massive black holes; several possible cases have been reported in the literature. Tidal disruptions of stars by massive black holes in relatively nearby galaxies will be detectable in approximately 10 galaxies per year. Such M.C. Miller and D.P. Hamilton, 2004, Production of intermediate-mass black holes in globular clusters, Mon. Not. R. Astron. Soc. â ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ 330:232-240. S. Komossa, J. Halpern, N. Schartel, G. Hasinger, M. Santos-Lleo, and P. Predehl, 2004, A huge drop in the x-ray luminosity of the nonac- â tive galaxy RX J1242.6-1119A, and the first postflare spectrum: Testing the tidal disruption scenario, Astrophys. J. 603:L17-L20. S. Komossa, 2002, X-ray evidence for supermassive black holes at the centers of nearby, non-active galaxies, Rev. Mod. Astron. 15:27.
20 NASAâS BEYOND EINSTEIN PROGRAM events will provide critical evidence regarding the rates at which massive black holes grow and the conditions most favorable for their growth. Contribution to Other Science The sources of the x-ray background up to 10 keV have been identified by the Chandra X-ray Observatory. However, the bulk of the energy in the cosmic x-ray background resides in the higher energy regime, and the nature of the objects emitting between 10 and 600 keV still is not determined. By providing a census of extraga- lactic hard x-ray sources, BHFP can help determine whether the background is due to massive black holes or to some other set of point sources. A key reason for extending the BHFP energy range up to 600 keV is to study the poorly resolved 511 keV electron-positron annihilation line in the Galaxy; BHFP will have the angular resolu- tion to study the spatial distribution of sources in the direction of the Milky Way bulge and will conduct sensitive searches for point components. BHFPâs ability to monitor the variability of blazars (black holes with jets oriented along our line of sight) over a wide range of time scales in the crucial hard x-ray band will be, when combined with gamma-ray and radio data, important to understanding how these phenomenal jets are formed and how they accelerate particles to high energy. Finally, BHFP will use its low-resolution spectroscopic capability to measure spectral lines from supernova remnants and neutron stars, thus inferring the local supernova rate. Table 2.4 sum- marizes some of the supplementary science for BHFP, as well as its capability for making unexpected discoveries (see the following subsection). Opportunity for Unexpected Discoveries The primary opportunity for unexpected discoveries will come from the unprecedented measurements of hard x-ray time variability made possible by BHFP. At any given instant, the field of view of BHFP will be more than 10 percent of the entire sky (19 percent of 4Ï instantaneous FOV for EXIST, full 4Ï coverage for EXIST or CASTER during a day), with a sensitivity of roughly 1 mCrab over the course of a day. Thus, BHFP will have an unprecedented sensitivity to rare events giving rise to hard x-ray flares. Possible flaring sources might include new types of magnetars and x-ray pulsars, association of gamma-ray bursts with new types of supernovas, ultraluminous x-ray sources in merger galaxies, and x-ray flares associated with black hole mergers detected by LISA. Since the sky at hard x-ray energies has never been surveyed by an instrument with BHFP sensitivity and positional accuracy of tens of arcseconds, entirely new classes of quasi-steady sources of hard x-rays also may be identified. Assessment of Scientific Impact The BHFP will be unique among current or planned missions in high-energy x-ray sensitivity combined with large field of view and frequent coverage of the sky. The resulting hard x-ray sky maps, temporal variability data, and the large number of short-lived transient detections will have direct impact on a number of important astro- physical questions. Some of the most significant are described below. Because of the great advances that BHFP will make in measuring the variable high-energy sky, which to date has only been crudely mapped, some of the impact is certain to come from new phenomena that have not yet been anticipated. The deepest hard x-ray (above ~20 keV) surveys to date, by the Swift and INTEGRAL spacecraft, have yielded only a few hundred sources,10 not enough to probe very far into the hard x-ray luminosity function. The increase to tens of thousands of hard x-ray sources found by BHFP will be an advance similar to the improvement from â W.N. Brandt and G. Hasinger, 2005, Deep extragalactic x-ray surveys, Ann. Rev. Astron. Astrophys. 43:827-859, and references therein. â G. Weidenspointner, C.R. Shrader, J. Knoedlseder, P. Jean, V. Lonjou, N. Guessoum, R. Diehl, W. Gillard, M.J. Harris, G.K. Skinner, P. von Ballmoos, G. Vedrenne, J.-P. Roques, S. Schanne, P. Sizun, B.J. Teegarden, V. Schoenfelder, and C. Winkler, 2006, The sky distribution of positronium annihilation continuum emission measured with SPI/INTEGRAL, Astron. Astrophys. 450:1013-1021. L.M. Winter, R.F. Mushotzky, J. Tueller, C.S. Reynolds, and C. Markwardt, 2007, Early results from Swiftâs BAT AGN Survey, Presenta- 10â tion Number 002.25, 210th Meeting of the American Astronomical Society, Bull. Am. Astron. Soc. 39.
SCIENCE IMPACT 21 TABLE 2.4â Black Hole Finder Probe (BHFP): Broader Science Examples Program Program Characteristics Program Significance Galactic 511 Science Origin of the 511 keV electron-positron The universe contains localized sources of antimatter; keV emission question annihilation line toward the center of the one set of such sources is indicated by the 511 keV Milky Way electron-positron annihilation line detected toward the center of the Milky Way. Study of the distribution of Measurements 511 keV line flux vs. position and time in the 511 keV sources may indicate whether energetic the galactic center direction positrons are produced by extreme physics such as dark matter annihilation or injection by massive Quantities Distribution of 511 keV sources toward cosmic strings. determined the center of the Milky Way Galactic Science Rate of supernova explosions in the Milky Because we live inside the disk of the Milky Way supernova question Way Galaxy, dust extinction makes it difficult to determine rate the rate of stellar explosions in our Galaxy, which Measurements Detection of hard x-ray lines such as the has an impact on theories of cosmic-ray acceleration 68 and 78 keV 44Ti lines expected from and other basic astrophysics. BHFP will improve supernova remnants the assessment of the Milky Way supernova rate by measuring the dust-penetrating hard x-ray lines from Quantities Line flux vs. location in the Milky Way; supernova remnants. determined count of associated supernova remnants Serendipitous Science New types of hard x-ray sources revealed BHFP will perform a hard x-ray survey that is more science question by a high-sensitivity survey than an order of magnitude more sensitive than any done previously. This new discovery space may enable Measurements Hard x-rays and/or rapid variability not detection of completely new types of sources, such as associated with known source classes extreme magnetars or highly variable ultraluminous x-ray sources. Quantities Identification of new hard x-ray sources determined with previously unknown types of emitters the 300 gamma-ray sources detected by the Compton Gamma-Ray Observatory 11 to the approximately 10,000 gamma-ray sources that will be found by the Gamma-ray Large Area Space Telescope (GLAST) after its launch in late 2007. A major quest in astrophysics is to understand how galaxies and their constituent components evolve over the age of the universe. Supermassive black holes play a central role in this process through mechanisms not yet fully understood. In order to study this connection, the number, size, and evolution of black holes must first be determined. Although BHFP will not measure the entire black hole population in isolationâmany electromagnetic wave bands from radio to infrared to x-ray, as well as gravitational waves, provide signals that may be combined to obtain a complete black hole censusâit will provide crucial information on the local obscured population that will not be provided by any other mission. The BHFP contribution to the understanding of this component of galaxies will have broad impact on our knowledge of how black holes form and grow and how they influence the growth and evolution of galaxies. BHFP also will have significant impact on our knowledge of the population of explosive transients and may well enable the employment of these to probe the transition of the universe from the âdark agesâ to the present- day ionized structures. Because of the large detection rate for short transients, BHFP can detect rare events in numbers that will enable us to understand their distribution and frequency. Since many of these events are likely to be binary black hole mergers, the observations can impact knowledge of the event rates for the production of the R.C. Hartman, D.L. Bertsch, S.D. Bloom, A.W. Chen, P. Deines-Jones, J.A. Esposito, C.E. Fichtel, D.P. Friedlander, S.D. Hunter, L.M. 11â McDonald, P. Sreekumar, D.J. Thompson, B.B. Jones, Y.C. Lin, P.F. Michelson, P.L. Nolan, W.F. Tompkins, G. Kanbach, H.A. Mayer-Has- selwander, A. MÃ¼cke, M. Pohl, O. Reimer, D.A. Kniffen, E.J. Schneid, C. von Montigny, R. Mukherjee, and B.L. Dingus, 1999, The third EGRET catalog of high-energy gamma-ray sources, Astrophys. J. Suppl. Ser. 123:79-202.
22 NASAâS BEYOND EINSTEIN PROGRAM gravitational radiation that would be detected by LISA and the Laser Interferometer Gravitational Wave Observa- tory (LIGO). If a sufficient number of bright gamma-ray bursts are detected at high redshift, 12 BHFP localizations combined with ground-based optical follow-up spectroscopy will reveal the chemical enrichment of the universe in the dark ages. Such observations are difficult to make with quasars (which are relatively rare) and are beyond the reach of Type Ia and Type II supernova surveys. These measurements would broadly impact understanding of the evolution of structures, another major objective of modern astrophysics. A unique scientific niche for the BHFP concepts is in the studies of variable hard x-ray sources, includ- ing GRBs, x-ray binaries, ultraluminous x-ray sources, magnetars, AGNs, and other potential sources. With the e Â xceptions of the continuing GRB work with Swift and the upcoming GLAST mission, relatively little evolution of knowledge about these variable sources is expected over the next decade. In addition, as stated previously, the opportunity for unexpected discoveries is greatest among the highly variable sources that will be detected by BHFP. This science will not be incremental, but rather it will provide a unique window into the properties and evolution of astronomical objects, the physical processes of which are dominated by strong gravity. Science Readiness and Risk It may be quite difficult to make quantitative statements about the growth of black holes in the universe on the basis of BHFP observations. Inferences about black hole masses and their evolution frequently make use of the assumptions that the massive black holes are accreting at or near their Eddington limit and that approximately 10 percent of the mass-energy accreted is turned into radiation. In fact, both assumptions are known to be incorrect in many circumstances. âStarvedâ black holes may accrete at much less than the Eddington limit due to a paucity of local material, and many active galaxies fall orders of magnitude short of the canonical 10 percent radiative efficiency factor. The black hole at the center of our own Milky Way, as well as the more luminous black holes of low radiative efficiency that reside at the centers of many other galaxies, contradict at least one and possibly both of the standard assumptions for converting x-ray luminosity into a black hole growth rate. 13 Thus, the conversion of a hard x-ray luminosity to a black hole mass or accretion rate could be in error by a factor of 10 or more. As a result, BHFP may enable the derivation of an x-ray luminosity function versus lookback time, but most likely not a black hole mass function versus lookback time. Another risk factor that could affect the achievement of BHFP science goals is the level of positional accuracy that may be achieved with feasible implementations of coded-aperture imaging. The best accuracies cited by the two candidate missions are 11 arcsec for the EXIST LET and 42 arcsec for the CASTER LEI, roughly calculated as the angular resolution of the instruments (see Table 2.2) divided by the signal-to-noise ratio of the source detection. Experience with deep integrations from the Sub-millimeter Common User Bolometer Array (SCUBA), combined with deep optical images, indicates that there may be several moderate- to high-redshift candidates for the host galaxies of submillimeter sources having approximately 5-15 arcsec position accuracy; only deep centimeter radio images with sub-arcsecond positions have broken the degeneracy in host-galaxy identification. 14 A similar situa- tion may exist for hard x-ray sources in distant galaxies. Thus, BHFP may detect a number of high-redshift black holes but may not be able to identify the host galaxies and determine their redshifts. To fully realize the BHFP scientific potential, it may be important either to improve the source location accuracy to 5 arcsec or better (which is technically quite challenging), or to combine BHFP detections with follow-up observations using a focusing hard x-ray telescope or wide-field infrared and radio surveys.15 The uncertain availability in the complementary V. Bromm and A. Loeb, 2006, High-redshift gamma-ray bursts from Population III progenitors, Astrophys. J. 642:382-388. 12â F. Yuan, S. Markoff, and H. Falcke, 2002, A Jet-ADAF Model for Sgr A*, Astron. Astrophys. 383:854-863. 13â R. Ivison, T. Greve, I. Smail, J. Dunlop, N. Roche, S. Scott, M. Page, J. Stevens, O. Almaini, A. Blain, C. Willott, M. Fox, D. Gilbank, 14â S. Serjeant, and D. Hughes, 2002, Deep radio imaging of the SCUBA 8-mJy survey fields: Submillimetre source identifications and redshift distributions, Mon. Not. R. Astron. Soc. 337:1-25. Examples of complementary observations or missions include the Constellation-X Hard X-ray Telescope, the NUSTAR Small Explorer 15â mission, the upcoming Wide-field Infrared Survey Explorer MidEx mission, and the existing Very Large Array survey âFaint Images of the Radio Sky at Twenty cm.â
SCIENCE IMPACT 23 position information during the time frame of BHFP operations is potentially a significant scientific limitation for the presented BHFP mission concepts. Since the x-ray luminosity thresholds are rather high at high redshifts, it also is important to combine BHFP measurements with sensitive hard x-ray surveys of narrow regions of sky to access the z ~1-2 population over a wider luminosity range. This may be done by combining BHFP information with surveys that could be done by the Con-X Hard X-ray Telescope instrument or by Simbol-X (a proposed hard x-ray focusing mission). Although the continued development of the lanthanum bromide scintillators being studied by the CASTER team may improve high-energy sensitivity, this will not reduce the detection thresholds in the critical band below about 200 keV. Steps for Moving Forward Further science planning work, perhaps by the proposing teams, will be important to determine the ancillary observations at other wavelengths that will help in identifying the x-ray sources and then improving their charac- terization. As noted in the preceding subsection, the conversion from x-ray luminosity to accretion rate has a large uncertainty. Multiwavelength observations of the brighter hard x-ray sources detected by INTEGRAL and Swift and related theoretical developments may lead to better accretion models that will enable an improved conversion from BHFP x-ray luminosities to mass accretion rates. Since multiwavelength information is critical to achiev- ing the primary science objectives, this planning work should be incorporated into the mission at an early stage. Combining the BHFP data with the multiwavelength observations, within a solid theoretical framework, will be necessary for BHFP to realize its full scientific potential. BHFP was originally proposed as one of the three Einstein Probes in the original Beyond Einstein Program. These missions were envisioned as medium-scale projects that could be executed in less time, and for consider- ably less money (up to about $600 million), than would be needed for the flagship LISA and Con-X missions. However, the independent assessments produced for the committee estimate that the BHFP probe concepts have costs well above a billion dollars (see the section âMission Cost Assessmentsâ in Chapter 3). Furthermore, the BHFP candidates are quite massive spacecraft that will require expensive launch vehicles in the Atlas V class. Thus, BHFP costs become a significant factor in the ability to realize most or all of the Beyond Einstein science portfolio. Since BHFP sensitivity scales with the square root of the collecting area, a decrease by a factor of 4 in the detector area would reduce the source-detection threshold by only a factor of 2, with a large savings in Âdetector mass and potential savings in launch vehicle cost. This possible scope reduction should be considered as a means of accelerating the time scale in which BHFP can be implemented. If the predicted masses of the candidate BHFP missions remain near 10,000 kg, a requirement for either BHFP mission to be viable is that the relevant high- capacity launch systems remain available (or be developed) at a reasonable cost for the approximate time frame of a BHFP launch. Science Assessment Summary The BHFP concepts presented are both hard x-ray all-sky surveys covering a range from a few kiloelectron- volts to 600 keV. Since massive black holes already are known in many galaxies, finding more such objects would not constitute a revolutionary contribution to Beyond Einstein science. However, detecting the formation of black holes through gamma-ray bursts in the early universe would be a revolutionary new discovery of relevance for Beyond Einstein. The science risk for Beyond Einstein is rather high. Although a census of massive black holes in g Â alaxies can be achieved, only very-high-luminosity and very-high-mass black holes will be seen at high redshifts. In addition, the very uncertain conversion from x-ray luminosity to black hole growth rate implies that BHFP will not provide a unique value (to better than a factor of 10) of the black hole growth rate (e.g., in solar masses per year) in any individual galaxy. Finally, the difficulty in identifying host galaxies also yields significant risk in the interpretation of BHFP results. A hard x-ray survey mission such as BHFP will be a unique facility, unmatched by any other space- or ground-based facilities. Thus, it provides an opportunity for the discovery of new types of variable x-ray sources that may relate to the Beyond Einstein Program in unpredictable ways. For example, studies of the power spectra
24 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.5â Black Hole Finder Probe (BHFP): Summary of Scientific Evaluation Potential Contributions to Science Beyond Einstein Broader Science Factors Revolutionary Massive black holes already are known in many galaxies. Hard x-ray variability on time scales of milliseconds discovery BHFP may find such black holes in different types of to days provides the potential for detecting entirely potential galaxies, where they might not follow the canonical new types of x-ray emitters, such as extreme relation between black hole mass and galaxy bulge magnetars or highly variable ultraluminous x-ray characteristics. In addition, the possibility of detecting sources. In addition, unexpected new classes of gamma-ray bursts at redshifts higher than 7 could sources may be found to be major contributors to provide insight on the stages of black hole formation in the hard x-ray background. the early universe. Science readiness Three major areas of science risk have been identified: The likelihood of finding unknown types of variable and risk (1) BHFP sensitivity is adequate to detect only the most sources with a significant astrophysical impact is luminous hard x-ray sources at high redshift, making it unknown. However, BHFP certainly will measure difficult to infer the evolution of black hole masses or hard x-ray variability on a variety of time scales x-ray emission over time; (2) the conversion from x-ray that are associated with the evolution of accretion luminosity to black hole growth rate is uncertain by at disks and relativistic jets near massive black holes. least an order of magnitude, depending on unknown Although individual supernova remnants will be accretion rates and radiative efficiencies, making the identified through their hard x-ray spectral lines, assessment of black hole growth dependent on very these identifications may not translate into a strong poorly constrained models; and (3) the achievable constraint on the overall supernova rate in the position accuracy may be inadequate to identify the host Galaxy. objects for x-ray sources, particularly at high redshifts. Mission Uniqueness Versus other BHFP would perform an all-sky hard x-ray survey a No other hard x-ray surveys in the past or future space missions factor of 10 to 100 more sensitive than any previous have sensitivity and cadence comparable to those satellite, detecting approximately 100 times more x-ray- of the BHFP, so BHFP has a unique capability to emitting black holes than Swift or INTEGRAL. It will find new types of variable x-ray sources. Further, no detect several times more gamma-ray bursts than seen by missions in prospect have the ability to detect and Swift. No other proposed U.S. or international missions locate the sources of the 511 keV electron-positron will have comparable capabilities. annihilation line as well as the supernova remnant sources of lines in the ~100 keV range. Versus ground- Because of the opaqueness of the atmosphere, no ground- No hard x-ray observations are possible from the based instruments based instrument can perform hard x-ray observations. ground. NOTE: See Appendix G in this report for definitions of acronyms. of hard x-ray variability in as many as a thousand massive black holes may enable the direct determination of the black hole masses. Studies of the duty cycles of ultraluminous x-ray sources will allow quantitative studies of the populations of these unusual objects and of the number density of the intermediate-mass black holes that they may represent. BHFP will make significant contributions to several broad science goals by resolving the source(s) of the hard x-ray background and the galactic 511 keV positron-electron annihilation line, as well as identifying new supernova remnants by means of their hard x-ray spectral lines such as 44Ti at 68 and 78 keV (Table 2.5).
SCIENCE IMPACT 25 CONSTELLATION-X Introduction X-ray emission is characteristic of the most violent and energetic objects in the universe, including accreting black holes of all sizes, neutron stars, supernovas and their remnants, events such as gamma-ray bursts associated with the formation of stellar-mass black holes, and mergers of clusters of galaxies. In addition, the gravitational growth of large-scale structure has heated most of the normal matter (baryons) in the universe to high temperatures (~105-8 K), at which the primary emission and absorption occur in the ultraviolet (UV) and x-ray spectral bands. This intergalactic gas is seen most prominently in the densest regions, clusters of galaxies, where the gas is a par- ticularly hot and bright emitter of x-rays. An advantage of x-rays over some other radiation is that hard x-rays have the property of penetrating significant amounts of matter (hence their use in medical diagnosis), which means that x-rays associated with accretion around black holes can escape from these very dense regions and be observed. X-ray astronomy began in the late 1940s with the detection of x-rays from the Sun using instruments on sound- ing rockets.16 The first detection of extrasolar sources of x-rays occurred in 1962 when a point source of x-rays (Sco X-1) and the diffuse x-ray background were discovered.17 Early work in x-ray astronomy was limited by the very short exposures possible with sounding rockets. The launch of the Uhuru satellite in 1970 revolutionized the subject, providing a survey of the entire sky and allowing detailed studies of individual sources. X-ray satellites flown during the following 37 years have provided profound insights into the nature of the most energetic objects in the universe. Perhaps the most important instrumental developments have involved the launch of x-ray tele- scopes with imaging detectors, starting with the Einstein X-ray Observatory, and culminating with Chandra, which has arcsecond angular resolution. Two areas that are ripe for further exploration are very high spectral resolution observations with a sufficiently high throughput to study a wide range of sources, and hard x-ray imaging. Constellation-X (Con-X) is one of the two Great Observatories within the Beyond Einstein Program. 18 Its pri- mary new capability is very high spectral resolution, high-throughput x-ray spectroscopy, representing an increase in these capabilities of roughly two orders of magnitude over missions currently flying (Tables 2.6 and 2.7). A secondary strength of Con-X is imaging and spectroscopy capability in the hard x-ray region of the spectrum. A single satellite will contain four high-throughput Spectroscopic X-ray Telescopes (SXTs), each equipped with an X-ray Microcalorimeter Spectrometer (XMS), which is an array of nondispersive, high-resolution spectrometers (see Table 2.7). The total collecting area will be about 15,000 cm 2 at a photon energy of 1.25 keV. One or two of the SXTs will also host dispersive X-ray Grating Spectrometers (XGSs), which provide high spectral resolution in the 0.3 to 1 keV band. Con-X will also have one or two Hard X-ray Telescopes (HXTs), which will extend the band-pass up to 40 keV. All of the instruments will operate simultaneously, which increases the observing efficiency and makes it possible to obtain simultaneous spectral information across the 0.3 to 40 keV band for variable objects, such as accreting black holes. Con-X is a facility-class astronomical observatory. In addition to its key science projects, it will contribute to many other astronomical areas as a result of observations proposed by general observers. Con-X was rated as the second highest priority among new space observatories (after the James Webb Space Telescope [JWST]) in the previous NRC decadal survey Astronomy and Astrophysics in the New Millennium, and was strongly endorsed by the NRCâs Connecting Quarks with the Cosmos report. The decadal survey said that Con-X âwill become the premier instrument for studying the formation and evolution of black holes of all sizes.â 19 T.R. Burnight, 1949, Soft x-ray radiation in the upper atmosphere, Phys. Rev. 76:165; H. Friedman, S.W. Lichtman, and E.T. Byram, 1951, 16â Photon counter measurements of solar x-rays and extreme ultraviolet light, Phys. Rev. 83:1025. R. Giacconi, H. Gursky, F.R. Paolini, and B.B. Rossi, 1962, Evidence for x-rays from sources outside the solar system, Phys. Rev. Lett. 17â 9:439. The Beyond Einstein Great Observatories are major, facility-class missions with broad applications to problems throughout astrophysics 18â and physics, similar in their expected impact to the Hubble Space Telescope and the Chandra X-ray Observatory (National Aeronautics and Space Administration, Beyond Einstein: From the Big Bang to Black Holes, Washington, D.C., January 2003, p. 5). National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001, p. 19â 11.
26 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.6â Constellation-X (Con-X): Mission Description Parameter Value Primary measurement X-ray spectroscopy Observatory type X-ray telescopes, 0.3-40 keV Projected years in orbit >5 yr, with 10 yr of consumables Type of orbit L2 halo orbit Mission phases One phase, facility-class observatory Science operations Guest observer programs with key projects Other mission characteristics All instruments operate simultaneously TABLE 2.7â Constellation-X (Con-X): Mission Instrument Properties Spectral Range Spatial Resolution Spectral Resolution Collecting Area Field of View Instrument (keV) (HPD arcsec) (E/âE) (cm2) (arcmin2) Microcalorimeter 0.3-10 15 2,400 at 6 keV 15,000 at 1.25 keV 7 spectrometerâ 6,000 at 6 keV core array Microcalorimeter 0.3-10 15 300 15,000 at 1.25 keV 21 spectrometerâ 6,000 at 6 keV outer array Grating 0.3-1 15 1,250 1,000 Not spectrometer applicable Hard x-ray 6-40 30 10 150 25 telescope NOTE: HPD, half power diameter. Connecting Quarks with the Cosmos cited Con-X and LISA as holding âgreat promise for studying black holes and for testing Einsteinâs theory in new regimes.â20 Mission Science Goals Contribution of the Mission Directly to Beyond Einstein Science Goals Con-X will test general relativity in the strong field limit by time-resolved spectroscopy of material being accreted just outside the horizon of black holes. The key feature here is high-resolution spectroscopy with high throughput, allowing good time resolution to observe the motions of individual hotspots in the accretion disk. The most useful spectral feature for this capability is the Fe K line, emitted at 6.4 keV. 21 In addition to the motions National Research Council, 2003, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, The National 20â Academies Press, Washington, D.C., p. 7. Y. Tanaka, K. Nandra, A.C. Fabian, H. Inoue, C. Otani, T. Dotani, K. Hayashida, K. Iwasawa, T. Kii, H. Kunieda, F. Makino, and M. 21â Matsuoka, 1995, Gravitationally redshifted emission implying an accretion disk and massive black hole in the active galaxy MCG-6-30-15, Nature 375:659; T.J. Turner, L. Miller, I.M. George, and J.N. Reeves, 2006, Evidence for orbital motion of material close to the central black hole of Mrk 766, Astron. Astrophys. 445:59.
SCIENCE IMPACT 27 of individual hotspots, the composite line profiles will be used to determine the spins of the black holes. A wide spectral bandwidth is important for separating the emission lines from the continuum; the hard x-ray capability of Con-X is particularly useful in this regard. A concern with the Con-X test of strong gravity around black holes is that the physics of accretion may turn out to be quite complex, and hotspot accretion disks may not move bal- listically. However, Con-X should provide very detailed information on the behavior of accreting matter, thus ad- dressing the last part of the Beyond Einstein Program key question: How do black holes manipulate space, time, and matter? With its very large collecting area, Con-X will allow the study of the evolution of accretion processes over a significant fraction of cosmic time. Con-X will also provide at least two measurements of the nature and evolution of dark energy. Both techniques involve the study of clusters of galaxies. Clusters can be used to determine distances (independent of redshift) if one assumes that the gas mass fraction in clusters (essentially, the baryon fraction) is independent of redshift. A geometric measure of dark energy and its evolution comes when these distances are compared to the expansion velocity (redshift).22 A second type of measurement of dark energy comes from its effect on the growth of struc- ture in the universe. Structure growth will be assessed through observations of the mass distribution function of clusters as a function of redshift. The constraints on dark energy from clusters may be comparably restrictive to those from some other techniques and will have different confidence regions in terms of the relevant cosmologi- cal parameters. Cosmic nucleosynthesis, clusters, and the Wilkinson Microwave Anisotropy Probe (WMAP) provide consis- tent measurements of the contribution of baryons to the cosmological density. All of the known material (stars, galaxies, gas in galaxies and clusters of galaxies) accounts for less than 40 percent of the baryons expected in the low-redshift universe, however.23 Cosmological simulations indicate that most of the baryons should be in the form of diffuse intergalactic gas.24 At low redshifts, this gas is heated by the gravitational growth of structure to temperatures of 105 to 3 Ã 107 K. The intergalactic gas is so diffuse that it would be very difficult to detect in emission. Con-X should be able to detect this Warm-Hot Intergalactic Medium (WHIM) by detecting absorption in the x-ray spectra of AGNs, mainly through the O VII and O VIII x-ray absorption lines. Although most of the mass in the WHIM is in hydrogen and helium, oxygen is the most common heavier element. It may also be possible to detect some of the WHIM by UV absorption measurements in the O VI line with the Cosmic Origins Spectrograph to be installed on the Hubble Space Telescope (HST) during the next servicing mission. However, the bulk of the baryons probably are at higher temperatures and are observable only with x-rays. Although the scientific projects discussed above and at the top of Table 2.8 represent the core science defini- tion program for Con-X, there are several other research projects that directly address Beyond Einstein science goals. For example, Con-X will study the evolution of supermassive black holes in the universe. Many of the AGNs associated with these objects are optically faint and strongly absorbed, so Con-Xâs high throughput to hard x-rays will be essential to studying these sources. Also, detailed studies of bright AGNs may lead to a deeper understanding of accretion physics, which would help scientists convert the observed x-ray luminosities of AGNs into more accurate estimates of the growth rates of their supermassive black holes. Con-X will help to determine the nature of dark matter by mapping the dynamics of clusters of galax- ies. It should also detect or strongly limit the masses of sterile neutrinos and other decaying warm dark matter candidates. S.W. Allen, R.W. Schmidt, H. Ebeling, A.C. Fabian, and L. van Speybroeck, 2004, Constraints on dark energy from Chandra observations 22â of the largest relaxed galaxy clusters, Mon. Not. R. Astron. Soc. 353:457. M. Fukugita and P.J.E. Peebles, 2006, The cosmic energy inventory, Astrophys. J. 616:643. 23â R. Cen and J.P. Ostriker, 2006, Where are the baryons? II. Feedback effects, Astrophys. J. 650:560. 24â
28 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.8â Constellation-X (Con-X): Beyond Einstein Science Programs Science Program Program Characteristics Program Significance Science Test general Science Motion near black holes Does Einsteinâs theory correctly describe gravity definition relativity in question near black holes? Measuring black hole spins programs the strong- will also help to determine how they formed and field limit Measurements Observe motion of hotspots in evolved. The physics of accretion will also be and measure black hole accretion disks studied. black hole spins Quantities Blob velocities, black hole determined spins Measure the Science What is the nature of dark Is the dark energy just Einsteinâs hypothetical evolution question energy? cosmological constant or a new force in the of dark universe? Learning how dark energy evolved will energy using Measurements Cluster distance as a function tell us about its nature and help predict the future clusters of of redshift; cluster abundance of the universe. galaxies evolution Quantities Cluster distances; cluster determined masses Detect the Science Where are most of the atoms? Only a small fraction of the atoms in the universe baryons question have yet been seen. X-ray absorption should allow in the us to determine the distribution of most of these Warm-Hot Measurements Absorption of quasar x-rays by thus-far invisible atoms. Intergalactic excited oxygen atoms Medium (WHIM) Quantities Amount and metallicity of gas determined in filaments Additional Evolution of Science Relation of SMBH growth to SMBH masses are observed to be closely related to Beyond supermassive question formation of galactic spheroids those of their host galactic spheroids. Do SMBHs Einstein black holes regulate the growth of spheroids or vice versa? science (SMBHs) Measurements X-rays from SMBHs hidden How important are mergers vs. accretion in SMBH within clouds of gas and dust growth? Quantities X-ray luminosity and determined attenuation Probing the Science Does dark matter emit energy Dark matter constitutes most of the mass of the nature of question via decay or annihilation? universe, but we still do not know what it is. dark matter Detection of energy inputs from dark matter in Measurements Line emission in galaxy clusters could help determine its nature. clusters Quantities Line energies, luminosities, determined and widths Contribution of the Mission to Other Science Because Con-X is a facility-class Great Observatory, it is likely that many of the most important scientific contributions of Con-X will be in areas outside those key projects and other Beyond Einstein science mentioned above. Some specific questions are listed in Table 2.9. For example, it has become clear recently that energy input from supermassive black holes helps to regulate the formation of large galaxies and the gas in clusters. Observa-
SCIENCE IMPACT 29 TABLE 2.9â Constellation-X (Con-X): Broader Science Examples Program Program Characteristics Program Significance Determine Science question Equation of state of neutron stars Determining the nature of matter within neutron properties stars will tell us about the strong interaction, of matter at Measurements X-ray line redshifts and widths and could discover a new state of matter at high extremely density. high densities Quantities determined Masses and radii of neutron stars Magnetar Science question How large are the magnetic fields in Magnetars are young neutron stars that are magnetic young neutron stars? believed to have very strong (~1014 G) magnetic fields fields. These fields can break down the Measurements Detect proton cyclotron lines vacuum, and provide a strong test of quantum electrodynamics. Quantities determined Magnetic-field strength Cosmic Science question How do supermassive black holes The formation of galaxies is strongly affected by feedback from affect galaxies? energy inputs from the supermassive black holes supermassive in galaxy centers. black holes Measurements Measure velocities and densities induced by jets and winds Quantities determined Velocity, density, kinetic energy input Supernovas Science question Where do heavy elements originate? Heavy elements are made by fusion in stars and the origin and dispersed by stellar explosions. Con-X will of heavy Measurements X-ray emission lines from heavy determine the origin of heavy elements (including elements elements in supernovas and remnants some beyond iron) by detecting their x-ray lines in supernova debris. Quantities determined Abundance of elements Stellar activity Science question How active are Sun-like stars, Solar flares and other activity from the Sun affect on Sun-like and how do they affect their life on Earth. By observing other similar stars, we stars environments? can learn about the likely history of the Sun and the effect of stellar activity on forming planetary Measurements X-ray emission and motions in stellar systems. flares and corona Quantities determined Densities, temperatures, velocities Interactions Science question How do comets and planets interact Observations with ROSAT and Chandra showed of comets and with the solar wind? that comets and planets emit surprising amounts of planets with x-rays. These occur by interactions with the solar the solar wind Measurements Line emission by charge exchange wind. Quantities determined Composition, density, and ionization tions with Chandra have provided the first clear information on this feedback;25,26 high-spectral-resolution x-ray observations with Con-X are important to understanding the detailed physics of all forms of AGN feedback. A.C. Fabian, J.S. Sanders, G.B. Taylor, S.W. Allen, C.S. Crawford, R.M. Johnstone, and K. Iwasawa, 2006, A very deep Chandra observation 25â of the Perseus cluster: Shocks, ripples, and conduction, Mon. Not. R. Astron. Soc. 366:417; D.A. Rafferty, B.R. McNamara, P.E.J. Nulsen, and M.W. Wise, 2006, The feedback-regulated growth of black holes and bulges through gas accretion and starbursts in cluster central dominant galaxies, Astrophys. J. 652:216. S. Kaspi, W.N. Brandt, I.M. George, H. Netzer, D.M. Crenshaw, J.R. Gabel, F.W. Hamann, M.E. Kaiser, A. Koratkar, S.B. Kraemer, G.A. 26â Kriss, S. Mathur, R.F. Mushotzky, K. Nandra, B.M. Peterson, J.C. Shields, T.J. Turner, and W. Zheng, 2002, The ionized gas and nuclear environment in NGC 3783. I. Time-averaged 900 kilosecond Chandra grating spectroscopy, Astrophys. J. 574:643.
30 NASAâS BEYOND EINSTEIN PROGRAM Con-X will discover close pairs of orbiting supermassive black holes by detecting pairs of iron K emission lines produced by the Doppler shifts due to the orbital motions. Con-X will constrain the properties of matter at extremely high densities by determining the masses and radii of neutron stars. These will be determined by measuring the redshifts (z, essentially GM/R) and pressure-broadened widths (g, essentially GM/R2) of x-ray absorption lines and by pulse shapes during burst oscillations. The masses and radii will constrain the equation of state of ultradense matter in neutron stars and determine the role of exotic phases of matter, such as quark-gluon plasma. Con-X should be able to detect ion cyclotron lines from magnetars and confirm that these are very highly magnetized neutron stars. The strong fields may provide a unique test of quantum electrodynamics through changes in behavior at the quantum critical magnetic field, which is given by B = me2 c3/(he) = 4.4 Ã 1013 G. However, the complex physics of the x-ray emission mechanisms near magnetars and the possibly complicated magnetic field geometry may make these tests ambiguous. Con-X should resolve the mystery of the nature of ultraluminous x-ray sources (ULXs) seen in many nearby galaxies. These are x-ray sources that, although not located at the centers of galaxies, have luminosities which are too large to be due to simple accretion by neutron stars or stellar-mass black holes. One theory is that these are binary stars with intermediate-mass (100 to 10,000 solar masses) black holes. Spectra of supernova remnants and other explosive phenomena will provide important dynamical information on these events. Con-X should give the first measurement of abundances of heavy elements beyond the iron peak and may determine the sites of heavy-element nucleosynthesis. Con-X would extend the study of stellar coronae, flares, and other stellar activity to other Sun-like stars. Solar activity affects communications and other aspects of life on Earth, and stellar activity may influence the conditions under which planets form. On the other end of the astronomical spectrum, x-ray spectra of comets and Jovian planets will provide new information on their composition and interactions with the solar wind. Opportunity for Unexpected Discoveries Because Con-X is a facility-class general observatory, the probability that it will enable unexpected discoveries is very high. It is important to keep in mind that many of the most important discoveries by Chandra and HST, two previous Great Observatories, were completely unanticipated. (Note that even the first-light image with Chandra of the supernova remnant Cas-A discovered a probable neutron star remnant at its center, which has never been detected in any other waveband.27) In addition to the chances for serendipitous discoveries, the general observer program of Con-X will harness the ingenuity of the entire astronomical community. Past experience has shown that many extremely clever and innovative ideas emerge when the entire world of astronomers and physicists has access to an observatory with vastly increased capabilities, such as Con-X. The recent history of astronomy has shown the great value of multiwavelength observations. High-throughput and high-spectral-resolution x-ray measurements with Con-X will complement observations with the Atacama Large Millimeter Array (ALMA) in the millimeter and submillimeter, the JWST in the infrared, and the large ground-based optical/infrared observatories being planned for the same time period. It should be noted, however, that x-ray astronomy has become a mature scientific area, and Con-X is not a survey instrument. Thus, Con-X may have less potential than some other missions do for discovering entirely new classes of objects, but it will nevertheless make fundamental contributions to the understanding of currently known phenomena and to basic physics. H. Tananbaum, 1999, Cassiopeia A. IAU Circ. 7246; G.G. Pavlov, V.E. Zavlin, B. Aschenbach, J. Truemper, and D. Sanwal, 2000, The 27â compact central object in Cassiopeia A: A neutron star with hot polar caps or a black hole? Astrophys. J. 531:L53.
SCIENCE IMPACT 31 Assessment of Scientific Impact Overall Assessment Revolutionary Nature of the Scienceâ Although the capabilities of Con-X represent an evolution of x-ray satel- lite technology and it is not a survey instrument, its very large collecting area and high-resolution spectrometry capability could lead to fundamental discoveries. The science goals for Con-X include tracing baryonic matter in the WHIM, determining the mass and radius of neutron stars and the mass and spin of stellar-mass black holes, studying the formation and evolution of supermassive black holes (SMBHs) and their roles in galaxy and clus- ter formation, and measuring cosmological parameters using clusters of galaxies. Con-X could find potentially revolutionary surprises in any of these areasâfor example, discovery of a new state of matter deep in neutron stars or deviations from the expected Kerr metric around black holes. However, interpreting any of these potential observations may be complicated because of the complex physics involved. Precision Measurement of Fundamental Quantitiesâ The best opportunity to make a precision measurement of a fundamental quantity is probably the determination of the dark energy equation-of-state parameter w by measur- ing the growth in the number of clusters of given mass as redshift decreases. There is no question that Con-X will enable greatly improved measurements of temperature, metallicity, and other properties of clusters. However, the interpretation of these measurements is likely to be somewhat uncertain, since clusters are complex. Cosmologi- cal dark matter simulations determine the number density of clusters of a given mass accurately as a function of cosmological parameters, but the challenge is to determine cluster masses accurately from observable quantities. While theory is improving, there are still serious difficulties in understanding cluster energy input and its con- sequences. In particular, the energy input from the growth of SMBHs is likely to be important in explaining the absence of cooling flows in low-redshift clusters and in preventing the overcooling of baryons at higher redshifts. The nature and timing of such energy inputs and how the energy couples to the cluster baryons remain uncertain, and one of the key advances from Con-X observations would be to help answer these questions. It is possible that the improved theoretical understanding of clusters will indeed enable a high-precision measurement of w, but the level of attainable precision is difficult to estimate at present. Another way that Con-X could improve the measurement of w is to measure the distance to clusters indepen- dent of their redshifts, assuming that the cluster baryon fraction is independent of redshift. The open question here is whether the uncertainties will be small enough to be competitive with other methods. However, it is important to measure fundamental quantities by several independent methods that have different sources of uncertainty, and the methods using clusters are quite different from the other methods being pursued using supernovas, weak lens- ing, and baryon acoustic oscillations. Advances in Basic Astrophysicsâ Unquestionably, Con-X would advance astrophysics on a broad front. Besides its science driversâtesting strong-field general relativity, determining the dark energy parameter w, and observing WHIMâCon-X will provide important new information on many other key astrophysical questions. Its unprec- edented spectral capabilities and high-energy x-ray sensitivity will allow Con-X to clarify the evolution of SMBHs and their role in the evolution of galaxies and clusters. Con-X can detect close SMBH binaries by means of their spectra. It can also constrain the nature of dark matter and clarify how heavy elements are formed. It can determine the nature of the ULXs that have recently been detected in nearby galaxies; the presence of relativistic iron K-shell lines and the variability time scales could solidify the interpretation of these observations as intermediate-mass black holes (IMBHs). And Con-X could also improve the understanding of flares on the Sun and nearby stars and could probe the magnetospheres of the Jovian planets in our solar system and the composition of comet comas. Breadth of the Science Impactâ It should be clear that Con-X would have an extremely broad impact on astrophys- ics and beyond. Its observations of magnetars could test quantum electrodynamics in the strong-magnetic-field regime, and new data on dark matter properties could be a significant new input for fundamental particle physics.
32 NASAâS BEYOND EINSTEIN PROGRAM But the main impact of the Con-X would be to extend the enormous progress of x-ray astronomy by enabling high-spectral-resolution measurements of a wide range of phenomena. Context of Science and Mission Unique Capabilitiesâ Con-X will be unique. Given its roughly two-order-of-magnitude increase in spectral resolu- tion and collecting area, no other existing x-ray observatory can match its high-throughput spectral capability. Complementary Role with Other Missionsâ The large ground-based optical telescopes such as the W.M. Keck Observatoryâs twin 10-m-diameter telescopes and the Very Large Telescope (VLT) have complemented the high angular resolution of HST with high-resolution optical spectrometry. If Con-X is active during the period when new instruments such as ALMA and JWST are available, the opportunities for complementary measurements will be increased, since the universe is relatively transparent to submillimeter, infrared, and x-ray photons that these instruments will observe. For example, JWST observations and Con-X x-ray spectra could together characterize the AGN population out to high redshift, while ALMA and Con-X could determine the role of obscured AGNs in submillimeter-bright galaxies. Thirty-meter-class ground-based optical telescopes are expected to begin operating within about a decade, and Con-X would also complement these instruments, for example, by measuring galaxy outflow winds driven by starbursts and AGNs. Can the Science Questions Be Answered by Other Space Missions and/or by Ground-Based Capabilities?â Some of the science questions that Con-X will address (e.g., the nature and evolution of dark energy, or the structure of spacetime near black holes) can be addressed by other missions. In these cases, it would still be valuable to make these measurements in several different ways. In addition, many of the science questions that Con-X will address, including the nature of dark matter in clusters and detecting the majority of baryons in intergalactic gas, require x-ray observations. Since x-rays do not penetrate the atmosphere, it is essential to put x-ray telescopes in space. The other proposed international x-ray telescope that may most closely match or exceed Con-X capabilities is the European X-ray Evolving Universe Spectroscopy (XEUS) mission; however, projected European Space Agency (ESA) funding would not permit XEUS to be started for perhaps a decade. Japanâs New X-ray Telescope (NeXT) mission, with a possible launch in about 2013, would cover roughly the same hard x-ray energies as are covered by the Con-X HXT, but with an effective area at least an order of magnitude smaller, poorer angular resolution, and a smaller field of view, limiting it to the study of only the brightest sources. Science Readiness and Risk Risks to Achieving Science Goals With the foundations laid by Chandra and X-ray Multi-Mirror Mission-Newton (XMM-Newton), the field of x-ray astronomy has become quite mature. These previous observatories have provided information on thousands of sources. As mentioned earlier, Con-X represents an evolution in technology through high spectral resolution and throughput. These advances will enable a number of high-science-return measurements on known sources. For example, the high throughput will allow sensitive measurements of time-varying sources critical to the study of compact objects. Aside from the well-known risks of satellite implementation, a number of technical risks have been identi- fied by the Con-X team, as discussed in detail in Chapter 3. Other than these technical risks, the science risks are moderate to low. As discussed above, the most significant science risks are associated with the possible physical complexity of the systems (accretion disks around black holes, clusters of galaxies) that will be used to probe strong gravity and dark energy.
SCIENCE IMPACT 33 Required Enabling Science The time and spectral resolution capabilities of Con-X stand alone in the study of black hole properties. In order to constrain the evolution of dark energy using clusters of galaxies, Con-X will require larger samples of high-redshift clusters than currently exist. However, it is very likely that such samples will be available before they are needed, as a result of Planck or ground-based Sunyaev-Zeldovich surveys, or from x-ray detections with current observatories or the extended Roentgen Survey with an Imaging Telescope Array (eROSITA). Although these surveys will provide high-redshift cluster samples, the high throughput and spectral resolution of Con-X will be needed to determine the cluster properties accurately enough for strong dark energy evolution constraints. Con-X will be able to observe far more clusters than Chandra or XMM-Newton can. The ~15 arcsecond resolu- tion provided by Con-X will allow for some discrimination of merging and otherwise complex clusters from the relaxed clusters to be used for measuring dark energy parameters. However, it is likely that even higher resolution imaging of the clusters at other wavelengths will be required to fully achieve the stated dark energy goals. Finding the missing baryons by measuring absorption features on background continuum sources in the WHIM is a very powerful tool enabled by Con-X. The target AGNs for this measurement have already been identified to allow the detection of approximately 100 filaments. There are no other measurements needed to achieve this goal. However, continued modeling of the expected measurements will certainly help to make the best use of observing time. Evolution of Knowledge Versus Potential Mission Start The field of x-ray astronomy is defined by the current and planned missions. At the present time and for the period leading up to a Con-X deployment, the state of the art in x-ray astronomy is determined by Chandra and XMM-Newton data. Consequently, the state of the field at the start of the mission is easy to predict. While other missions will continue to make progress, the capabilities of Con-X will yield a significant step in the x-ray field. The only caveat to this is the potential of instrumental technology advances that would go beyond the Con-X stated goals. Steps for Moving Forward Con-X is one of the best-studied missions in the Beyond Einstein Program. Because of this high level of development, the committeeâs suggestions for moving forward are more focused than for some of the less-devel- oped missions. The Technology Readiness Levels (TRLs) of the key components of Con-X are in the TRL 3-5 range (in Chapter 3, see the section entitled âTechnology Readiness and Degree of Difficultyâ for the definition of TRLs). This high level of pre-Phase A readiness can be attributed to the heritage of the flight technology, strong community participation and support, and, finally, to the availability of significant resources for technology and mission development. All of the components have strong heritage with previous missions, most notably Chandra and XMM-Newton. The team has produced a large volume of studies to back up its plans to bring these compo- nents to flight status. The committee notes that the technological requirements to achieve the mission goals appear to have been purposely kept conservative. The positive side is that the path to achieving the requirements (such as an Âangular resolution of ~15 arcsec) is well defined. The significant progress achieved both at the laboratories and by u Â niversity-based groups indicates that a more aggressive influx of resources in key areas such as the mirror development, the staged cooler system, and the large microcalorimeter arrays would be of significant benefit to developments in these areas. Science Assessment Summary The primary strength of Con-X, one of the two Great Observatories included in the Beyond Einstein Program, is the ability to carry out x-ray astronomy with very high spectral resolution and high throughput, representing
34 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.10â Constellation-X (Con-X): Summary of Scientific Evaluation Potential Contributions to Science Beyond Einstein Broader Science Factors Revolutionary discovery Measure growth of structure and distance- Discovery of exotic phases of matter in potential redshift relation using clustersârevolutionary neutron starsâe.g., quark-gluon plasma. if w â â1. Potential discovery of small-separation orbiting Test general relativity in strong fields by supermassive black holes. measuring motions in accretion disks around black holes. Test of quantum electrodynamics in strong magnetic fields with magnetars. Science readiness and risk Unclear whether definitive measurement of Complex physics may make interpretation of cosmological parameters is possible using data difficult. clusters due to complex gas physics. Interpretation of data on accretion disk motion may be difficult. Mission Uniqueness Versus other space missions Detecting the bulk of baryons in the warm-hot The high-throughput, high-resolution intergalactic medium. capabilities of Con-X ensure that it will make unique and broad contributions to astrophysics. Versus ground-based instruments X-ray astronomy can only be done from space. X-ray astronomy can only be done from space. an improvement of two orders of magnitude over current missions. It will make the broadest and most diverse contributions to astronomy and physics of any of the Beyond Einstein missions. It also has the potential to make very strong contributions to Beyond Einstein science. However, other missions address the measurement of dark energy parameters and tests of strong-field general relativity in a more focused and definitive manner. A summary of the committeeâs evaluation of the scientific merit of Con-X within the Beyond Einstein Program is given in Table 2.10. However, given the very strong, very broad contribution that Con-X will make to basic astrophysics, the committee concluded that its merits can only be fully assessed when it is judged as a major astrophysics mis- sion in a context broader than that of the Beyond Einstein Program. INFLATION PROBE Introduction Inflation, the term for an era of early universe exponential expansion, has been proposed as a solution to several fundamental problems in cosmology. Among these is the âHorizon Problem,â the difficulty that apparently caus- ally disconnected regions appear to have almost identical conditions, as though they had been in thermal contact. Another is the âFlatness Problem,â the fact that the universe appears to be very close to being geometrically flat despite the fact that it should evolve away from flat as the universe expands.28 Inflation also naturally explains the generation of âseedsâ of structure formation from quantum fluctuations and predicts a nearly scale-free power spec- A. Guth, 1981, Inflationary universe: A possible solution to the horizon and flatness problems, Phys. Rev. D. 23:347. 28â
SCIENCE IMPACT 35 TABLE 2.11â Inflation Probe (IP): Mission Description Parameter CMB Polarization CIP Primary measurement CMB B-mode Survey HÎ± galaxy survey Observatory type Millimeter telescope Passively cooled slitless grating spectrograph Projected years in orbit 1 3 Type of orbit L2 or IRAS/COBE L2 or IRAS/COBE Mission phases One phase, full-time scanning One phase, full-time scanning Science operations Full-time scanning Full-time scanning Other mission characteristics Cryogenic NOTE: See Appendix G in this report for definitions of acronyms. TABLE 2.12â Inflation Probe (IP): Mission Instrument Properties Spectral Resolution Instrument Spectral Range Spatial Resolution (Î½/Î´Î½) Collecting Area Field of View EPIC-F 30-300 GHz 0.25-2.5 degrees 3 0.4 m2 5 degrees CMBPol 30-300 GHz 1 degree 3 0.2 m2 ~15 degrees EPIC-I 30-250 GHz 1 degree 3 0.002-0.1 m2 7 degrees CIP 2.5-5 Âµm 0.2 arcsec 600 2.54 m2 20 arcmin NOTE: See Appendix G in this report for definitions of acronyms. trum (P(k)).29 During the inflationary era the universe expanded by 30 orders of magnitude in linear scale, creating nearly all of the particles and radiation in the current universe. Evidence for a flat universe is very well established experimentally.30,31 More recently the power spectrum slope of the cosmic microwave background (CMB) has been measured with high precision and is, as expected, close, but possibly not exactly, scale invariant. 32 The Inflation Probe (IP) seeks to study the conditions that existed during this crucial phase in the history of the universe. Its objectives are challenging, since direct observational connections with this early era are difficult to find. Four proposed missions in the Beyond Einstein Program fall under the âInflation Probeâ title. Three are aimed at learning about the inflationary period using the signal imparted to the polarization of the CMB radiation by gravitational waves induced during the inflationary period. The fourth mission measures the structure in the universe on various length scales, which arises from the primordial density fluctuations induced by the inflation potential. The polarization mission concepts are CMB Polarimeter (CMBPol), the Experimental Probe of Infla- tionary Cosmology (called EPIC-F below, because it is a filled aperture telescope), and the Einstein Polarization Interferometer for Cosmology (called EPIC-I below, since it is an interferometric experiment). The polarization J.M. Bardeen, P.J. Steinhardt, and M.S. Turner, 1983, Spontaneous creation of almost scale-free density perturbations in an inflationary 29â universe, Phys. Rev. D. 28:679. P. de Bernardis, P.A.R. Ade, J.J. Bock, J.R. Bond, J. Borrill, A. Boscaleri, K. Coble, B.P. Crill, G. De Gasperis, P.C. Farese, P.G. Ferreira, 30â K. Ganga, M. Giacometti, E. Hivon, V.V. Hristov, A. Iacoangeli, A.H. Jaffe, A.E. Lange, L. Martinis, S. Masi, P. Mason, P.D. Mauskopf, A. Melchiorri, L. Miglio, T. Montroy, C.B. Netterfield, E. Pascale, F. Piacentini, D. Pogosyan, S. Prunet, S. Rao, G. Romeo, J.E. Ruhl, F. Scar- amuzzi, D. Sforna, and N. Vittorio, 2000, A flat universe from high-resolution maps of the cosmic microwave background radiation, Nature 404:955. D.N. Spergel, L. Verde, H.V. Peiris, E. Komatsu, M.R. Nolta, C.L. Bennett, M. Halpern, G. Hinshaw, N. Jarosik, A. Kogut, M. Limon, S.S. 31â Meyer, L. Page, G.S. Tucker, J.L. Weiland, E. Wollack, and E.L. Wright, 2003, First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Determination of cosmological parameters, Astrophys. J. Suppl. Ser. 148:175. D.N. Spergel, R. Bean, O. DorÃ©, M.R. Nolta, C.L. Bennett, J. Dunkley, G. Hinshaw, N. Jarosik, E. Komatsu, L. Page, H.V. Peiris, L. Verde, 32â M. Halpern, R.S. Hill, A. Kogut, M. Limon, S.S. Meyer, N. Odegard, G.S. Tucker, J.L. Weiland, E. Wollack, and E.L. Wright, 2006, Wilkinson Microwave Anisotropy Probe (WMAP) three year results: Implications for cosmology, Astrophys. J. Suppl. Ser. 170:377.
36 NASAâS BEYOND EINSTEIN PROGRAM missions are collectively designated âCMB polarization,â while the power spectrum mission is referred to as the Cosmic Inflation Probe (CIP). There are two types of CMB polarization patterns: E-modes, produced both during and after inflation by electron scattering, and B-modes, generated by small distortions in the E-mode pattern either from gravitational waves or gravitational lensing. The E-mode polarization has been detected with the predicted characteristics by several experiments.33,34 Tables 2.11 and 2.12 list the mission and instrument characteristics of the four IP mission concept studies. The importance of these measurements was detailed in the NRC report Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century35 and the joint National Science Foundation (NSF)/ NASA/Department of Energy (DOE) CMB Taskforce report.36 Mission Science Goals Contribution to Beyond Einstein Science Goals Each of the proposed Inflation Probe concepts will test the existence and properties of inflation. They are expected to shed light on the specific Beyond Einstein question, What powered the big bang? Specifically, the gravitational waves measured by the polarization missions will determine the magnitude of the potential during inflation, while the matter power spectrum measured by CIP gives information about the shape of the potential. In addition, understanding the accelerating expansion during the inflationary era might help to understand the dark energy that is causing the current acceleration of the expansion. The more detailed statement of the Beyond Einstein goal on what powered the big bang sets forth the goal of searching for gravitational waves from inflation and phase transitions in the big bang. While this statement indi- cates that the authors of the Beyond Einstein roadmap were expecting that the Inflation Probe would be a CMB polarization mission, the Cosmic Inflation Probe also seems relevant to the more general Beyond Einstein goal in the 2003 roadmap. Table 2.13 lists the Beyond Einstein science that would be performed by the four mission concepts. Observations of the polarization of the CMB can distinguish between different models of the early universe. The critical signal is the B-mode polarization of CMB fluctuations, imprinted on the CMB by gravitational waves generated by inflation. The B-mode amplitude is proportional to the energy density during inflation. If the inflation- ary model is correct, the successful detection of large-angular-scale B-mode polarization in the CMB produced by gravitational waves from inflation will therefore measure the energy scale of inflation. 37 As inflation ends, the energy density declines, locking in the shape of the power spectrum of primordial density fluctuations. The shape of the potential function for the inflation fields can be constrained by precise measurements of the spectral index (or slope) and curvature (or running) of the fluctuation power spectrum. 38 This well-under- stood technique has already been implemented with the CMB temperature fluctuations at large angular scales and surveys such as the Sloan Digital Sky Survey (SDSS). The CIP project proposes to substantially improve the knowledge of these quantities. E.M. Leitch, J.M. Kovac, N.W. Halverson, J.E. Carlstrom, C. Pryke, and M.W.E. Smith, 2004, DASI Three-year cosmic microwave back- 33â ground polarization results, Astrophys. J. 624:10. A. Kogut, D.N. Spergel, C. Barnes, C.L. Bennett, M. Halpern, G. Hinshaw, N. Jarosik, M. Limon, S.S. Meyer, L. Page, G.S. Tucker, E. 34â Wollack, and E.L. Wright, 2003, First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Temperature-polarization cor- relation, Astrophys. J. Suppl. Ser. 148:161-173. National Research Council, 2003, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, The National 35â Academies Press, Washington, D.C. R. Weiss, J. Bock, S. Church, M. Devlin, G. Hinshaw, A. Lange, A. Lee, L. Page, B. Partridge, J. Ruhl, M. Tegmark, P. Timbie, B. Winstein, 36â and M. Zaldarriaga, 2005, Task Force on Cosmic Microwave Background Research: Final Report, available at http://www.nsf.gov/mps/ast/ tfcr_final_report.pdf. M. Kamionkowski and A.H. Jaffe, 2001, Detection of gravitational waves from inflation, Int. J. Mod. Phys. A 16:116-128. 37â M. Takada, E. Komatsu, and T. Futamase, 2006, Cosmology with high-redshift galaxy survey: Neutrino mass and inflation, Phys. Rev. D 38â 73:83520.
SCIENCE IMPACT 37 TABLE 2.13â Inflation Probe (IP): Beyond Einstein Science Programs Science Program Program Characteristics Program Significance Science All-sky CMB Science question Detect gravitational waves The inflation model of the early universe definition polarization sourced by inflation predicts two types of fluctuations: density programs map (CMB perturbations that evolve into structure, polarization) Measurements All sky CMB B-mode and tensor or gravitational perturbations. polarization study The ratio of the amplitude of the two perturbations is a measure of the energy Quantities Gravitational-wave amplitude scale of inflation. determined and energy scale of inflation 2.5-5.5 Âµm Science question Constrain the physics of inflation Models of inflation and what drives it galaxy redshift have distinct predictions for the matter survey at z = 3 to Measurements Measure the galaxy power fluctuation power spectrum. An accurate 5 using H-alpha spectrum at scales ranging from measure of the spectrum constrains line (CIP) the CMB to optical galaxy the possible inflation mechanisms. CIP surveys when combined with CMB constraints significantly narrows the possible inflation Quantities Power spectrum slope from 5 to models. determined 500 hâ1 Mpc Additional Baryonic Measurements Detect baryonic oscillations in The properties of dark energy can be Beyond oscillations at the matter power spectrum probed using geometry. The baryon Einstein high redshift acoustic oscillations have a known scale. science (CIP) Quantities Angular diameter distance Measuring their angular size at redshift of determined 3<z<5 3-5 will constrain the properties of dark energy. Science question Dark energy properties NOTE: See Appendix G in this report for definitions of acronyms. Contributions of the Mission to Other Science Two kinds of mission have been proposed as Inflation Probes, and their contributions to other science differ. The CMB polarization experiments will need to achieve a very good understanding of the magnetic field in the Milky Way and the properties of interstellar dust. Thus, the CMB polarization experiments will contribute sub- stantially to both the study of galactic magnetic fields and to the study of the properties of interstellar grains. The E-mode polarization at large angular scales will also provide information about the history of reionization. The scattering of CMB photons by free electrons can only produce E-mode polarization, and WMAP has detected the large-angular-scale E-mode polarization produced since the universe was reionized 400 million years after the big bang. The Inflation Probe would provide a much higher signal-to-noise ratio and would be able to study the time history of the reionization of the universe. The Cosmic Inflation Probe studies galaxies at high redshift. It will generate a very large catalog of high- redshift emission-line galaxies, which will aid in understanding the assembly of galaxies and the star-formation history of the universe. This catalog will provide many interesting targets for follow-up studies with the JWST. In addition, the experiment is sensitive to the growth of structure on scales from 10 to 1,000 megaparsecs (Mpc). Questions that can be touched on with concurrent CMB and nearby Large-Scale Structure (LSS) surveys are the neutrino mass, dark energy constraints, galaxy clustering properties, and galaxy evolution. Table 2.14 lists the broader science capabilities of the missions. Opportunity for Unexpected Discoveries CIPâs high-redshift LSS survey provides many possibilities for unexpected discoveries. It will catalog 107 high- redshift galaxies, providing unprecedented information about the star-formation history of the universe. The catalog
38 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.14â Inflation Probe (IP): Broader Science Examples Program Program Characteristics Program Significance Polarized galactic Science questions What is the nature of galactic dust, galactic The types of emission from galactic foreground magnetic fields, electron spectrum? dust are not known. The distribution (CMB polarization) of grain sizes and temperature Measurements Polarization of galactic emission will be better determined with polarized measurements through the Quantities determined Dust grain properties, dust thermal submillimeter wavelength range. The environment, global maps of galactic nature of high galactic latitude dust magnetic fields will be characterized. Ionization history Science question When was the universe reionized? Energy injected into the universe by of the universe the formation of the first massive stars (CMB polarization) Measurements E-mode polarization of the CMB caused it to become reionized. The redshift of the epoch is an important Quantities determined Total optical depth to scattering of CMB detail in understanding the evolution of photons in the nearby universe. Possibly structure. some constraints on the reionization history. High-redshift star Science question What is the history of star formation for The star formation rate in early formation rate 3 < z < 6? galaxies is uncertain. How galaxies (CIP) formed and how and when they Measurements Measure the star formation rate in 107 generated the elements that we see are galaxies 3 < z < 6 not yet known. Quantities determined Not applicable Neutrino mass Science question What are the masses of the three kinds of Neutrinos are now known to have (CIP) neutrinos? masses. The differences between the mass squared of types of neutrinos are Measurements The matter power spectrum from 1,000 to known, but the actual masses have not 5 hâ1 Mpc is sensitive to the neutrino yet been determined. masses. Quantities determined The total mass of all neutrinos to a 2Ï level of 0.05 eV NOTE: See Appendix G in this report for definitions of acronyms. will serve as a target list for JWST and other instruments. The CMB experiments with beams larger than 1 degree have fewer chances for unexpected discoveries, since they will have been preceded by WMAP and Planck. Mission Characteristics Three of the four Inflation Probe missions measure the B-mode CMB radiation polarization. B-modes have a twistiness or handedness that cannot be produced by the polarization dependence of electron scattering and are generated only in the presence of spatial distortions arising from either gravity waves or gravitational lensing. This signal is generated by gravitational waves with wavelengths on the order of the speed of light times the age of the universe and from gravitational lensing on much smaller scales. Processes occurring during the CMB emission do not generate B-mode polarization. The B-mode signal is generated subsequently as the photons travel through the spacetime distorted by the gravitational waves. The signal from inflation is strongest at angular scales of several degrees, and, because of reionization, the B-mode signal can also be seen at scales greater than 20 degrees, though the amplitude depends on the optical depth due to electron scattering that occurred after reionization. Instruments
SCIENCE IMPACT 39 designed to make this B-mode measurement must have sensitivity about a factor of 10 better than any current measurement. Even more challenging is the required rejection of foreground signal from the galaxy 39 and the required rejection of leakage of temperature and E-mode polarization signal into the B-mode signal. 40 All three CMB polarization missions address these requirements and their challenges. Experimental Probe of Inflationary Cosmologyâ The EPIC-F is a cryogenic, bolometric instrument with angular resolution of about 1 degree operating at frequencies from 30 to 300 GHz. It employs six 30 cm telescopes, each at a different frequency band, with a total of 830 bolometers. The angular resolution scales with wavelength. The probe operates at the second Earth-Sun Lagrange point (L2) for a year. This mission will use a phased array of slot antennas coupled to transition edge sensor (TES) bolometers. This planar detector technology is to be tested in proposed and ongoing ground-based and balloon instruments in the next 5 to 10 years. The focal plane array is more compact than an array of horns with the same number of detectors. This detector system, though still untested in real observational situations at this time, will likely be more mature by the close of this decade. 41 Einstein Polarization Interferometer for Cosmologyâ EPIC-I is a Fizeau interferometric instrument operating in a 900-km-altitude polar orbit with an operating lifetime of 1 year. The synthesized beam resolution is 1 degree in all bands, and the instrument uses 1,024 detectors. The proposed system could be made with either bolometric or amplified detector systems. The proposed interferometric technique, which may offer immunity to some types of systematic errors, requires extensive field and flight testing before it could be considered for a space mission. In addition, this team has not yet selected between bolometric and heterodyne sensors. In either case, considerable development is needed to integrate the detectors with the interferometer system. At this time, no specific projects are in development that will test the Fizeau interferometric technique in a full-scale astronomical instrument. CMBPolâ CMBPol is a general mission concept for measuring CMB polarization at large angular scales. The pro- posed concept would fly in a Cosmic Background Explorer (COBE) 900-km-altitude polar orbit using about 1,000 bolometers covering 6 bands from 30 to 300 GHz and having about a 1 degree resolution. The mission concept study discusses foreground removal, polarization techniques, and the choice of orbit. A candidate detector system using an array of horn-fed waveguide planar ortho-mode antennas is presented. This detection system requires a very large field of view. The system has not yet been used in an astronomical measurement but a balloonborne system using this technology is in development.42 Cosmic Inflation Probeâ CIP will generate a 140 square degree survey of galaxies at redshift from 3 to 6.5 in HÎ± emission. The goal is to measure the primordial power spectrum at spatial scales smaller than is possible with CMB anisotropy. Together with both low-redshift surveys and high-quality CMB anisotropy information, CIP provides tight constraints on inflation models. The mission consists of a 1.8 meter cooled telescope with a slitless grating spectrometer with resolution of 600, operating at wavelengths from 2.5 to 5 micrometers. The mission would operate at the Sun-Earth L2 Lagrange point for 3 to 5 years. M. Tegmark, D.J. Eisenstein, W. Hu, and A. de Oliveira-Costa, 2000, Foregrounds and forecasts for the cosmic microwave background, 39â Astrophys. J. 530:133. W. Hu, M.M. Hedman, and M. Zaldarriaga, 2003, Benchmark parameters for CMB polarization experiments, Phys. Rev. D 67:043004. 40â C.L. Kuo, J.J. Bock, G. Chattopadthyay, A. Goldin, S. Golwala, W. Holmes, K. Irwin, M. Kenyon, A.E. Lange, H.G. LeDuc, P. Rossinot, 41â A. Vayonakis, G. Wang, M. Yun, and J. Zmuidzinas, 2006, Antenna-coupled TES bolometers for the SPIDER experiment, Nucl. Instrum. Methods A 559:608. A. Kogut, D.T. Chuss, D. Fixsen, G.F. Hinshaw, M. Limon, S.H. Moseley, N. Phillips, E. Sharp, E.J. Wollack, K. U-Yen, N. Cao, T. Ste- 42â venson, W. Hsieh, M. Devlin, S. Dicker, C. Semisch, and K. Irwin, 2006, PAPPA: Primordial Anisotropy Polarization Pathfinder Array, New Astron. Rev. 50:1009.
40 NASAâS BEYOND EINSTEIN PROGRAM Assessment of Scientific Impact Overall Assessment Revolutionary Nature of the Scienceâ The question of the nature of the inflationary era is most assuredly fun- damental. The initial conditions for all the subsequent evolution were set during inflation or are a direct conse- quence of the physics of inflation. The Inflationary Big Bang Model predicts that the expansion was propelled by a quantum mechanical vacuum energy. A better understanding of this era will help to answer the question, What powered the big bang? Advances in Basic Astrophysicsâ The inflationary era generated the seed fluctuations for the long period of structure growth that followed. A fundamental understanding of the nature and spectrum of the fluctuations would underpin our knowledge of the structure formation processes. Breadth of Science Impactâ Models of inflation explain the largest structures in the universe in terms of quantum fluctuations and phenomena at the smallest scales. Physics and astronomy are both tied directly to an understanding of the inflationary period. The Inflation Probe is the next step along the path to that understanding. The accelerat- ing expansion that occurred during inflation may have a connection to the accelerating expansion occurring today because of the presence of dark energy. A deeper understanding of inflation and dark energy is needed to explore that similarity. Context of the Science and Mission Unique Capabilitiesâ Very few observational probes exist to verify and characterize inflation. The inflationary scenario was inspired by cosmological observations that indicated in a very indirect way that something was amiss in the then-current picture of the early universe. But the problems with the standard big bang model were subtle, and the development of the theory of inflation followed the observational facts by more than a decade. With the introduction of the Inflation Probe missions, some of the few direct observational predictions of inflation can be tested. In view of the fundamental nature of the idea of inflation, these predictions should be explored as soon as possible. Complementary Role with Other Missionsâ The study of the early universe involves the fusion of many research paths. These include past, present, and future missions such as COBE, WMAP, and Planck. The interpretation of the results from any of the IP missions will depend on and complement the results from these prior efforts. In addition, major ground-based and suborbital work is under way to fill in the technological, observational, and theoretical gaps that will be required to fully realize the potential of the Inflation Probe. Can the Science Questions Be Answered by Other Space Missions and/or Ground-Based Capabilities?â Since the angular scale of the B-mode signal from inflation is on the order of a few degrees or more, ground-based ex- periments are unlikely to be able to provide the highest signal-to-noise ratio, systematic-error-free power spectra. Atmospheric emission, emission from warm optics, and ground emission are all difficult observational problems that have yet to be demonstrably overcome at the needed levels of sensitivity. While the Planck mission is space- based, the scan pattern is not optimized for large-angular-scale polarization measurements and may compromise the large-angle polarization fidelity. Ground-based and balloonborne experiments are important stepping-stones toward the detection of B-mode polarization. They are likely to be the first to detect the B-mode signal from gravitational lensing of the CMB at smaller angular scales, but such observations do not probe inflation. CIP will extend precision measurements of the galaxy power spectrum by about a factor of 5 higher in wave number. This range of wave numbers is covered by ground-based measurements of galaxy clustering and the Lyman-alpha forest, but the corrections for nonlinear structure growth have prevented an accurate determination of the primordial power spectrum using these data. The development of an accurate nonlinearity correction for
SCIENCE IMPACT 41 Lyman-alpha forest data could reduce the value of CIP data. WMAP data plus current galaxy data from the SDSS have already made a preliminary measurement of the power spectrum spectral index. Planck will improve the data in the low-wave-number region currently covered by WMAP, leading to a severalfold improvement beyond cur- rent knowledge of the power spectrum. Adding CIP data to Planck plus SDSS data will yield another severalfold improvement in the determination of the power spectrum. Science Readiness and Risk Science Readiness Risk to Achieving Science Goalsâ The theoretical framework for understanding the results of both the CMB and high-redshift galaxy observations exists. The observations will fit readily into models of the universe and provide useful constraints on cosmological parameters. Required Enabling Scienceâ The polarization missions will need to extract a B-mode signal, which is a factor of 30 below the estimated signal from galactic foregrounds. Extensive research into the characterization and modeling of polarized galactic emission will be required to mitigate this mission risk. Science Risk One concern about the B-mode polarization experiments is based on the fact that the B-mode power varies as the fourth power of the energy scale during inflation, so there is only a factor of 3 range in energy scale between the current limits on the B-mode power and the likely detection limits of the Inflation Probe. 43 Possibly mitigat- ing this concern is the fact thatâfor the current best estimates for the spectral index of the primordial power spectrumâthe energy scale for inflation may be in this range for typical inflation models. The CIP proposes to measure this spectral index to much greater precision. However, there are other inflation models in which the B- mode power is disconnected from the spectral index. Steps for Moving Forward The CMB polarization Inflation Probes collectively are in an early stage of development. The three proposals outline detector and instrument concepts that are extrapolations from existing experiments. No detailed engineer- ing or budgeting plans have been presented. No instrument focal planes of the complexity or sensitivity proposed are in operation on any platform. The CMB polarization experiments EPIC-F, EPIC-I, and CMBPol all require extremely sensitive millimeter- wave continuum detectors and extremely effective rejection of the common-mode noise from the anisotropy signal. All three of these missions have proposed to use state-of-the-art detectors to reach the required high sensitivity. The polarization, stability, and characterization of the instrument needed to achieve a successful B-mode spec- trum measurement are at levels far beyond what has been reached with currently existing instruments. EPIC-F proposes to use doped germanium resistance thermometers in bolometers that are very similar to the detectors in Planck. The committeeâs assessment of the TRLs for EPIC-F are discussed in Chapter 3 and range from TRL 3 to 6+ for various components. A successful Planck mission will go much of the way, but not all the way, toward proving the readiness of the detector technology. The EPIC-I and CMBPol concepts were less detailed, and they contemplate using detectors that have less heritage and have not been developed for spaceflight. The committee did not have enough information to assess the TRL of CMBPol and EPIC-I. Further support of detector and ultracool c Â ryocoolers (sub-100 mK) is needed to push these missions along. The three CMB missions have proposed three different approaches for modulating the polarization signal to separate the desired polarized signal from the much A. Amblard, A. Cooray, and M. Kaplinghat, 2007, Search for gravitational waves in the CMB after WMAP3: Foreground confusion and 43â the optimal frequency coverage for foreground minimization, Phys. Rev. D 75:083508.
42 NASAâS BEYOND EINSTEIN PROGRAM larger temperature anisotropy. Ground-based and balloonborne demonstrations of these techniques would be a cost-effective way to demonstrate these techniques. The CIP concept is mature, and much of the design for the mission is a modification of existing missions. The detectors are very similar to the JWST Near-Infrared Camera (NIRCAM) long-wavelength detectors, but CIP requires eight times as many detectors as NIRCAM. While there may be a need for further theoretical advances to obtain the most from the mission, there are no major hurdles to overcome in order to start the mission. Impacts on Institutional Relationships None of the missions proposed here involves partnerships outside of NASA. Thus, the impacts of either a 2009 start or a deferred start on relationships with NSF, DOE, or ESA are minimal. TABLE 2.15â Inflation Probe (IP): Summary of Scientific Evaluation Potential Contributions to Science Beyond Einstein Broader Science Factors Revolutionary Knowing the energy scale is crucial for understanding Interstellar dust and galactic magnetic-field properties discovery inflation (CMB polarization). interesting to a small community (CMB polarization). potential Improved measurement of spectral index and running Large IR spectroscopic survey will find many unusual constrains the shape of the inflationary potential and interesting objects that will be good targets for JWST (CIP). (CIP). Science The energy scale of inflation could be outside the 3Ã Low risk, since foreground signal will be strong (CMB readiness and range, between the current limit and the foreground polarization). risk subtraction limit. Foreground subtraction could be too difficult (CMB polarization). Low risk, since such a large spectroscopic survey will certainly find many fascinating sources such as high-z Improved understanding of nonlinearities in P(k) quasars (CIP). and/or the Lyman-alpha forest could reduce the value of the result (CIP). Mission Uniqueness Versus The Big Bang Observer (follow-on to LISA) could Planck will provide information on the galactic magnetic other space measure the gravitational waves from inflation (CMB fields and interstellar dust, but not the large-angular-scale missions polarization). B-modes (CMB polarization). Other large-scale spectroscopic surveys such as Objects in similar classes could be found in other large- ADEPT could duplicate some CIP science. Planck scale spectroscopic surveys, but missions such as ADEPT will also improve our knowledge of the spectral will not duplicate CIP bands and fields of view (CIP). index, but in a different part of the spectrum (CIP). Versus Ground-based experiments are unlikely to measure Ground-based (and balloon) experiments could measure ground-based the large-angular-scale B-modes from inflation (CMB IS dust properties and B-modes from lensing (CMB instruments polarization). polarization). SKA, MWA, and LOFAR could measure P(k) at high Sensitive measurements in CIP band are not possible from z using high-redshift 21 cm spectra. Ground-based the ground (CIP). spectroscopic surveys will improve on the SDSS measurement of P(k) (CIP). NOTE: See Appendix G in this report for definitions of acronyms.
SCIENCE IMPACT 43 Science Assessment Summary The Inflation Probe has a diverse set of mission concepts, using two very different types of observations to probe two quite different aspects of the vacuum energy density or potential that powered inflation. The CMB polarization mission concepts seek to measure the absolute level of the potential, while the high-redshift galaxy power-spectrum mission concept seeks to measure the shape (normalized derivatives) of this potential. The CMB polarization is very difficult to measure; doing so requires unprecedented detector sensitivity and foreground rejection accuracy, but it will provide a unique view of the inflationary era. These CMB mission concepts require continued technology development and the acquisition of more data about the galactic foreground (Table 2.15). ESAâs Planck mission and ongoing ground-based and balloonborne CMB polarization experiments will provide both platforms for testing technology and more foreground data. The galaxy power-spectrum measurement im- proves on existing data by a factor of about 5, limiting its revolutionary science potential, but it is technically ready to proceed. THE JOINT DARK ENERGY MISSION Introduction NASA and DOE are developing the Joint Dark Energy Mission primarily to investigate the dark energy of the universe. Three mission concepts were considered by the committee: the Supernova Acceleration Probe (SNAP), the Dark Energy Space Telescope (DESTINY), and the Advanced Dark Energy Physics Telescope (ADEPT). The committee reviewed each of these candidate missions in order to evaluate the potential scientific impact of JDEM, with the understanding that the eventual JDEM mission resulting from a request for proposals could be one of these three, or a mission based on a different combination of techniques. Each of the proposed JDEM candidates is based on an optical-to-near-infrared wide-field survey telescope. SNAP is a 1.8 m telescope concept with 0.7 square-degree field of view and optical and near-IR (NIR) imaging, plus spectroscopy and multiband photometry capability for the study primarily of Type Ia supernovas and weak lensing. DESTINY is a proposed 1.65 m telescope designed for near-IR spectrophotometry of high-redshift Âsupernovas and for weak lensing with multiband photometry. ADEPT would employ a 1.3 m telescope operating in the near-IR focusing on baryon acoustic oscillations as well as Type Ia supernovas. Each of these missions would be capable of high-precision studies of dark energy out to redshifts on the order of 1.7. A brief description of the mission and a listing of the instrument properties are provided in Tables 2.16 and 2.17. TABLE 2.16â Joint Dark Energy Mission (JDEM): Mission Description Parameter Value Primary measurement Optical/near-IR imaging and spectroscopy Observatory type Optical/near-IR wide-field survey telescope Projected years in orbit 3-yr primary, 5-yr goal Type of orbit LEO (ADEPT); L2 (DESTINY/SNAP) Mission phases ADEPT: full-sky survey DESTINY: 24 months SN survey, 12 months weak-lensing survey SNAP: 22 months SN survey, 12 months weak-lensing survey Science operations Continuous survey NOTE: See Appendix G in this report for definitions of acronyms.
44 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.17â Joint Dark Energy Mission (JDEM): Mission Instrument Properties Spectral Range Spatial Resolution Spectral Resolution Collecting Area Field of View Instrument (microns) (arcsec) (Î»/âÎ») (diameter in meters) (sq. deg) SNAP imager 0.35-1.7 0.14 5 1.8 0.7 SNAP spectrometer 0.35-1.7 0.14 100 (visible) 1.8 Not applicable 70 (NIR) DESTINY imager 0.85-1.7 0.15 5 1.65 0.12 DESTINY grism 0.85-1.7 0.15 75 1.65 0.12 ADEPT slitless spectrograph 1.3-2.0 Not available Not available 1.3 Not available NOTE: See Appendix G in this report for definitions of acronyms. Mission Science Goals Over the past decade, conclusive evidence has been assembled indicating that the expansion of the universe is accelerating.44â50 Within the standard cosmological model, this expansion implies that some 70 percent of the mass- energy density of the universe is in the form of a mysterious dark energy that counters the attractive gravitational force of matter and radiation. The accelerating expansion of the universe is one of the great discoveries in the history of cosmology, and it could have profound implications for elementary particle physics, general relativity, and astronomy. JDEM will address one of the central questions of the Beyond Einstein Program: What is the mysterious dark energy pulling the universe apart? Little is known at present about dark energy. Whether dark energy is due to a cosmological constant term in Einsteinâs equation for general relativity, a dynamically evolving quantum field, a modification of general relativity, or some other new physics cannot be determined from the data currently available. To explore the nature of dark energy, JDEM must determine to high precision whether the accelerating expansion is consistent with a cosmological constant, or whether the dark energy density is evolving with time. Comparison A.G. Riess, A.V. Filippenko, P. Challis, A. Clocchiattia, A. Diercks, P.M. Garnavich, R.L. Gilliland, C.J. Hogan, S. Jha, R.P. Kirshner, B. 44â Leibundgut, M.M. Phillips, D. Reiss, B.P. Schmidt, R.A. Schommer, R.C. Smith, J. Spyromilio, C. Stubbs, N.B. Suntzeff, and J. Tonry, 1998, Observational evidence from supernovae for an accelerating universe and a cosmological constant, Astron. J. 116:1009. S. Perlmutter, G. Aldering, G. Goldhaber, R.A. Knop, P. Nugent, P.G. Castro, S. Deustua, S. Fabbro, A. Goobar, D.E. Groom, I. M. Hook, 45â A.G. Kim, M.Y. Kim, J.C. Lee, N.J. Nunes, R. Pain, C.R. Pennypacker, R. Quimby, C. Lidman, R.S. Ellis, M. Irwin, R.G. McMahon, P. Ruiz- Lapuente, N. Walton, B. Schaefer, B.J. Boyle, A.V. Filippenko, T. Matheson, A.S. Fruchter, N. Panagia, H.J.M. Newberg, and W.J. Couch, 1999, Measurements of omega and lambda from 42 high-redshift supernovae, Astrophys. J. 517:565. D.N. Spergel, L. Verde, H.V. Peiris, E. Komatsu, M.R. Nolta, C.L. Bennett, M. Halpern, G. Hinshaw, N. Jarosik, A. Kogut, M. Limon, S.S. 46â Meyer, L. Page, G.S. Tucker, J.L. Weiland, E. Wollack, and E.L. Wright, 2003, First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Determination of cosmological parameters, Astrophys. J. Suppl. Ser. 148:175. D.N. Spergel, R. Bean, O. DorÃ©, M.R. Nolta, C.L. Bennett, J. Dunkley, G. Hinshaw, N. Jarosik, E. Komatsu, L. Page, H.V. Peiris, L. Verde, 47â M. Halpern, R.S. Hill, A. Kogut, M. Limon, S.S. Meyer, N. Odegard, G.S. Tucker, J.L. Weiland, E. Wollack, and E.L. Wright, 2007, Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Implications for cosmology, Astrophys. J. Suppl. Ser. 170:377. A.G. Riess, L.-G. Strolger, S. Casertano, H.C. Ferguson, B. Mobasher, B. Gold, P.J. Challis, A.V. Filippenko, S. Jha, W. Li, J. Tonry, R. Foley, 48â R.P. Kirshner, M. Dickinson, E. MacDonald, D. Eisenstein, M. Livio, J. Younger, C. Xu, T. DahlÃ©n, and D. Stern, 2007, New Hubble Space Telescope discoveries of Type Ia supernovae at z â¥ 1: Narrowing constraints on the early behavior of dark energy, Astrophys. J. 659:98. D.J. Eisenstein, I. Zehavi, D.W. Hogg, R. Scoccimarro, M.R. Blanton, R.C. Nichol, R. Scranton, H.-J. Seo, M. Tegmark, Z. Zheng, S.F. 49â Anderson, J. Annis, N. Bahcall, J. Brinkmann, S. Burles, F.J. Castander, A. Connolly, I. Csabai, M. Doi, M. Fukugita, J.A. Frieman, K. Gla- zebrook, J.E. Gunn, J.S. Hendry, G. Hennessy, Z. Ivezi, S. Kent, G.R. Knapp, H. Lin, Y.-S. Loh, R.H. Lupton, B. Margon, T.A. McKay, A. Meiksin, J.A. Munn, A. Pope, M.W. Richmond, D. Schlegel, D.P. Schneider, K. Shimasaku, C. Stoughton, M.A. Strauss, M. SubbaRao, A.S. Szalay, I. Szapudi, D.L. Tucker, B. Yanny, and D.G. York, 2005, Detection of the baryon acoustic peak in the large-scale correlation function of SDSS luminous red galaxies, Astrophys. J. 633:560. W.J. Percival, S. Cole, D.J. Eisenstein, R.C. Nichol, J.A. Peacock, A.C. Pope, and A.S. Szalay, 2007, Measuring the Baryon Acoustic 50â Oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, Mon. Not. R. Astron. Soc. 381:1053-1066.
SCIENCE IMPACT 45 of the effect of dark energy on the expansion history of the universe with its effect on the history of the growth of structure will address both the nature of dark energy and the correctness of general relativity. The wide-field optical-NIR surveys required for exploring dark energy will also produce data sets of unprec- edented richness for the investigation of a very broad range of other astrophysical questions. Contribution of the Mission Directly to Beyond Einstein Goals JDEM will probe the nature of dark energy by measuring its effects on the expansion history of the universe and on the history of the growth of structure. Several observational techniques exist for the exploration of dark energy. The report of the Dark Energy Task Force (DETF),51 established by the Astronomy and Astrophysics Ad- visory Committee (AAAC) and the High Energy Physics Advisory Panel (HEPAP), discussed four techniques: 1. Supernova (SN) surveys use Type Ia supernovas as standard candles to determine the luminosity distance versus redshift relation. 2. Weak lensing (WL) surveys measure the bending of light as it passes galaxies or galaxy clusters. WL is sensitive to dark energy through dark energyâs effect on the growth rate of structure. 3. Baryon acoustic oscillations (BAOs) are observed through surveys of the spatial density and distribution of galaxies. This technique is sensitive to dark energy through dark energyâs effect on the angular-diameter distance versus redshift relation. 4. Galaxy cluster (CL) surveys measure the distances, distribution, and spatial density of clusters. CL is sensitive to dark energy through the angular-diameter distance versus redshift relation and the growth rate of structure. The use of two or more of these techniques provides improved sensitivity and important cross-checks. Fur- thermore, as discussed in the Dark Energy Task Force report,52 a comprehensive dark energy program should provide measures of both the homogeneous (geometric) and inhomogeneous (growth of structure) effects of dark energy, in order to provide the potential to test whether an acceleration of expansion arises from the modification of general relativity. It is also important to have both types of tests, especially when considering that a simple parameterization of dark energy may be incomplete. Each of the proposed JDEM concepts would employ at least two of the first three techniques. DESTINY and SNAP would rely equally on supernovas (a geometric test) and weak lensing (both a geometric test and a growth-of-structure test). ADEPT would rely mainly on baryon acoustic oscillations (a geometric test), and also includes a study of supernovas. The galaxy cluster technique would be employed by Con-X. A principal goal with each observational technique is to measure accurately the ratio of the dark energy pres- sure P to its energy density Ï, w(a) = P(a)/Ï(a), as a function of the scale factor a = 1/(1 + z) (where z is redshift), or equivalently as a function of time. If the presence of a cosmological constant term in general relativity (GR) is an accurate model for dark energy, then the energy density is uniform in space and constant in time, and w = â1 for all times. If instead, a dynamical field is responsible for the dark energy, then w could take on other values and vary with time. The Dark Energy Task Force adopted a simple, two-parameter description of w(a), and defined a figure of merit for its measurement in terms of the inverse area of the 95 percent confidence-limit ellipse in the space of these two parameters. The task force called for a factor-of-10 gain over current accuracy in this figure of merit for any JDEM-generation dark energy project, a somewhat arbitrary but not unreasonable goal for advancing the understanding of dark energy. ADEPT would determine the expansion history of the universe using a full-sky spectroscopic survey that measures baryon acoustic oscillations derived from redshifts and positions of approximately 100 million galaxies A. Albrecht, G. Bernstein, R. Cahn, W.L. Freedman, J. Hewitt, W. Hu, J. Huth, M. Kamionkowski, E.W. Kolb, L. Knox, J.C. Mather, S. 51â Staggs, and N.B. Suntzeff, 2006, Report of the Dark Energy Task Force, available at http://arxiv.org/ftp/astro-ph/papers/0609/0609591.pdf. A. Albrecht, G. Bernstein, R. Cahn, W.L. Freedman, J. Hewitt, W. Hu, J. Huth, M. Kamionkowski, E.W. Kolb, L. Knox, J.C. Mather, S. 52â Staggs, and N.B. Suntzeff, 2006, Report of the Dark Energy Task Force, available at http://arxiv.org/ftp/astro-ph/papers/0609/0609591.pdf, Section IX, pp. 53-78.
46 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.18â Joint Dark Energy Mission (JDEM): Beyond Einstein Science Programs Science Program Program Characteristics Program Significance Science SNAP and Science question What is the nature of dark Combining SN light curves with WL results definition DESTINY energy? will provide a measure of the expansion programs Measurements Light curves of Type Ia rate of the universe to ~1%. This level will supernovas (SN) with 0.3 < z < provide over a factor-of-10 improvement 1.7 via deep-field survey of 3-7.5 compared to the current knowledge of sq. deg; gravitational WL via the dark energy contribution and may wide-field survey of 1,000-4,000 establish that dark energy does not arise sq. deg from a cosmological constant, that it varies dynamically with time, or that it arises from Quantities determined Expansion history of the universe; a modification of general relativity. history of growth of structure ADEPT Science question What is the nature of dark ADEPT combines BAOs with SN energy? light curves to provide a measure of the expansion rate of the universe to Measurements Baryon acoustic oscillations approximately 1%. This level will provide (BAOs) derived from redshifts and over a factor-of-10 increase compared to positions of 100 million galaxies the current knowledge of the dark energy with 1 < z < 2 and light curves of contribution and may establish that dark Type Ia SN with 0.8 < z < 1.3 via energy does not arise from a cosmological a full-sky spectroscopic survey constant or that it varies dynamically with time. Quantities determined Expansion history of the universe NOTE: See Appendix G in this report for definitions of acronyms. with redshifts in the range 1 < z < 2 and that measures light curves of approximately 1,000 Type Ia supernovas with redshifts in the range 0.8 < z < 1.3. Combining these measurements, ADEPT would determine the expansion rate of the universe to approximately 1 percent, providing at least a factor-of-10 improvement compared to current knowledge. This level of improvement may reveal that dark energy does not arise from a cosmological constant or that it varies dynamically with time. Given the very large volume surveyed and given that the BAO signal is quite free from systematic errors, the measurements of ADEPT should be very robust. SNAP or DESTINY would determine the expansion history of the universe by way of a deep-field survey of 3-7.5 square degrees that measures light curves of Type Ia supernovas with redshifts in the range 0.3 < z < 1.7. They would also determine the expansion history and the history of the growth of structure through a wide-field survey of 1,000-4,000 square degrees that measures gravitational weak lensing of galaxies. Combining these measurements, either of these missions would determine the expansion rate of the universe to approximately 1 percent, providing at least a factor-of-10 improvement in accuracy over current measurements. This level of im- provement, along with measurement of the histories of both expansion and growth of structure, may reveal that dark energy does not arise from a cosmological constant, that it varies dynamically with time, or that it arises from a modification of general relativity. A brief summary of the Beyond Einstein science goals of each of the proposed JDEM concepts is provided in Table 2.18. Contribution of the Mission to Other Science Dark energy manifests itself only on large scales; consequently, any JDEM mission will probe large volumes of space, which will naturally lead to a substantial observational data set that can be used to address a significant range of astrophysics questions. This broader science program of JDEM will appeal to many astrophysicists. A brief summary of some examples of the broader science goals of each of the proposed JDEM concepts is provided in Table 2.19. An imaging survey, such as DESTINY or SNAP, would provide a large, deep survey in multiple
SCIENCE IMPACT 47 TABLE 2.19â Joint Dark Energy Mission (JDEM): Broader Science Examples Program Program Characteristics Program Significance SNAP and Science question How did galaxies form and evolve? After HST there will be no large diffraction-limited DESTINY optical or near-IR telescope in space. The low Measurements Photometric surveys in 5 (DESTINY) background and large field of views offered by SNAP to 9 (SNAP) optical and NIR bands and DESTINY will provide the most detailed and important information ever for understanding how Quantities determined Deep-field survey over 3 sq. deg galaxies formed and acquired their mass. (DESTINY) to 7.5 sq. deg (SNAP); wide-field survey over 1000 sq. deg (DESTINY) to 1,000-4,000 sq. deg (SNAP) ADEPT Science question At what rate did stars form, and how There has never been a full-sky spectroscopic survey did that rate depend on environment? from space; consequently, ADEPT has large discovery potential. It will characterize the star formation rate of Measurements Full-sky IR spectroscopic survey the universe down to a sensitive limiting flux, finding the most extreme star-forming galaxies in the universe. Quantities determined Redshift and emission fluxes for over The epoch that ADEPT probes is the most active when 100 million galaxies galaxies acquire their mass. Very little is known about star formation in the smallest galaxies. NOTE: See Appendix G in this report for definitions of acronyms. bands. A spectroscopic survey, such as ADEPT, would provide a full-sky survey of the near-IR emission-line uni- verse. Large-scale surveys have had substantial impact on basic astrophysics in the past. For instance, SDSS, 53,54 which has been operating since 1998 and whose survey was recently extended, has resulted in many hundreds of publications with many thousands of citations. The scientific impact of such surveys typically extends well beyond their initial goals. The significant potential impact of JDEM imaging studies would derive from the very wide and deep fields that they image. Requirements for dark energy studies using weak lensing demand a very-wide-field survey, typi- cally at least 1,000 square degrees, and requirements for dark energy studies using supernovas demand multiple images of a wide field, typically 15 square degrees, that provide a very-deep-field survey. For example, the SNAP supernova survey would cover an area of 7.5 square degrees, 2,000 times larger than the Hubble Ultra-Deep-Field (HUDF) survey,55 and deeper in each of nine color bands from optical through NIR wavelengths. It would reach much greater depths than SDSS. The SNAP weak-lensing survey would cover an area of 1,000 (4,000 in the full extended mission) square degrees, at least 500 times larger than the Cosmological Evolution Survey (COSMOS) 56 field and to similar depth. DESTINY would provide similar results. Significant impact of a JDEM imaging study would also derive from enhanced sensitivity in the NIR relative to present and future ground-based surveys. Such sensitivity would allow studies such as those currently under way with SDSS to be extended to the high-redshift universe. A large imaging survey such as DESTINY and SNAP would greatly complement the exquisite detail obtained from HST wide-field camera WF3 and JWST. The benefits of deep optical and NIR images are easily seen from the advances made with HST. Understanding how galaxies form, acquire their mass, and evolve has been a prime focus of HST studies. The low-background, D.G. York, J. Adelman, J.E. Anderson, Jr., S.F. Anderson, J. Annis, N.A. Bahcall, J.A. Bakken, R. Barkhouser, S. Bastian, E. Berman, W.N. 53â Boroski, et al., 2000, The Sloan Digital Sky Survey: Technical summary, Astron. J. 120:1579. J.K. Adelman-McCarthy, M.A. AgÃ¼eros, S.S. Allam, K.S.J. Anderson, S.F. Anderson, J. Annis, N.A. Bahcall, I.K. Baldry, J.C. Barentine, 54â A. Berlind, M. Bernardi, et al., 2006, Fourth data release of the Sloan Digital Sky Survey, Astrophys. J. Suppl. Ser. 162:38. S.V.W. Beckwith, M. Stiavelli, A.M. Koekemoer, J.A.R. Caldwell, H.C. Ferguson, R. Hook, R.A. Lucas, L.E. Bergeron, M. Corbin, S. 55â Jogee, N. Panagia, M. Robberto, P. Royle, R.S. Somerville, and M. Sosey, 2006, The Hubble Ultra Deep Field, Astron. J. 132:1729. N. Scoville and the COSMOS Survey Group, 2007, Large scale structure in COSMOS, 210th Meeting of the American Astronomical 56â Society, Presentation Number 039.01, Bull. Am. Astron. Soc. 39.
48 NASAâS BEYOND EINSTEIN PROGRAM high-spatial-resolution images have been invaluable for quantifying morphology, which has been the major obstacle for ground-based studies. A data set that is over three orders of magnitude larger than that obtained from HST will allow a direct comparison with ground-based studies (present and future) of the nearby universe. A JDEM imaging survey, such as DESTINY or SNAP, would dominate the studies of how galaxies acquire their mass over time, reaching back through more than 90 percent of the age of the universe, from redshift zero to ~3.5. Data from JDEM imaging surveys would enable a wide range of other astrophysical studies in addition to studies of galaxy evolution and morphology. For example, with a deeper field and near-infrared capability, the unobscured quasar luminosity function could be mapped to z ~10, far beyond the z <6-6.5 range of SDSS and the planned Dark Energy Survey (DES). With identification of high-redshift quasars and galaxies, the era of reioniza- tion could be probed in great detail, and, in combination with spectroscopic studies from the ground or JWST, measurements of the proximity effect and the spatial structure of reionization could be performed. Imaging data would also enable studies of stellar populations, distributions, and evolution. Exploiting near-infrared capabilities of the imaging studies would also enable a census of nearby low-mass L and T stars and brown dwarfs in the Milky Way. Faint, cool objects in the outer solar system could also be discovered in the time series data of the imaging studies. The imaging studies would also provide important information and identification of targets such as quasars, galaxies, and gamma-ray bursts for JWST. With large imaging surveys and repetitive pointing on the same fields, a JDEM mission such as DESTINY or SNAP would have significant potential for unexpected discoveries. A spectroscopic near-infrared JDEM survey would also offer significant discovery potential. For instance, ADEPT would produce a full-sky slitless-grism survey at moderate resolution. Its spectral range is 1-2 microns, corresponding to various emission lines over redshifts of 0.8 < z < 8, or higher. This redshift range is one of the most important for studies of star formation, because it is during those eras that most stars formed. Having a flux- limited spectrographic survey with no selection effects is essential to an understanding of where stars are formed and the processes that control star formation. Such a survey would find the most prolific star-forming objects in the universe and the pure emission-line objects, allowing the most robust measure of where stars are formed. By providing the largest-effective-volume survey of the universe, a full-sky, spectroscopic JDEM survey such as ADEPT would perform studies of many phenomena in addition to star formation. For instance, it could be used to measure the power spectrum of density fluctuations, to study high-order n-point correlation functions, to improve the determination of matter density, and for high-statistics studies of active galactic nuclei. Such a full-sky, spectroscopic survey has never been obtained; consequently, such a JDEM survey also offers significant potential for unexpected discoveries. A JDEM infrared imaging or spectroscopic large-format telescope could also prove invaluable in locating infrared transients associated with LISA signals indicating imminent supermassive black hole mergers. Thus, as a secondary but potentially equally important contribution to science, JDEM will produce an extraor- dinary database that, properly archived and made available to the community in a timely manner after acquisition, would provide the basis for a broad archival research program leading to opportunities for unexpected discoveries in many areas of astrophysics. The broader science potential of JDEM has been critical to the high urgency that the committee has assigned to JDEM, and developing this potential will continue to have great value regardless of which JDEM mission concept may be selected. Opportunity for Unexpected Discoveries In summary, JDEM will offer the opportunity for unexpected discoveries both through its dark energy measure- ments and through its broader science program. By performing a precision study of the expansion history of the universe, JDEM will provide the possibility for unexpected, fundamental discoveries regarding the nature of dark energy. JDEM may establish that the expansion rate is consistent with a cosmological constant, or may alternatively discover that the history of expansion demands the existence of a new dynamical field or that it demands modifica- tion of the theory of general relativity. Such a discovery would be profound. Furthermore, in order to achieve the sensitivity required for its studies of dark energy, JDEM would establish an astrophysical reach greatly beyond that of present surveys. The rich data set from its large field surveyâwhether it be the wide-field and deep-field photometric imaging surveys of a JDEM mission such as DESTINY or SNAP or the full-sky spectroscopic survey
SCIENCE IMPACT 49 of a mission such as ADEPTâwould enable not only the broad program of astrophysical studies sketched above, but it would also open a window for new exploration and unexpected discoveries. Assessment of Scientific Impact Broad Science Impact The history of the expansion of the universe reflects the nature of the fundamental principles that govern the expansion. Recent studies have conclusively demonstrated that the universe is expanding ever more rapidly, rather than slowing because of the pull of gravity. Within the standard cosmological framework, the observed acceleration of expansion must be caused by an unknown entity, dark energy, that behaves as if it has negative pressure and that comprises 70 percent of the mass-energy of the universe. JDEM will perform precision studies of the history of expansion, shedding light on the nature of dark energy that will shape our understanding of gravity and the theories of fundamental particles and fields and of general relativity. Indeed, the present mystery of dark energy demonstrates that our current theories are incomplete or incorrect. Probing dark energy through astrophysical observations that enable the precision measurement of expansion is essential to progress in the understanding of these theories that are the foundation of our understanding of nature on both the largest and the smallest scales. Advances in Basic Astrophysics JDEM will advance basic astrophysics in several ways. Charting the history of the expansion of the universe is a basic astrophysical measurement that has been a mainstay of astrophysics since the work of Edwin Hubble in the early part of last century. Furthermore, JDEMâs wide-field surveys will provide a wealth of data over unprec- edented areas. This data sample will enable new measurements on many key astrophysical questions, for instance on galaxy formation and evolution using a photometric imaging survey, as proposed by SNAP and DESTINY, or on star formation using a spectroscopic survey, as proposed by ADEPT. A deep-field photometric survey, as proposed by SNAP and DESTINY, would also provide deep-field data over unprecedented areas. Precision Measurement The primary goal of JDEM, to deepen the understanding of dark energy and the accelerating expansion of the universe through precision measurement, may lead to revolutionary science. JDEM will measure fundamental properties, characterized by variables such as w(a), at an unmatched level of precisionâpossibly even illuminat- ing the source of dark energy. Such a result would be a major advance in basic astrophysics and cosmology and would have broad impact across all of fundamental physics. JDEMâs measurements will certainly shape future dark energy research. Scientific Context While present observational results from ground and space have revealed the existence of dark energy by determining that the expansion of the universe is accelerating, these results are not capable of distinguishing among the possible explanations of dark energy. Ongoing projects relevant to dark energy are also unlikely to distinguish successfully among the possible explanations, and, if dark energy is not a cosmological constant, will not distinguish among possible dynamical models. Increased observational sensitivity will be needed. Numerous observational projects are being developed, both near-term ground-based projects and longer-term projects on the ground and in space, for example, JDEM. Much research is also being invested in developing new techniques for measuring the effects of dark energy. Using its figure of merit to characterize sensitivity to dark energy parameters, the Dark Energy Task Force projected that near-term projects taken in combination may improve the figure of merit by a factor in the range of approximately 3 to 5 beyond the ultimate results of ongoing experiments, whereas DETF projected that JDEM could be capable of improving the figure of merit by at least a factor in the range of
50 NASAâS BEYOND EINSTEIN PROGRAM approximately 10 to 15. Proposed future large-scale ground-based observational projects, such as an optical Large Survey Telescope (LST), or eventually a radio Square Kilometer Array (SKA), might also be capable of an order- of-magnitude improvement in the DETF figure of merit. However, projections of the sensitivities for ground-based projects are considerably more uncertain than for JDEM. Much work on observational techniques has ensued since the DETF report, and proponents both of near-term projects and of JDEM concepts and other longer-term projects target improvements better than those projected by the DETF. The ultimate sensitivity of future experiments will depend largely on the capability of the experiments to control systematic uncertainties. The inability to forecast today the level of systematic uncertainties in future experiments gives rise to the ranges in the DETF projections. As part of its study, DETF included a careful discussion of ground- and spaced-based systematics for the four techniques listed above: BAOs, CL formation, Type Ia SNs, and WL.57 The projected improvements in sensitivity can only be achieved if systematic uncertainties can be adequately controlled, which is generally felt to be easier for a space-based mission such as JDEM. Systematic uncertainties are biasing effects arising from the environment, the methods of observation, or the instruments employed. Exploration of dark energy by any observational technique may be limited by systematic uncertainties, as future projects will greatly improve statistical samples. The sources of systematic uncertainty dif- fer among techniques. Sources can generally be categorized as observational or astrophysical, where observational uncertainties are intrinsic to the technique and astrophysical uncertainties are intrinsic to the astronomical objects (supernovas or galaxies) used by the technique. Spectroscopic BAO studies are less affected by observational un- certainties than other techniques are; however, they may be limited by two astrophysical uncertainties: nonlinear effects in the growth of structure and understanding of the difference (bias) between the distribution of galaxies and the distribution of matter. Photometric baryon acoustic oscillation surveys may also be limited by bias in the photometric redshift scale, an observational uncertainty. Galaxy cluster surveys may be limited by knowledge of the relationship between galaxy-cluster mass and observables used for the selection of clusters of galaxies, which has both observational and astrophysical contributions. Supernova surveys may be limited by wavelength-dependent errors in the astronomical flux scale, an observational uncertainty, or by any redshift dependency of properties, such as intrinsic luminosity, of supernovas or their host extinction that is not understood and correctedâan astrophysi- cal uncertainty. Surveys using the WL technique, which is not as developed as BAO and SN techniques, may be limited by both observational and astrophysical uncertainties. Limiting weak-lensing observational uncertainties may be miscalibration of the shear measurement as a function of redshift, bias in the photometric redshift scale, and effects of optics and anisotropies in the point-spread function of the optics. Limiting weak-lensing astrophysical uncertainties may arise from inaccuracy of the theoretically calculated power spectrum of dark matter and from intrinsic correlations of galaxy shapes with each other and local density. All dark energy experiments must limit systematic uncertainties. Space-based experiments, such as JDEM, are generally held to have better control of systematic uncertainties. By virtue of being space-based, JDEM will be able to reduce significantly systematic uncertainties with better angular resolution and using a wider spectrum of diagnostic data for supernova, weak lensing, and/or galaxy cluster surveys than is possible from the ground. Furthermore, JDEM capabilities in the NIR could strengthen constraints on dark energy parameters by studying supernovas and weak lensing of galaxies at higher redshifts than possible from the ground. However, a ground- based LST will face challenges arising from observational effects such as atmospheric fluctuations and possible biases in photometrically determined redshifts of large samples of galaxies. For measuring baryon acoustic oscil- lations, JDEM will be capable of surveying the full sky, providing a large statistical advantage over ground-based experiments. Thus, scientifically, JDEM is at present a lower-risk project than ground-based dark energy projects. It has a lower uncertainty in its projected sensitivity. It has superior capabilities for controlling systematic uncer- tainties for all primary techniques except baryon acoustic oscillations, for which it may have statistical advantages. Finally, it has the important capability of making measurements at higher redshift, which could be critical for probing small effects. A. Albrecht, G. Bernstein, R. Cahn, W.L. Freedman, J. Hewitt, W. Hu, J. Huth, M. Kamionkowski, E.W. Kolb, L. Knox, J.C. Mather, S. 57â Staggs, and N.B. Suntzeff, 2006, Report of the Dark Energy Task Force, available at http://arxiv.org/ftp/astro-ph/papers/0609/0609591.pdf, Section IX, pp. 53-78.
SCIENCE IMPACT 51 In practice, JDEM and ground-based projects are likely to be complementary. Systematic uncertainties will limit the ultimate level of sensitivity of both. The four primary observational techniques for exploring dark energy are sensitive in different and complementary ways to dark energy and other cosmological properties, and, although JDEM will implement a combination of at least two techniques, JDEM and a ground-based project together could implement a larger combination. Furthermore, the systematic challenges to space-based and ground-based proj- ects are somewhat different. Together, JDEM and ground-based projects are likely to yield important consistency checks and possibly improved sensitivity over that with JDEM alone. Science Readiness and Risk JDEM faces risks arising from systematic uncertainties and from competition. The principal science risk to JDEM arises from the challenge to control systematic uncertainties to the sub-percent level required to achieve at least the factor-of-10 improvement in sensitivity called for by the Dark Energy Task Force. None of the observa- tional techniques that may be employed by JDEM has yet demonstrated the ability to reach this level of control. Nonetheless, the expectation is that each technique can be calibrated to sufficient accuracy using existing theoretical and observational strategies. Considerable progress has been made in understanding sources of systematic uncer- tainty and in developing strategies to mitigate systematic effects. Factors that limit the ultimate JDEM sensitivity will be addressed by intermediate-term observational and theoretical projects, as well as by control data collected by JDEM itself and by other observations. Moreover, JDEM will benefit from two or more complementary ob- servational techniques with differing systematic limitations. Whereas the Dark Energy Task Force projects that a JDEM mission combining at least two techniques will produce at least a factor-of-10 improvement in sensitivity over present projects, it also projects an improvement of at least a factor of 8 under worst-case assumptions regard- ing the ability of JDEM to control systematic errors. Such a worst-case improvement factor will still represent a critical improvement in our understanding of the nature of dark energy. As discussed in the previous subsection, JDEM will face competition from ground-based dark energy experi- ments. Multiple ground-based experiments using a variety of techniques are being planned, some for the period preceding a JDEM launch and more aggressive experiments for the future. These experiments will significantly advance the sensitivity of dark energy measurements if they control their systematic uncertainties much better than has been possible to date on the ground. The unique scientific impact of JDEM could be reduced by these other experiments if they achieve their targeted sensitivities and if astrophysical systematic uncertainties prevent JDEM from achieving its sensitivity goals, although such an outcome seems unlikely. JDEM, by virtue of being based in space, will generally have better control of observational systematic uncertainties. It will also collect large samples of control data, and it will benefit from progress in understanding sources of systematic uncertainty as observational techniques and associated astrophysics theory advance. In practice, the implementation of a variety of measurement techniques, in space and on the ground, will provide a considerable degree of complementarity, which will improve overall sensitivity to dark energy and will provide important cross-checks of results. Steps for Moving Forward In order to prepare for implementation of one of the JDEM concepts, further progress should be made in understanding sources of systematic uncertainty and in developing strategies to mitigate systematic effects. All techniques for measuring effects of dark energy will benefit greatly from both observational and theoretical studies. For supernovas, issues include whether evolutionary effects are important, whether Type Ia supernova explosions are isotropic, and whether the light curve is standard. All of these effects can be empirically calibrated, but the issue is whether they can be calibrated to required sub-percent level accuracy. Both detailed observations and theoreti- cal modeling performed over the next few years will help substantially to reduce the uncertainties. Weak-lensing studies rely on extremely accurate measurements of the image profile and shape obtained from the telescopes over a large field and large time baseline. Weak-lensing results from ground-based facilities are not at the required level for the control of systematic errors, but space telescopes offer a significant improvement. Comparison of shear models, both from simulations and from ground-based observations using a variety of weak-lensing techniques,
52 NASAâS BEYOND EINSTEIN PROGRAM would improve understanding of the reliability of profile and shape measurements. For baryon acoustic oscillations, a concern is whether nonlinearities from gravitational effects bias the signal of the acoustic scale. Large redshift surveys from ground-based studies can provide knowledge of the systematic errors down to the percent level but will not explore sub-percent levels. However, theoretical models involving both analytic and numerical analysis will be very useful for understanding the systematic errors at the sub-percent level. The Dark Energy Task Force recommended high priority for near-term funding of projects that will improve the understanding of, or reduce, dominant systematic effects in dark energy measurements. With adequate support, substantial progress in theoretical and observational studies designed to calibrate the different distance estimators will be made within a few years. Any one of the three proposed JDEM concepts that the committee evaluated would strongly advance the understanding of dark energy. Nonetheless, as both observational techniques and theory rapidly advance, a differ- ent combination of observational techniques may be found to achieve better sensitivity. Ongoing analysis will be important for determining which combination of techniques can best achieve the Beyond Einstein goal. Science Assessment Summary Understanding the nature of dark energy is one of the most important scientific endeavors of the present era. JDEM will significantly advance both this endeavor and a broad array of other astrophysical studies. A central goal of JDEM is a precision measurement of the expansion history of the universe to determine whether the contribution of dark energy to the expansion rate varies with time. A measurement that discovers that the expan- sion history is not consistent with a cosmological constant will have a fundamental and revolutionary impact on physics and astronomy. While there are several current and planned dark energy experiments, JDEM will significantly improve sensitivity to the effects of dark energy. The principal science risk to JDEM arises from the challenge to control systematic uncertainties to the level required for significant improvement. Techniques for the control of observa- tional systematic uncertainties to required levels have not yet been demonstrated, and astrophysical systematic uncertainties for some measurement techniques may be irreducible. If JDEM is not able to control systematic uncertainties adequately, its improvement in dark energy sensitivity may be more modest than projected. JDEM will mitigate these risks by employing multiple complementary observational techniques and by collecting rich data sets to improve the control of systematic uncertainties and to provide valuable cross-checks. Many of JDEMâs advantages stem from the fact that it is space-based. Thus, JDEM could improve sensitivity to the effects of dark energy by an order of magnitude with respect to present measurements, and it is likely to improve significantly on new, ground-based experiments despite the challenges of controlling systematic uncertainties. In fact, the clarity provided by JDEMâs precision measurements is likely to be needed to confirm other dark energy measurements. Wide-field optical and NIR surveys required for dark energy studies will offer large, rich data sets for a broad array of other astrophysics studies, providing tremendous discovery potential. A full-sky, NIR spectroscopic survey such as ADEPT proposes for studying baryon acoustic oscillations, has never been performed, and no comparable mission is planned. This survey would open the emission-line universe, providing new probes of star formation during the era when galaxies grow, along with data for many other astrophysics studies (Table 2.20). A low-background, wide-field imaging survey, such as DESTINY and SNAP propose for studying weak lensing, would provide a much larger diffraction-limited NIR survey than is otherwise available. This survey would revo- lutionize understanding of how and when galaxies acquire their mass, as well as providing data for many other astrophysics studies. LASER INTERFEROMETER SPACE ANTENNA Introduction According to Einsteinâs general theory of relativity, mass in accelerated motion may lead to the emission of gravitational radiation. Like electromagnetic radiation (light, x-rays, and so on), gravitational waves travel at the speed of light, have two modes of polarization, and cause effects transverse to the direction of propagation.
SCIENCE IMPACT 53 TABLE 2.20â Joint Dark Energy Mission (JDEM): Summary of Scientific Evaluation Potential Contributions to Science Beyond Einstein Broader Science Factors Revolutionary discovery A measurement that discovers that the Wide-field optical and near-infrared (IR) surveys will potential expansion history of the universe is not offer tremendous discovery potential. A spectroscopic consistent with a cosmological constant survey would open the emission-line universe, and will have a fundamental and revolutionary an imaging survey would produce the richest data set impact on physics and astronomy. ever for studies of galaxy evolution. Science readiness and risk Systematic uncertainties may limit JDEM Because of the exquisite data sets that JDEM surveys to modest improvements over ground-based will produce, there is little risk to the broader science studies. impact. Mission Uniqueness Versus other space missions A comparable European space mission There are no comparable spectroscopic or imaging concept is under discussion but is not yet surveys to the proposed JDEMs. approved. Versus ground-based JDEM affords better control of systematic Wide-field cameras based on the ground cannot instruments uncertainties than ground-based experiments access the near-IR and have much poorer resolution at for supernova and weak-lensing studies, optical wavelengths due to atmospheric effects. and better statistics for baryon acoustic oscillations. NOTE: See Appendix G in this report for definitions of acronyms. But unlike electromagnetic radiation, which consists of varying electromagnetic fields in spacetime, gravitational radiation is the result of ripples in the fabric of spacetime itself. Electromagnetic radiation is strongly scattered or absorbed by dense regions of matter, and thus the radiation that we see, say from a supernova or a gamma-ray burst, often comes from secondary processes in the expanding shell of gas. By contrast, gravitational waves are extremely weakly absorbed, and thus they propagate directly to us from the region of accelerated bulk motions of massive objects. Gravitational waves are a uniquely powerful means of peering into those regions of the universe where the spacetime curvature is greatest and most rapidly changing and of seeing to the most distant reaches of the universe in space and time. Gravitational waves open a unique window onto the cosmos that will provide insights that cannot be gained from electromagnetic or cosmic-ray probes. There is compelling evidence from observations of the decaying orbits of binary pulsars that gravitational waves exist. For example, in the binary pulsar B1913+16, the rate of decrease of the orbital period agrees to bet- ter than half a percent with the prediction of general relativity of the loss of orbital energy through the emission of gravitational waves.58 The discovery of this system and the confirmation of Einsteinâs theory were recognized with the 1993 Nobel Prize to Joseph Taylor and Russell Hulse. Data from other binary pulsars confirm these conclusions. Nevertheless, despite considerable effort to build and operate gravitational-wave detectors on the ground, gravitational waves have not been detected directly to date, because the astrophysical signals are exceedingly weak in the frequency regime accessible to the ground-based experiments, currently operating at their design sensitivi- ties. Space-based instruments, which are not subject to Earthâs seismic noise, can âhearâ low-frequency gravity J.M. Weisberg and J.H. Taylor, 2005, The relativistic binary pulsar B1913+16: Thirty years of observations and analysis, pp. 25-32 in Binary 58â Radio Pulsars (F.A. Rasio and I.H. Stairs, eds.), ASP Conference Series 328, Astronomical Society of the Pacific, San Francisco, Calif.
54 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.21â Laser Interferometer Space Antenna (LISA): Mission Description Parameter Value Primary measurement Gravitational waves Observatory type Three satellites in triangular formation; intersatellite distance variations measured by laser interferometry Projected years in orbit 5 yr after beginning of science operation Type of orbit Heliocentric at 1 AU, 20Â° behind Earth Mission phases Single full-time data-collecting phase after commissioning Science operations Observation of total sky all the time; no pointing or scheduling needed or possible Other mission characteristics Drag-free proof masses TABLE 2.22â Laser Interferometer Space Antenna (LISA): Mission Instrument Properties Instrument Spectral Range Spatial Resolution Spectral Resolution Collecting Area Field of View Gravitational 3 Ã 10 Hz to â5 1Â° angular Measures waveform directly 10 sq. km 13 All sky, all the time wave antenna 0.1 Hz resolution for to fractions of a cycle over MBH mergers hundreds to thousands of cycles waves produced by a rich variety of known and exotic sources. The direct detection of gravitational waves will revolutionize our ability to observe the universe. The Laser Interferometer Space Antenna is a proposed gravitational-wave antenna in space whose goal is to detect gravitational waves, study their properties, and use them to create a radically new form of astronomy. A rich variety of strong low-frequency gravity wave signals is expected, and these can only be detected from space. LISA will consist of an array of three satellites orbiting the Sun, each satellite separated from its neighbor by about 5 million kilometers. The satellites will fly in an equilateral triangular formation in an Earth-like orbit, but trailing Earth by about 20Â°. The orbits are chosen to keep the spacecraft close to the vertices of an equilateral triangle throughout the mission. Launch of the three spacecraft will be on a single Atlas V rocket. A passing gravitational wave will cause minute changes in the relative distance between a fiducial or reference mass (called a proof mass) housed in one of the satellites and identical masses housed in each of the other satel- lites. Each proof mass is a 2 kg cube made of a gold-platinum alloy. These distance changes are to be measured using laser beams provided by 1 watt diode-pumped 1,064 nanometer Nd:Yag frequency-stabilized lasers coupled to 40-cm-aperture modified Cassegrain telescopes. Each satellite will house two such systems, with each beam directed at one of the two companion satellites. The six beams will be sent between the three satellites (one in each direction), with phases precisely referenced to the reflective surfaces of the proof mass associated with each laser, using an onboard phase measurement system (PMS). In order that the proof masses respond only to the spacetime strain induced by a gravitational wave, they will be maintained in purely gravitational orbits, protected from nongravitational disturbance forces such as solar radiation pressure, using a system of drag compensation. Electrostatic sensors will determine the location of each proof mass within its chamber and send signals to low-force thrusters (called micronewton thrusters), which will nudge the spacecraft to keep the proof masses at the centers of their respective chambers. This disturbance reduc- tion system (DRS) is a critical aspect of LISA technology, which will be tested on the LISA Pathfinder mission (see in Chapter 3 the section âLISA Missionâ). Employing phase-sensitive detection techniques, LISA will use the phases of each of the six laser beams to monitor the distance between the three pairs of proof masses (the relative location of the pair of proof masses within each satellite is monitored using internal laser optics). The changes in physical distance along each arm of the triangle induced by a gravitational wave will be reflected in phase changes in each of the six beams. Certain combinations of these six phase signals are directly related to the gravitational-wave amplitude, while another combination is insensitive to the waves but contains information
SCIENCE IMPACT 55 about instrumental noise sources. The basic mission characteristics and instrument properties are summarized in Tables 2.21 and 2.22. LISA will be sensitive to gravitational waves in the low-frequency band, between 3 Ã 10â5 and 0.1 Hz, with sensitivity to proof-mass displacements at the level of tens of picometers, corresponding to a fractional displace- ment sensitivity of 10â20. It is worth pointing out that the raw displacement sensitivity required for LISA is a mil- lion times less stringent than that already achieved by the ground-based laser interferometers, LIGO in the United States, and Virgo and GEO600 in Europe, although the ground-based instruments operate at higher frequencies. 59 But because of the long arms, the fractional sensitivity, or strain sensitivity, is so high that many of the target sources for LISA will be rather easy to detect, in the sense that their expected signal amplitudes will be between 10 and 10,000 times higher than the instrumental noise. Indeed, there are guaranteed detections: many known nearby binary star systems whose gravitational-wave signals are precisely calculable and are sufficiently strong that they will be used as verification and calibration sources.60 A gravitational-wave antenna of this sensitivity will open up a completely new window on many of the most interesting objects in the universe. During its proposed 5-year mission, LISA may be expected to detect gravita- tional waves from the inspiral and merger of massive black holes in the centers of galaxies or stellar clusters at cosmological distances, and from the inspiral of stellar-mass compact objects into massive black holes. Studying these waves will allow researchers to trace the history of the growth of massive holes and the formation of galactic structure, to test general relativity in the strong-field dynamical regime, and to verify if the black holes of nature are truly described by the predicted geometry of Einsteinâs theory of general relativity. LISA will measure the signals from close binaries of white dwarfs, neutron stars, or stellar-mass black holes in the Milky Way and nearby galaxies. These measurements will enable the construction of a census of compact binary objects throughout the Galaxy. There may also be waves from exotic or unexpected sources, such as cosmological backgrounds, cosmic string kinks, or boson stars. LISA will also be able to measure the speed of gravitational waves to very high preci- sion, and it may study whether there are more than the two polarizations predicted by general relativity. Mission Science Goals Contribution to Beyond Einstein Science LISA will contribute directly to Beyond Einstein goals by studying the properties of cosmic black holes, testing general relativity in new regimes, and making interesting cosmological measurements (see Table 2.23). There is strong and growing observational evidence for the existence of massive astrophysical black holes. The most convincing case comes from our own Galaxy, where a population of stars is seen orbiting a compact object of 3.7 million solar masses,61 but evidence supports the conclusion that black holes with masses between 10 5 and 109 solar masses reside in the centers of nearly all nearby massive galaxies. There is also a robust correlation between the mass of the central black hole and both the luminosity and velocity dispersion of the host galaxyâs central bulge.62 How such massive holes formed and what the origin of this correlation is are still mysteries. The leading scenario involves the repeated mergers of, and gas accretion by, galactic-center black holes following the merger of their respective host galaxies. However, it is not known whether the original âseedâ black holes were 30 to 300 solar mass holes formed from the collapse of heavy-element-free Population III stars in the early universe (redshift ~20), or 105 solar mass holes formed much later from the collapse of material in protogalactic disks. 59â F.J. Raab, representing the LIGO Scientific Collaboration and other laser interferometer groups, 2006, ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ The status of laser interferometer gravitational-wave detectors, J. Phys. Conf. Ser. 39:25-31. K. Danzmann, 1997, LISAâan ESA cornerstone mission for a gravitational-wave observatory, Class. Quantum Grav. 14:1399-1404. 60â R. SchÃ¶del, T. Ott, R. Genzel, A. Eckart, N. Mouawad, and T. Alexander, 2003, Stellar dynamics in the central arcsecond of our galaxy, 61â Astrophys. J. 596:1015-1034; A.M. Ghez, S. Salim, S.D. Hornstein, A. Tanner, J.R. Lu, M. Morris, E.E. Becklin, and G. Duchene, 2005, Stellar orbits around the galactic center black hole, Astrophys. J. 620:744-757. J. Kormendy and D. Richstone, 1995, Inward bound: The search for supermassive black holes in galaxy nuclei. Ann. Rev. Astron. Astrophys. 62â 33:581-628; K. Gebhardt, R. Bender, G. Bower, A. Dressler, S.M. Faber, A.V. Filippenko, R. Green, C. Grillmair, L.C. Ho, J. Kormendy, T.R. Lauer, J. Magorrian, J. Pinkney, and D. Richstone, 2002, The slope of the black-hole mass versus velocity dispersion correlation, Astrophys. J. 574:740-753.
56 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.23â Laser Interferometer Space Antenna (LISA): Beyond Einstein Science Programs Science Program Program Characteristics Program Significance Science Formation of Science How and when do massive black holes form? Observations will detect massive definition massive black question black hole binary mergers to z = programs holes 15 and shed light on when massive Measurements Gravitational waveform shape as a function of time black holes formed. from massive black hole binary inspiral and merger Quantities Mass and spin of black holes as a function of determined distance Test general Science Does general relativity correctly describe gravity Measurement of the detailed relativity in question under extreme conditions? gravitational waveform will the strong- test whether general relativity field regime Measurements Gravitational waveform shape as a function of time accurately describes gravity under from massive black hole binary inspiral and merger the most extreme conditions. Quantities Evolution of dynamical spacetime geometry, mass determined and spin of initial and final holes History of Science How is black hole growth related to galaxy Observations will trace the galaxy and question evolution? evolution of massive black hole black hole masses as a function of distance or co-evolution Measurements Gravitational waveform shape as a function of time time, and will shed light on how from massive black hole binary inspiral and merger black hole growth and galactic evolution may be linked. Quantities Mass as a function of distance determined Additional Map Science Are black holes correctly described by general Observations will yield maps of the Beyond black hole question relativity? spacetime geometry surrounding Einstein spacetimes massive black holes and will test science Measurements Gravitational waveform shape from small bodies whether they are described by the spiraling into massive black holes (EMRI) Kerr geometry predicted by general relativity. They will also measure Quantities Mass, spin, multipole moments, spacetime the parameters (mass, spin, shape) determined geometry close to hole of the holes, and test whether they obey the no-hair theorems of GR. Cosmological Science Are there gravitational waves from the early First-order phase transitions or backgrounds question universe? cosmic strings in the early universe could leave a background of Measurements Stochastic background of gravitational waves detectable waves. Quantities Effective energy density of waves vs. frequency determined Cosmography, Science What is the distance scale of the universe? If redshift of source or host galaxy dark energy question can be determined, then precise, calibration-free measurements of Measurements Gravitational waveform shape and amplitude the Hubble parameter and other measurements yield luminosity distance of sources cosmological parameters could be directly done, significantly constraining dark energy. Quantities Luminosity distance determined NOTE: See Appendix G in this report for definitions of acronyms.
SCIENCE IMPACT 57 Furthermore, it has proven difficult to find a process whereby the holes in the merged galaxy can efficiently find each other and merge on a fast-enough time scale. By studying massive black hole mergers beyond redshift 10 for holes between 105 and 107 solar masses and to redshift 10 to 20 for holes between 100 and 105 solar masses, LISA will be able to search for the earliest seed black holes. In addition, by matching the observed gravitational waveform to a bank of theoretically predicted template waveforms, a technique that has been developed for use in the ground-based interferometers, LISA will be able to make very precise measurements of black hole masses and distances. Furthermore, in the hierarchical merger scenarios, the rate of detectable mergers may be as high as two per week. Thus, LISA will be able to trace the history of the growth of black hole masses and thereby shed direct light on how their formation and growth may be linked to the evolution of galaxies. Because the final inspiral and merger of the two massive holes is dominated by the mutual gravity of the holes, which consist themselves of pure warped spacetime geometry, the orbit and gravitational-wave signal will reflect strong-field, dynamical, curved-spacetime general relativity in its full glory. Detailed comparisons between the measured waveforms and theoretical waveforms calculated from combinations of analytical and numerical solutions of Einsteinâs equations (a method called matched filtering) will give a rich variety of tests of the theory in a regime that has hitherto been inaccessible to experiment or observation. For example, there is now evidence from numerical solutions of Einsteinâs equations that the spin of the individual black holes may play a critical role in how they merge; depending on the magnitude and alignment of the spins, the mergers could be very rapid or could experience a momentary âhang-up,â with significant consequences for the observed waveform. 63 These are the consequences of âframe dragging,â a fundamental prediction of Einsteinâs theory that has been probed in the solar system using Gravity Probe B, Laser Geodynamics Satellites (LAGEOS), and lunar laser ranging; frame dragging has been hinted at in observations of accretion onto neutron stars and black holes. Observing the effects of frame dragging in such an extreme environment would be a stunning test of general relativity. Furthermore, with spinning progenitors, the final black hole could experience a substantial recoil resulting from the emission of linear momentum in the gravitational waves, large enough to eject it completely from the host galaxy. Matched filtering of the inspiral and merger waveforms will also provide measurements, some with very high precision, of such quantities as the masses and spins of the initial and final black holes, the distance to the system, and its location on the sky. For example, for two 106 solar mass nonspinning black holes merging at z = 10, the total mass of the system could be measured to 0.1 percent and the luminosity distance could be measured to 30 percent; at z = 1, the corresponding figures are 0.001 percent and 2 percent, respectively. 64 In addition, LISA will be able to detect âringdownâ waves, which are waves emitted by the distorted final black hole as it settles down to a stationary state. These waves have discrete frequencies and damping rates that depend on the mass and spin of the hole. By carrying out âblack holeâ spectroscopy on this discrete spectrum of ringdown waves, LISA will be able to test whether the geometry obeys the âno-hairâ theorem of the Kerr metric predicted by general relativity. If the basic ideas of massive black hole growth are qualitatively correct, LISA may expect to see tens to hundreds of events per year for inspirals at the high-mass end. For inspirals at the low-mass end, the rates are highly uncertain. Another class of sources, called extreme mass-ratio inspirals (EMRI), may provide additional quantitative tests of the spacetime geometry of black holes. These involve a stellar-mass compact object spiraling into a mas- sive (106 solar mass) black hole. Over the 104-105 eccentric, precessing orbits traced out by the smaller mass, the emitted waves encode details about the spacetime structure of the larger hole with a variety of distinct signatures. In addition to providing determinations of the black holeâs mass and angular momentum to fractions of a percent, the observations can also be used to test whether the spacetime that encodes the waves is the unique Kerr geometry that general relativity predicts for rotating black holes.65 M. Campanelli, C.O. Lousto, and Y. Zlochower, 2006, Spinning black-hole binaries: The orbital hang-up, Phys. Rev. D 74:041501. 63â E. Berti, A. Buonanno, and C.M. Will, 2005, Estimating spinning binary parameters and testing alternative theories of gravity with LISA, 64â Phys. Rev. D 71:084025. S.A. Hughes, 2006, (Sort of) testing relativity with extreme mass ratio inspirals, pp. 233-240 in Laser Interferometer Space Antenna: 65â 6th International LISA Symposium (S.M. Merkowitz and J.C. Livas, eds.), AIP Conference Proceedings, Volume 873, American Institute of Physics, College Park, Md.
58 NASAâS BEYOND EINSTEIN PROGRAM TABLE 2.24â Laser Interferometer Space Antenna (LISA): Broader Science Examples Program Program Characteristics Program Significance Galactic compact Science question What is the distribution of binary Could provide a census of compact binary binaries systems of white dwarfs and systems not achievable by electromagnetic means, neutron stars in our Galaxy? and could survey the systems that are progenitors of high-frequency gravitational-wave sources Measurements Sinusoidal gravitational waveforms detectable by ground-based interferometers. Population statistics could improve models of Quantities determined Orbital frequencies, sky distribution binary stellar evolution. LISA will also be able to test the nature of the gravitational waves and test specific alternative theories to general relativity. Using massive-black-hole inspiral data, LISA will be able to measure any hypothetical differ- ence in the speeds of gravitational waves and of light with a precision of parts in 10 17 and test whether or not the âgraviton,â the putative quantum particle of gravity, has a mass.66 Because the LISA spacecraft orbit the Sun, they will be sensitive to different mixtures of the polarization modes in the waves from a sufficiently longlasting source and may be able to test whether the general relativistic prediction of only two transverse quadrupolar modes is correct. These would constitute tests of Einsteinâs theory in an entirely new regime. Because binary black hole inspirals are controlled by a relatively small number of parameters, such as mass, spin, and orbital eccentricity, they are good candidates for standard candles.67 They are good candidates because the frequency and frequency evolution of the waves are determined only by the systemâs parameters, while the wave amplitude depends on those same parameters and on the luminosity distance to the source. No complex calibra- tions are needed. Matched filtering analyses have shown that, for nearly circular inspirals, LISA could measure luminosity distances to a few percent at redshift 2 and to tens of percent at z = 10. At the same time, because of the changing orientation of the LISA array with respect to the source, it can also determine the orientation, with precision of better than a degree for massive inspirals at z = 1. If this angular and distance resolution were enough to link a LISA event with a corresponding electromagnetic event in a host galaxy or quasar and thereby to yield a redshift, LISA would contribute a direct, absolute calibration of the cosmic distance scale (Hubble diagram) that relies only on fundamental physics rather than on the complex chain of largely empirical distance ladders on which researchers rely at present. A 2 percent measurement of distance combined with a redshift at z = 1 would give a 2 percent measurement of the dark energy parameter w. The combination of several such measurements could give a dark energy bound that begins to be competitive with JDEM. The main challenge will be in using LISAâs angular resolution to identify the host galaxy. Contributions to Other Science Because of the apparent close connection between galactic center black holes and the structure of their host galaxies, information on the formation and growth of massive black holes over cosmic time will feed into models of galactic formation and evolution. The study of EMRIs using coordinated gravitational-wave and electromagnetic observations will improve the understanding of the stars and gas in the close vicinity of galactic black holes. Within our own Galaxy, LISA will measure the orbits and determine the locations of up to 10,000 close binary systems consisting mainly of white dwarfs; as such systems are the precursors of Type Ia supernovas and millisecond pulsars, such a census will aid in understanding the evolution of such systems (see Table 2.24 for a summary). C.M. Will, 1998, Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries, Phys. Rev. 66â D 57:2061-2068. B.F. Schutz, 1986, Determining the Hubble constant from gravitational wave observations, Nature 323:310-311. 67â
SCIENCE IMPACT 59 Opportunity for Unexpected Discoveries Despite numerous expectations and predictions based on the current knowledge of the universe derived from electromagnetic observations, in fact, the direct knowledge of the gravitational-wave sky is precisely zero. The history of astronomy tells us that every new window on the universe has completely transformed the understanding of the cosmos. Such transformations took place when the first telescopes were invented, when radio astronomy began, and when x-ray astronomy started, to name just a few examples. It would be unreasonable to imagine that there will be no surprises when we open the gravitational-wave window. LISA may well observe signals from new sources that cannot be detected with electromagnetic radiation. Because gravitational waves may originate at very high redshift and propagate without absorption or scattering, LISA could provide the first information of any kind about some types of nonlinear motions of matter and energy. For example, first-order phase transitions of new forces or extra dimensions in the early universe could produce a detectable background of gravitational waves. Such events would occur between an attosecond (10 â18 seconds) and a nanosecond after the big bang, a period not directly accessible by any other technique. Other potential ex- otic sources include intersecting cosmic string loops or vibrations and collapses of âboson stars,â stars made of hypothetical scalar-type matter. Assessment of Scientific Impact LISA will open a revolutionary new window on the universe, using the rippling of spacetime itself rather than fields propagating through spacetime as its source of information about the activities of the sources. It will observe many phenomena that cannot be detected directly by electromagnetic means, such as the inspiral and merger of black holes. LISA will uncover how massive black holes formed and interacted, and it will yield for the first time precise measurements of their masses and spins. It will test how well general relativity accounts for extreme gravity, will verify the dragging of inertial frames in extreme situations, and will check whether black holes are indeed those described by general relativity, tests that cannot be done by any other means or that are prone to uncertainties owing to complex nongravitational physics phenomena. LISA will study how the earliest galactic structures formed in the early universe and will shed light on the merger history of galaxies. It will provide a census of compact binary systems in the Galaxy far beyond what can be done with electromagnetic techniques, and it will measure luminosity distances to high-redshift sources precisely and without complex calibrations. It will also make fundamental measurements of the properties of the gravitational waves themselves. Finally, it may detect waves from processes in the early universe or from exotic or unexpected sources. No other technique addresses some of the questions that LISA addresses, especially related to the gravitational dynamics of black holes, where only gravitational signals can escape the surrounding gas and dust unimpeded. It will also be studying directly the bulk, coherent motions of large masses, which dominantly produce gravitational waves. This production method contrasts with electromagnetic waves, which usually originate in the incoherent superposition of motions of charged particles. LISA may also provide the first direct detection of gravitational waves, a quest that began in the 1960s. Al- though the ground-based laser interferometers in the United States and Europe are operating on schedule and at their design sensitivities, they must successfully carry out a sequence of planned upgrades before they reach the level of sensitivity at which they can confidently be expected to see gravitational waves. There is no guarantee that this level will be achieved before the proposed launch and operation of LISA. At the same time, there is no direct competition from the ground-based interferometers even if they should detect waves first. The two approaches are complementary. The ground-based systems are sensitive to the high-frequency gravitational-wave band, between 10 and 1,000 Hz. Their target sources are stellar-mass black hole and neutron star inspirals and mergers, spinning pulsars, neutron star vibrations, and supernova core collapse in the relatively nearby universe. They do not address the same science that LISA does. However, there are some synergies between the two approaches: for example, some of the close compact binary systems that LISA is expected to detect in the millihertz band are the precursors to the kilohertz inspiral sources detectable by the ground-based interferometers.
60 NASAâS BEYOND EINSTEIN PROGRAM Science Readiness and Risk LISAâs quest to detect gravitational waves is based on our understanding of general relativity (indeed of any theory of gravity that is compatible with special relativity), where the emission of gravitational waves is required by the existence of a fundamental limiting speed for the propagation of information. But because the most interest- ing sources involve extreme gravity and relativistic speeds, it is important to ask whether techniques for solving Einsteinâs equations are sufficiently advanced to predict confidently the gravitational waves from the sources of interest and to interpret the data taken. Secondly, the mission is based on our understanding of sources that might actually exist, so we must ask whether the astrophysics is sufficiently well understood to predict with reasonable confidence that LISA will detect interesting sources during its proposed 5-year mission lifetime. During the past decade, a combination of analytical and numerical work has provided sufficient machinery to yield robust predictions from general relativity for the gravitational-wave signal from massive-black-hole coalescences, including the inspiral, merger, and ringdown phases. Indeed, recent breakthroughs in ânumerical relativityâ have been critical in providing solutions that link the inspiral signal, which is determined using ana- lytical approximation techniques (commonly known as post-Newtonian theory), with the ringdown signal, which is determined from perturbation theory of black holes.68 These new methods are now being applied to the more complex and interesting case of mergers of rapidly spinning black holes, and substantial progress is likely during the next few years, well in advance of LISA. The EMRI problem is somewhat different: there the small compact object can be viewed as a perturbation of the background spacetime of the large black hole, but one must take into account the âbackreactionâ of the small bodyâs gravitational field on itself, including the damping of the orbit due to the emission of gravitational waves. Despite considerable progress, substantial work remains to be done to develop waveform predictions for LISA that will cover the hundreds of thousands of expected orbits with sufficient accuracy. For the more conventional sources, such as the galactic close binary systems, textbook general relativity is completely adequate. Because of LISAâs high sensitivity, it is expected that many sources will have their signals superimposed simultaneously on the data stream. Recently, a program of LISA âmock data challengesâ has shown substantial promise in demonstrating the ability to extract multiple signals, ranging from inspiral âchirpsâ to steady sinusoidal signals from simulated data streams.69 On the astrophysics side, there are a number of assured sources, including well-documented, close binary systems in our Galaxy, which will be used as verification or calibration signals during the first year of science operation. A foreground of waves from galactic and extragalactic close white-dwarf binaries is expected to be detectable; in fact, in some frequency ranges this foreground will represent an unresolvable gravitational-wave noise stronger than the instrumental noise. Predicted event rates for massive-black-hole inspirals are uncertain by a factor of 10 but indicate that LISA is likely to detect them even in 1 year of operation. For EMRIs, the rates are even more uncertain; this could be a risk factor if the mission fails to achieve its 5-year lifetime. Steps for Moving Forward Because LISA is a joint NASA-ESA project, the committee considered how to maintain a level of synchron- icity between the schedules of the two agencies. In late 2009, ESA plans to select two candidate missions for an âL-1â class launch around 2018 from proposals submitted in response to its Cosmic Visions 2025 opportunity. As LISA is likely to be the most developed project among the possible contenders, it will be in a strong position for selection to enter ESAâs Definition Phase (roughly equivalent to NASAâs Phase B). The final selection of a single mission to enter the implementation phase is expected to occur in late 2012 and will include the Pathfinder results F. Pretorius, 2005, Evolution of binary black hole spacetimes, Phys. Rev. Lett. 95:121101; J.G. Baker, J. Centrella, D. Choi, M. Koppitz, 68â and J. van Meter, 2006, Gravitational wave extraction from an inspiralling configuration of merging black holes, Phys. Rev. Lett. 96:111102; A. Buonanno, G.B. Cook, and F. Pretorius, 2007, Inspiral, merger, and ringdown of equal-mass black-hole binaries, Phys. Rev. D 75:124018. K.A. Arnaud et al., 2006, ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Laser Interferometer Space Antenna: 6th Inter- 69â The mock LISA data challenges: An overview, pp. 619-624 in national LISA Symposium (S.M. Merkowitz and J.C. Livas, eds.), AIP Conference Proceedings, Volume 873, American Institute of Physics, College Park, Md.
SCIENCE IMPACT 61 TABLE 2.25â Laser Interferometer Space Antenna (LISA): Summary of Scientific Evaluation Potential Contributions to Science Beyond Einstein Broader Science Factors Revolutionary discovery LISA will open a unique new window on LISA could detect waves from exotic or unexpected potential the universe, will test general relativity sources, such as cosmic strings or early universe phase in the most extreme regimes, will study transitions. the formation and evolution of massive black holes, and will measure absolute distances on cosmological scales. Detection of gravitational waves is assured. Science readiness and Understanding of the underlying theory Low risk: detection of many galactic binaries is assured. risk and data analysis is robust. The main risk is the uncertainty in rates of mergers involving massive black holes. Mission Uniqueness No similar or competing missions are No similar or competing missions are envisioned. envisioned. in the evaluation process. Aggressive technology development will be needed to advance the technical readiness of the mission so that LISA will be ready to enter a NASA implementation phase in line with ESAâs schedule. Science Assessment Summary LISA promises to open a completely new window into the heart of the most energetic processes in the universe, with consequences fundamental to both physics and astronomy (see Table 2.25). During its proposed 5-year mission, LISA is expected to detect gravitational waves from the inspiral and merger of massive black holes in the centers of galaxies or stellar clusters at cosmological distances, and from the inspiral of stellar mass compact objects into massive black holes. The study of these waves can trace the growth of massive holes and the formation of galactic structure, test general relativity in the hitherto untested strong-field dynamical regime, and test whether the black holes found in nature are truly described by Einsteinâs theory. LISA can measure absolute distances to systems on the far side of the universe and could contribute to cosmological measurements, such as of dark energy. LISA will measure both the speed and the polarization states of gravitational waves. LISA could also detect waves from exotic sources such as cosmic strings or phase transitions in the early universe. LISA can measure signals from close binaries of white dwarfs, neutron stars, or stellar-mass black holes in the Milky Way and nearby galaxies. These measurements will enable the construction of a census of compact binary objects throughout the Galaxy. SCIENCE SUMMARY This section summarizes the committeeâs assessment of the contribution that the candidate missions would make to the Beyond Einstein science questions. The summary captures the strengths, scientific uncertainties, readi- ness, and uniqueness of the associated scientific programs. What Powered the Big Bang? Inflation Probe is the mission that most directly addresses the question, What powered the big bang? IP aims to study the conditions that existed during the time of inflation, when the universe expanded by 30 orders of mag-
62 NASAâS BEYOND EINSTEIN PROGRAM nitude, creating nearly all particles and radiation. The inflationary period cannot be observed directly. However, inflation does leave distinct imprints that can be observed to determine its properties. The IP mission concepts take one of two approaches. The first approach studies the imprint of gravitational waves on the cosmic microwave background. This measurement will probe the energy scale of inflation, possibly around 10 16 GeV, far beyond the capabilities of ground-based accelerators. The second approach measures inflationâs effect on primordial density fluctuations by observing the amount of structure in the universe on various length scales. It is also possible that LISA will observe the early universe during inflation directly by detecting a gravitational-wave background pro- duced during this era; however, most theories predict a signal that is beyond LISAâs reach. There are ongoing and vigorous efforts to develop technology and measurement techniques to achieve the estimated 30 nK sensitivity required for a CMB polarization mission. Control of instrumental and observational systematic effects has yet to be sufficiently understood. In addition, the polarized galactic foreground is an esti- mated 30 times bigger than the expected signal. It has yet to be proven that it can be removed with high enough precision to reveal an unambiguous primordial signal. These issues are being addressed with ground-based and suborbital missions. However, there is a clear need for more research in these areas. Finally, the theory indicates that the signal may be too small to be detected with the missions as they are currently defined. Advances in tech- nology, observations, and theory are likely to clarify this risk. This makes the selection of a CMB polarization mission premature at this time. The technique of using structure measurements is less subject to systematic and measurement uncertainties than the polarization measurement. It must be combined with accurate low-redshift surveys and high-quality CMB anisotropy data that either exist or will be mature in the near future. The result will be a strong constraint on inflationary models but not a measurement of the energy scale of inflation. Significant progress measuring the amount of structure in the universe on various length scales has already been made from the ground with Sloan Digital Sky Survey (SDSS). More importantly, ongoing and future ground-based optical measurements of galaxies using Lyman-alpha emission could prove to be as significant as space-based approaches. Finally, no matter how the structure method is carried out, the energy scale of inflation would still need to be measured. How Do Black Holes Manipulate Space, Time, and Matter? Gravitational waves and black holes are among the most interesting predictions of Einsteinâs theory of grav- ity. LISA will use its high-signal-to-noise detector to test Einsteinâs theory of general relativity in the strong-field dynamical regime and to map spacetime around a black hole by detailed studies of low-frequency gravitational waveforms. By observing the mergers of pairs of massive black holes, LISA will test whether general relativity accurately describes gravity under the most extreme possible conditions. These will provide fundamentally new measurements of the distortion of spacetime near a black hole. As small bodies spiral into massive black holes, they trace tens of thousands of orbits and emit waves that encode details of the spacetime structure around the massive black hole. By detecting these waves, LISA will provide a rigorous and clean test of whether spacetime is described by the Kerr geometry predicted by general relativity for rotating holes and measure black hole masses and spins to a fraction of a percent. The main science risk to LISAâs ability to test general relativity is the event rates, which may be smaller than predicted. Predicted rates for massive-black-hole inspirals are uncertain by a factor of 10, but they indicate that LISA is likely to detect them even in 1 year of operation. But for small-body inspirals into massive black holes, the rates are even more uncertain: this could be a risk factor if the mission fails to achieve its 5-year lifetime. In terms of scientific readiness, the framework for interpreting LISA waveforms has recently been made more robust.70 The theory for inspirals has been adequate for quite some time, but recently numerical relativity techniques have advanced to the point that black hole merger waveforms can be predicted with confidence. F. Pretorius, 2005, Evolution of binary black hole spacetimes, Phys. Rev. Lett. 95:121101; J.G. Baker, J. Centrella, D. Choi, M. Koppitz, 70â and J. van Meter, 2006, Gravitational wave extraction from an inspiralling configuration of merging black holes, Phys. Rev. Lett. 96:111102; A. Buonanno, G.B. Cook, and F. Pretorius, 2007, Inspiral, merger, and ringdown of equal-mass black-hole binaries, Phys. Rev. D 75:124018.
SCIENCE IMPACT 63 LISA is unique in that no other facility can probe the low-frequency regime that contains the majority of interesting astrophysical signals. Seismic noise prevents ground-based detectors such as LIGO and VIRGO from accessing this regime. Constellation-X will also probe the geometry of the region near black holes by observing hot, x-ray emitting material as it spirals into the hole in an accretion disk. The motion of hot blobs in the disk can be observed using time-resolved, high-resolution x-ray spectroscopy, and overall distortions in the shapes of composite lines from the disk can be modeled to determine the spacetime geometry and measure the black hole spin. The science risk lies in understanding the magnetohydrodynamics that may be needed to connect the x-ray observations to the detailed properties of the black holeâs spacetime metric. If the orbits of hot blobs are ballistic in the inner regions of the accretion disk, the measurements will be simpler to interpret. Con-X will be able to constrain black hole masses and spins given a Kerr metric, providing important information on black hole formation scenarios. If, however, the observations do find deviations from the expected spacetime geometry it will be difficult to confidently ascribe these to deviations from general relativity because of the uncertainty in the accretion physics. For this reason, the committee found LISAâs measurements of spacetime surrounding black holes to be a better precision test of general relativity. Current x-ray missions have already detected the line shape distortions due to general relativistic effects; however, no proposed x-ray facility other than Con-X has the needed combination of efficiency and resolution to extend this technique to time-resolved measurements. In addition to understanding how black holes distort spacetime, the Beyond Einstein Program seeks to under- stand how they are formed and evolve and how they interact with galaxies and clusters. LISA, Con-X, BHFP, and JDEM will all make significant contributions to different aspects of this important problem. Theory tells us that very massive (Mh > 107 Msun) black holes in the centers of galaxies should become increasingly rare at high redshift; however, there are currently no observational constraints on the black hole mass distribution from above z ~7. Here LISA promises to be revolutionary, by detecting massive-black-hole binary mergers out to z ~15-20, measuring the high-redshift mass distribution in the range 104-108 Msun. This will be crucial to revealing how galaxies with black holes formed and merged in the early universe, and how black hole growth and galactic evolution may be linked. The gravitational-wave signals unambiguously yield masses for both the merging black holes and the luminosity distance, which can be converted to redshift given a cosmology. The principal uncertainty in the quality of this measurement is the unknown merger rates. However, even a few detections will be very interesting. JDEM will also constrain the high-z luminous black hole population by using its near-infrared sensitivity to extend SDSS-like surveys well beyond redshifts of 6.4 (the highest redshift quasar identified by SDSS). Limits on these bright objects (the high-mass end of the black hole spectrum) are particularly constraining to galaxy formation models. X-ray spectral follow-up observations with Con-X SXTâs large collecting area will, however, be critical to confidently identifying these objects as black holes and to determining their bolometric accretion luminosity. At low redshifts (z <1), BHFP will use the penetrating power of high-energy x-rays to locate those accreting massive black holes that are hidden behind large columns of dust and gas over the entire sky and over a relatively wide luminosity (and therefore mass) range, providing another key component of a black hole census. Con-X, with the excellent sensitivity of its hard x-ray telescope, can also detect obscured massive black holes out to z >2 over more limited areas of sky, helping to determine how these objects evolve. The JDEM, BHFP, and Con-X black hole measurements are all evolutionary in the sense that they extend current optical, infrared, and x-ray surveys to a broader population. However, there is little risk that these mea- surements will not provide substantial new insights, given the expected data quality. Since they measure accretion luminosity, they are all subject to uncertainties in the conversion of accretion luminosity to hole mass, and this may limit the determination of black hole evolution. However, studies with Con-X and BHFP will likely improve our understanding of accretion physics and therefore the luminosity to mass conversion. These missions are each unique in their ability to uncover specific portions of the black hole content of the universe using wavelength bands only accessible from space. Finally, BHFP will detect gamma-ray bursts, many of which signal the formation of a stellar mass black hole, out to high redshifts, and through variability measurements can observe stars being shredded as they plunge
64 NASAâS BEYOND EINSTEIN PROGRAM into black holes. The rate and high-energy x-ray luminosity of these events are uncertain, but detection would be exciting and unique. What Is the Mysterious Dark Energy Pulling the Universe Apart? The Joint Dark Energy Mission and Constellation-X will make measurements that characterize the effect of dark energy on the geometry of the universe and/or on the growth of structure. This will yield the ratio of the dark energy pressure to its energy density as a function of time, enabling researchers to distinguish between a cosmo- logical constant, a dynamical evolving field, a modification of general relativity, or some other new physics. The primary purpose of the JDEM missions is to employ at least two of the following three techniques for the explora- tion of dark energy: (1) using Type Ia supernovas as standard candles to determine the luminosity-distance versus redshift relation; (2) using weak lensing to measure the angular-diameter versus redshift relation, as well as the growth of structure; and (3) using baryon acoustic oscillations to measure angular diameter versus distance. Con- X will use galaxy clusters in two different ways to measure the evolution of dark energy. The first is to determine cluster distances independent of redshift (assuming the gas mass fraction is redshift-independent) and compare these distances to the measured redshift. The second is to measure the effect of dark energy on the growth of structure by determining the mass distribution of clusters as a function of redshift. For the latter measurement, Con-X relies on wide-area cluster surveys from other experiments and will provide the follow-up observations required to accurately determine the cluster masses. LISA also has the potential to measure the dark energy equation of state, along with the Hubble constant and other cosmological parameters. Through gravitational-waveform measurements, LISA can determine the luminos- ity distance of sources directly. If any of these sources can be detected and identified as infrared, optical, or x-ray transients and if their redshift can be measured, this would revolutionize cosmography by determining the distance scale of the universe in a precise, calibration-free measurement. The science risk of the JDEM and Con-X dark energy evolution measurements is the uncertainty in the level of precision and control of the systematic effects. At the present time, weak lensing and baryon acoustic oscillation measurements appear most likely to provide the requisite factor-of-10 improvement over currently available constraints, and each of the proposed JDEM missions employs one of these techniques. The complex astrophysics associated with clusters makes the understanding of systematic effects particularly challenging for this measurement; however, it is possible that detailed x-ray observations of individual clusters with Con-X will improve theoretical understanding sufficiently to allow a precision measurement of w. It is important to use several independent methods of measurement, since they can lead to almost orthogonal constraints and have very differ- ent uncertainties. However, because of the importance of controlling systematics, the committee favors the JDEM missions over Con-X for this measurement. One risk to the success of cosmography with gravitational waves from merging supermassive black holes is the uncertain merger rate. Also at the present time researchers do not know if it will be possible to determine optical counterparts in order to measure redshifts. While the prospect is very exciting, since it would be precise and free of systematic uncertainties, it may not be achievable if, for example, counterparts do not exist. The committee notes that both a wide-FOV near-IR space telescope, such as JDEM, and the Con-X mission would enhance the prospects of counterpart identification if they flew simultaneously with LISA. All of the JDEM dark energy measurements are being pursued by other experiments. Ground-based telescopes are currently improving statistics of the supernova and baryon oscillation measurements, and future wide-field telescopes will make progress on weak lensing. Space measurements are, however, unique for access to the near-IR, redshift coverage, and stable point spread function, all of which are important for the control of systematics crucial for these measurements. For cluster studies, the eROSITA x-ray mission and ground-based Sunyaev-Zeldovich experiments will significantly improve the dark energy measurements, but it is unlikely that the ultimate precision will be reached without Constellation-Xâs spectroscopic capability. Improving measurements of the amount of dark and baryonic matter in the universe is also essential to un- derstanding the amount of dark energy. All of the JDEM mission concepts can contribute to this goal. Wide FOV optical and NIR imaging telescopes can study the large-scale distribution of mass via weak lensing and clarify
SCIENCE IMPACT 65 how galaxies and clusters acquired their mass through both weak lensing and optical photometric surveys. Al- ternatively, the full-sky NIR spectroscopic survey could revolutionize the understanding of how and when star formation occurred in galaxies. Constellation-X will make important contributions by detecting and characterizing the warm hot intergalactic medium, believed to contain most of the atoms in the present-day universe. Existing measurements of the baryon content in the early universe from the CMB would allow determination of the present-day distribution of baryonic matter. Con-X also has the potential to probe the nature of dark matter, which constitutes most of the mass of the universe, by observing its effect in galaxy clusters. The JDEM and Con-X measurements of the matter content and distribution in the universe would be synergistic with the many other efforts in this area being pursued by other ground- and space-based facilities. Conclusions As a whole, the suite of five Beyond Einstein missions has tremendous potential to unambiguously answer the three fundamental questions at the core of the program. In its consideration of which mission should fly first, the committeeâs primary science-evaluation criterion was how directly and unambiguously the different missions would answer one or more of the three questions put forward in NASAâs Beyond Einstein roadmap. This evaluation involved balancing breadth, depth, and scientific risk. The committee gave priority to those missions that promise significant advances, even if on a single question, over missions providing more incremental but broader progress touching on many areas, although both sets of contributions were valued. The committee determined that Inflation Probe is the candidate offering the greatest potential for progress in addressing the question, What powered the big bang? JDEM is the mission providing the measurements most likely to determine the nature of dark energy, and LISA provides the most direct and cleanest probe of spacetime near a black hole. Constellation-X, in contrast, provides measurements promising progress on at least two of the three Beyond Einstein questions, but does not provide the most direct, cleanest measurement on any of them. It is, however, an outstanding general astrophysics observatory that will make important advances on other questions set forth in NASAâs Beyond Einstein roadmap. The Black Hole Finder Probe will contribute to a black hole census, but it provides less direct measurements of black hole properties than LISA measurements. It was the committeeâs judgment that for a focused program like Be- yond Einstein, it is most important to provide the definitive measurement against at least one of the questions. With any bold scientific venture there is always risk. For Inflation Probe, the scientific risk is, at the current time, unacceptably high for an investment of the scale of the proposed missions. Uncertain signal levels, fore- grounds, and measurement sensitivities suggest that it is premature to proceed with an IP at this time. However, progress from the ground and suborbital platforms will likely be rapid in the next few years, and the maturation of theory and observation in this area will likely make it an exciting future opportunity. JDEM provides the best constraints on the nature of dark energy; however, there is risk that the systematic uncertainties associated with astronomical phenomena will limit the ultimate precision at a level less constraining than what the missions cur- rently estimate, representing less of an advance over ground-based measurements than would be desirable for an investment of this scale. However, it is certainly the case that the ultimate precision and best control of systematics in constraining the DE equation of state will be achieved by space-based observations. Also mitigating the over- all scientific risk of the mission is the fact that JDEM is guaranteed to make advances in other areas of Beyond Einstein science, such as the evolution of black holes and matter content of the universe. These two factors, in the committeeâs view, make a strong case for a JDEM in spite of the risk posed by uncertain systematic effects. On purely scientific grounds, LISA is the mission that is most promising and least scientifically risky. Even with pessimistic assumptions about event rates, it should provide unambiguous and clean tests of the theory of general relativity in the strong-field dynamical regime and be able to make detailed maps of spacetime near black holes. Thus, the committee gave LISA its highest scientific ranking.