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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 NASA’s Beyond Einstein Program.1 Einstein’s theory of general relativity predicted the expansion of the universe from a big bang and the phenomenon of black holes. Einstein’s general relativity equation contains a term associated with a “cosmological constant” that may describe dark energy. Investigating the nature of these phenomena—going beyond Einstein—will take space missions that harness the ingenuity, creativity, and technical sophistication of current and future generations. The Beyond Einstein roadmap2 lays out specific research goals related to each of the three fundamental questions 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. 1 National Aeronautics and Space Administration, Beyond Einstein: From the Big Bang to Black Holes, Washington, D.C., January 2003, p. 5. 2 National Aeronautics and Space Administration, Beyond Einstein: From the Big Bang to Black Holes, Washington, D.C., January 2003. This document was part of NASA’s 2003 roadmapping effort required under the Government Performance Results Act of 1993 (Public Law No. 103-62).
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation The mission suite designed in the Beyond Einstein roadmap to carry out the program’s 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 Probes—the Cosmic Inflation Probe (CIP), the Black Hole Finder Probe (BHFP), and the Joint Dark Energy Mission (JDEM)—are 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. Advancement of Beyond Einstein research goals. The primary assessment criterion is how directly and unambiguously a mission candidate addresses the Beyond Einstein research goals. Broader science contributions. 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. Potential for revolutionary discovery. Will the mission candidate’s measurements truly alter current paradigms, or discover new and unexpected phenomena? Science risk and readiness.3 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)? Uniqueness of the mission candidate for addressing the scientific questions. 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 “How did black holes form and grow?” 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 3 This criterion is focused on scientific challenges inherent in the investigation, assuming that the technology challenges are or can be met. The technology challenges for each mission are addressed in Chapter 3.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation TABLE 2.1 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 “subtelescopes,” 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 somewhat 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 5σ sources) for the Low Energy Telescope (LET) and 56 arcsec for the High Energy Telescope (HET), while CASTER predicts position accuracies (for 10σ 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 Council’s (NRC’s) decadal survey report Astronomy and Astrophysics in the New Millennium, published in 2001.4 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. 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 4 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation TABLE 2.2 Black Hole Finder Probe: Mission Instrument Properties Instrument Spectral Range (keV) Angular Resolution (arcmin) Spectral Resolutiona(∆E/E) Collecting Areab(m2) Field of View (degrees) EXIST-HET 10-600 6.9 2% at 100 keV 1% at 511 keV 2.7 at 10 keV 2.7 at 100 keV 0.7 at 511 keV 65 × 154 EXIST-LET 3-30 1.4 5% at 10 keV 0.6 at 10 keV 64 × 160 CASTER-HEI 200-600 12 ~5% at 511 keV 1.4 at 511 keV 40 × 160 CASTER-LEI 10-200 7 ~35% at 10 keV ~10% at 100 keV 3.1 at 10 keV 3.1 at 100 keV 40 × 160 NOTE: See Appendix G in this report for definitions of acronyms. aThe spectral resolution for CASTER has not yet been optimized; values cited are intermediate between those for the current prototype and the best published results. bOnly 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.5 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 luminosities (Lx) ~1044 ergs s−1 out to redshifts (z) of 0.25 and (Lx) ~1046 ergs s−1 out to z ~2. Previous studies indicate that AGNs with x-ray luminosities ~1046 ergs s−1 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. 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 sensitivity—perhaps 10 times more sensitive than Swift. Thus, it will detect the formation of stellar-mass black holes throughout the universe by both core-collapse (“long” GRBs) and merging compact objects (likely associated with “short” GRBs). The BHFP will re-image large portions of the 5 C.B. Markwardt, J. Tueller, G.K. Skinner, N. Gehrels, S.D. Barthelmy, and R.F. Mushotzky, 2005, The Swift/BAT High-Latitude Survey: First results, Astrophys. J. 633:L77-L80.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation TABLE 2.3 Black Hole Finder Probe (BHFP): Beyond Einstein Science Programs Science Program Program Characteristics Program Significance Science definition programs All-sky hard x-ray survey Science question Perform a census of black holes throughout the universe The BHFP all-sky survey will detect tens of thousands of hard x-ray sources, determining the population distribution of massive black holes in external galaxies and their contribution to the x-ray background. The x-ray luminosities also will help determine how black holes evolve (see science question below) by providing a characterization of the accretion rates of massive black holes. In addition, the all-sky survey will detect and characterize the emission from several thousand stellar-mass black holes in our Galaxy, undoubtedly finding new rare objects. Measurements All-sky survey in a 10-600 keV (CASTER) or a 3-600 keV (EXIST) range Quantities determined X-ray flux at low and high energies; source localization of tens of arcsec; location and widths of strong x-ray lines Hard x-ray variability study Science question Determine how black holes evolve; observe stars and gas plunging into black holes The study of variability of extragalactic hard x-ray sources will be used to assess the accretion rates, and hence the rate of growth, of massive black holes. In addition, BHFP will detect rare events in which massive black holes shred and capture the matter from stellar-mass objects that approach too closely. Measurements Variability of hard x-ray sources Quantities determined Flux vs. energy for hard x-ray sources around the sky, on time scales from milliseconds to days Gamma-ray bursts (GRBs) Science question Determine how black holes are formed The formation rate of stellar-mass black holes over cosmic time, including their possible formation earlier than the first galaxies that scientists have detected to date, can be probed by detecting a significant population of GRBs at high redshift. These distant GRBs may herald the formation of stellar-mass black holes that provide seeds for the eventual evolution to the massive black holes seen at the centers of galaxies. Measurements Detection and characterization of GRBs Quantities determined Flux vs. time of over a thousand GRBs, with telemetry 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 “intermediate” masses between tens and thousands of solar masses,6 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. 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 “swallowing” of stars by massive black holes; several possible cases have been reported in the literature.7 Tidal disruptions of stars by massive black holes in relatively nearby galaxies will be detectable in approximately 10 galaxies per year. Such 6 M.C. Miller and D.P. Hamilton, 2004, Production of intermediate-mass black holes in globular clusters, Mon. Not. R. Astron. Soc. 330:232-240. 7 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 nonactive 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.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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.8 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 extragalactic 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;9 BHFP will have the angular resolution to study the spatial distribution of sources in the direction of the Milky Way bulge and will conduct sensitive searches for point components. BHFP’s 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 summarizes 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 4π instantaneous FOV for EXIST, full 4π 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 astrophysical 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 8 W.N. Brandt and G. Hasinger, 2005, Deep extragalactic x-ray surveys, Ann. Rev. Astron. Astrophys. 43:827-859, and references therein. 9 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. 10 L.M. Winter, R.F. Mushotzky, J. Tueller, C.S. Reynolds, and C. Markwardt, 2007, Early results from Swift’s BAT AGN Survey, Presentation Number 002.25, 210th Meeting of the American Astronomical Society, Bull. Am. Astron. Soc. 39.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation TABLE 2.4 Black Hole Finder Probe (BHFP): Broader Science Examples Program Program Characteristics Program Significance Galactic 511 keV emission Science question Origin of the 511 keV electron-positron annihilation line toward the center of the Milky Way The universe contains localized sources of antimatter; one set of such sources is indicated by the 511 keV electron-positron annihilation line detected toward the center of the Milky Way. Study of the distribution of the 511 keV sources may indicate whether energetic positrons are produced by extreme physics such as dark matter annihilation or injection by massive cosmic strings. Measurements 511 keV line flux vs. position and time in the galactic center direction Quantities determined Distribution of 511 keV sources toward the center of the Milky Way Galactic supernova rate Science question Rate of supernova explosions in the Milky Way Because we live inside the disk of the Milky Way Galaxy, dust extinction makes it difficult to determine the rate of stellar explosions in our Galaxy, which has an impact on theories of cosmic-ray acceleration and other basic astrophysics. BHFP will improve the assessment of the Milky Way supernova rate by measuring the dust-penetrating hard x-ray lines from supernova remnants. Measurements Detection of hard x-ray lines such as the 68 and 78 keV 44Ti lines expected from supernova remnants Quantities determined Line flux vs. location in the Milky Way; count of associated supernova remnants Serendipitous science Science question New types of hard x-ray sources revealed by a high-sensitivity survey BHFP will perform a hard x-ray survey that is more than an order of magnitude more sensitive than any done previously. This new discovery space may enable detection of completely new types of sources, such as extreme magnetars or highly variable ultraluminous x-ray sources. Measurements Hard x-rays and/or rapid variability not associated with known source classes Quantities determined Identification of new hard x-ray sources with previously unknown types of emitters the 300 gamma-ray sources detected by the Compton Gamma-Ray Observatory11 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 isolation—many 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 census—it 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 “dark ages” 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 11 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. 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-Hasselwander, A. Mücke, 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.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation gravitational radiation that would be detected by LISA and the Laser Interferometer Gravitational Wave Observatory (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, including GRBs, x-ray binaries, ultraluminous x-ray sources, magnetars, AGNs, and other potential sources. With the exceptions 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. “Starved” 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 situation 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 12 V. Bromm and A. Loeb, 2006, High-redshift gamma-ray bursts from Population III progenitors, Astrophys. J. 642:382-388. 13 F. Yuan, S. Markoff, and H. Falcke, 2002, A Jet-ADAF Model for Sgr A*, Astron. Astrophys. 383:854-863. 14 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, 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. 15 Examples of complementary observations or missions include the Constellation-X Hard X-ray Telescope, the NUSTAR Small Explorer mission, the upcoming Wide-field Infrared Survey Explorer MidEx mission, and the existing Very Large Array survey “Faint Images of the Radio Sky at Twenty cm.”
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 characterization. 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 achieving 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 considerably 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 “Mission Cost Assessments” 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 detector 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 kiloelectronvolts 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 galaxies 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
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation TABLE 2.5 Black Hole Finder Probe (BHFP): Summary of Scientific Evaluation Potential Contributions to Science Beyond Einstein Broader Science Factors Revolutionary discovery potential Massive black holes already are known in many galaxies. BHFP may find such black holes in different types of galaxies, where they might not follow the canonical relation between black hole mass and galaxy bulge characteristics. In addition, the possibility of detecting gamma-ray bursts at redshifts higher than 7 could provide insight on the stages of black hole formation in the early universe. Hard x-ray variability on time scales of milliseconds to days provides the potential for detecting entirely new types of x-ray emitters, such as extreme magnetars or highly variable ultraluminous x-ray sources. In addition, unexpected new classes of sources may be found to be major contributors to the hard x-ray background. Science readiness and risk Three major areas of science risk have been identified: (1) BHFP sensitivity is adequate to detect only the most luminous hard x-ray sources at high redshift, making it difficult to infer the evolution of black hole masses or x-ray emission over time; (2) the conversion from x-ray luminosity to black hole growth rate is uncertain by at least an order of magnitude, depending on unknown accretion rates and radiative efficiencies, making the assessment of black hole growth dependent on very poorly constrained models; and (3) the achievable position accuracy may be inadequate to identify the host objects for x-ray sources, particularly at high redshifts. The likelihood of finding unknown types of variable sources with a significant astrophysical impact is unknown. However, BHFP certainly will measure hard x-ray variability on a variety of time scales that are associated with the evolution of accretion disks and relativistic jets near massive black holes. Although individual supernova remnants will be identified through their hard x-ray spectral lines, these identifications may not translate into a strong constraint on the overall supernova rate in the Galaxy. Mission Uniqueness Versus other space missions BHFP would perform an all-sky hard x-ray survey a factor of 10 to 100 more sensitive than any previous satellite, detecting approximately 100 times more x-ray-emitting black holes than Swift or INTEGRAL. It will detect several times more gamma-ray bursts than seen by Swift. No other proposed U.S. or international missions will have comparable capabilities. No other hard x-ray surveys in the past or future have sensitivity and cadence comparable to those of the BHFP, so BHFP has a unique capability to find new types of variable x-ray sources. Further, no missions in prospect have the ability to detect and locate the sources of the 511 keV electron-positron annihilation line as well as the supernova remnant sources of lines in the ~100 keV range. Versus ground-based instruments Because of the opaqueness of the atmosphere, no ground-based instrument can perform hard x-ray observations. No hard x-ray observations are possible from the 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).
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 particularly 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 sounding 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 telescopes 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 primary 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 cm2 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 NRC’s Connecting Quarks with the Cosmos report. The decadal survey said that Con-X “will become the premier instrument for studying the formation and evolution of black holes of all sizes.”19 16 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, Photon counter measurements of solar x-rays and extreme ultraviolet light, Phys. Rev. 83:1025. 17 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. 9:439. 18 The Beyond Einstein Great Observatories are major, facility-class missions with broad applications to problems throughout astrophysics 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). 19 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001, p. 11.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 × 10−5 and 0.1 Hz, with sensitivity to proof-mass displacements at the level of tens of picometers, corresponding to a fractional displacement sensitivity of 10−20. It is worth pointing out that the raw displacement sensitivity required for LISA is a million 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 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. 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 Einstein’s 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 precision, 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 105 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 galaxy’s 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 “seed” 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. 59 F.J. Raab, representing the LIGO Scientific Collaboration and other laser interferometer groups, 2006, The status of laser interferometerThe status of laser interferometer gravitational-wave detectors, J. Phys. Conf. Ser. 39:25-31. 60 K. Danzmann, 1997, LISA—an ESA cornerstone mission for a gravitational-wave observatory, Class. Quantum Grav. 14:1399-1404. 61 R. Schödel, T. Ott, R. Genzel, A. Eckart, N. Mouawad, and T. Alexander, 2003, Stellar dynamics in the central arcsecond of our galaxy, 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. 62 J. Kormendy and D. Richstone, 1995, Inward bound: The search for supermassive black holes in galaxy nuclei. Ann.Rev.Astron.Astrophys. 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.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation TABLE 2.23 Laser Interferometer Space Antenna (LISA): Beyond Einstein Science Programs Science Program Program Characteristics Program Significance Science definition programs Formation of massive black holes Science question How and when do massive black holes form? Observations will detect massive black hole binary mergers to z = 15 and shed light on when massive black holes formed. Measurements Gravitational waveform shape as a function of time from massive black hole binary inspiral and merger Quantities determined Mass and spin of black holes as a function of distance Test general relativity in the strong-field regime Science question Does general relativity correctly describe gravity under extreme conditions? Measurement of the detailed gravitational waveform will test whether general relativity accurately describes gravity under the most extreme conditions. Measurements Gravitational waveform shape as a function of time from massive black hole binary inspiral and merger Quantities determined Evolution of dynamical spacetime geometry, mass and spin of initial and final holes History of galaxy and black hole co-evolution Science question How is black hole growth related to galaxy evolution? Observations will trace the evolution of massive black hole masses as a function of distance or time, and will shed light on how black hole growth and galactic evolution may be linked. Measurements Gravitational waveform shape as a function of time from massive black hole binary inspiral and merger Quantities determined Mass as a function of distance Additional Beyond Einstein science Map black hole spacetimes Science question Are black holes correctly described by general relativity? Observations will yield maps of the spacetime geometry surrounding massive black holes and will test whether they are described by the Kerr geometry predicted by general relativity. They will also measure the parameters (mass, spin, shape) of the holes, and test whether they obey the no-hair theorems of GR. Measurements Gravitational waveform shape from small bodies spiraling into massive black holes (EMRI) Quantities determined Mass, spin, multipole moments, spacetime geometry close to hole Cosmological backgrounds Science question Are there gravitational waves from the early universe? First-order phase transitions or cosmic strings in the early universe could leave a background of detectable waves. Measurements Stochastic background of gravitational waves Quantities determined Effective energy density of waves vs. frequency Cosmography, dark energy Science question What is the distance scale of the universe? If redshift of source or host galaxy can be determined, then precise, calibration-free measurements of the Hubble parameter and other cosmological parameters could be done, significantly constraining dark energy. Measurements Gravitational waveform shape and amplitude measurements yield luminosity distance of sources directly Quantities determined Luminosity distance NOTE: See Appendix G in this report for definitions of acronyms.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 Einstein’s 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 Einstein’s 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 “hang-up,” with significant consequences for the observed waveform.63 These are the consequences of “frame dragging,” a fundamental prediction of Einstein’s 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 “ringdown” 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 “black hole” spectroscopy on this discrete spectrum of ringdown waves, LISA will be able to test whether the geometry obeys the “no-hair” 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 massive (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 hole’s 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 63 M. Campanelli, C.O. Lousto, and Y. Zlochower, 2006, Spinning black-hole binaries: The orbital hang-up, Phys. Rev. D 74:041501. 64 E. Berti, A. Buonanno, and C.M. Will, 2005, Estimating spinning binary parameters and testing alternative theories of gravity with LISA, Phys. Rev. D 71:084025. 65 S.A. Hughes, 2006, (Sort of) testing relativity with extreme mass ratio inspirals, pp. 233-240 in Laser Interferometer Space Antenna: 6th International LISA Symposium (S.M. Merkowitz and J.C. Livas, eds.), AIP Conference Proceedings, Volume 873, American Institute of Physics, College Park, Md.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation TABLE 2.24 Laser Interferometer Space Antenna (LISA): Broader Science Examples Program Program Characteristics Program Significance Galactic compact binaries Science question What is the distribution of binary systems of white dwarfs and neutron stars in our Galaxy? Could provide a census of compact binary systems not achievable by electromagnetic means, and could survey the systems that are progenitors of high-frequency gravitational-wave sources detectable by ground-based interferometers. Population statistics could improve models of binary stellar evolution. Measurements Sinusoidal gravitational waveforms Quantities determined Orbital frequencies, sky distribution 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 difference in the speeds of gravitational waves and of light with a precision of parts in 1017 and test whether or not the “graviton,” 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 Einstein’s 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 system’s parameters, while the wave amplitude depends on those same parameters and on the luminosity distance to the source. No complex calibrations 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 LISA’s 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). 66 C.M. Will, 1998, Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries, Phys. Rev. D 57:2061-2068. 67 B.F. Schutz, 1986, Determining the Hubble constant from gravitational wave observations, Nature 323:310-311.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 (10−11 seconds) and a nanosecond after the big bang, a period not directly accessible by any other technique. Other potential exotic sources include intersecting cosmic string loops or vibrations and collapses of “boson stars,” 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. Although 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.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation Science Readiness and Risk LISA’s 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 interesting sources involve extreme gravity and relativistic speeds, it is important to ask whether techniques for solving Einstein’s 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 “numerical relativity” have been critical in providing solutions that link the inspiral signal, which is determined using analytical 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 “backreaction” of the small body’s 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 LISA’s high sensitivity, it is expected that many sources will have their signals superimposed simultaneously on the data stream. Recently, a program of LISA “mock data challenges” has shown substantial promise in demonstrating the ability to extract multiple signals, ranging from inspiral “chirps” 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 synchronicity between the schedules of the two agencies. In late 2009, ESA plans to select two candidate missions for an “L-1” 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 ESA’s Definition Phase (roughly equivalent to NASA’s 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 68 F. Pretorius, 2005, Evolution of binary black hole spacetimes, Phys. Rev. Lett. 95:121101; J.G. Baker, J. Centrella, D. Choi, M. Koppitz, 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. 69 K.A. Arnaud et al., 2006, The mock LISA data challenges:An overview, pp. 619-624 inThe mock LISA data challenges: An overview, pp. 619-624 in Laser Interferometer Space Antenna: 6th International LISA Symposium (S.M. Merkowitz and J.C. Livas, eds.), AIP Conference Proceedings, Volume 873, American Institute of Physics, College Park, Md.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation TABLE 2.25 Laser Interferometer Space Antenna (LISA): Summary of Scientific Evaluation Potential Contributions to Science Beyond Einstein Broader Science Factors Revolutionary discovery potential LISA will open a unique new window on the universe, will test general relativity in the most extreme regimes, will study the formation and evolution of massive black holes, and will measure absolute distances on cosmological scales. Detection of gravitational waves is assured. LISA could detect waves from exotic or unexpected sources, such as cosmic strings or early universe phase transitions. Science readiness and risk Understanding of the underlying theory and data analysis is robust. The main risk is the uncertainty in rates of mergers involving massive black holes. Low risk: detection of many galactic binaries is assured. Mission Uniqueness No similar or competing missions are envisioned. No similar or competing missions are 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 ESA’s 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 Einstein’s 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 committee’s assessment of the contribution that the candidate missions would make to the Beyond Einstein science questions. The summary captures the strengths, scientific uncertainties, readiness, 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-
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 1016 GeV, far beyond the capabilities of ground-based accelerators. The second approach measures inflation’s 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 produced during this era; however, most theories predict a signal that is beyond LISA’s 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 estimated 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 technology, 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 Einstein’s theory of gravity. LISA will use its high-signal-to-noise detector to test Einstein’s 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 LISA’s 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. 70 F. Pretorius, 2005, Evolution of binary black hole spacetimes, Phys. Rev. Lett. 95:121101; J.G. Baker, J. Centrella, D. Choi, M. Koppitz, 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.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 hole’s 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 LISA’s 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 understand 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 SXT’s 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 measurements 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
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 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 cosmological 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 exploration 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. ConX 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 luminosity 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 different 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-X’s spectroscopic capability. Improving measurements of the amount of dark and baryonic matter in the universe is also essential to understanding 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
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation how galaxies and clusters acquired their mass through both weak lensing and optical photometric surveys. Alternatively, 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 committee’s primary science-evaluation criterion was how directly and unambiguously the different missions would answer one or more of the three questions put forward in NASA’s 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 NASA’s 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 committee’s judgment that for a focused program like Beyond 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, foregrounds, 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 currently 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 overall 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 committee’s 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.