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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011)

Chapter: 8 Report of the Panel on Particle Astrophysics and Gravitation

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Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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8
Report of the Panel on Particle Astrophysics and Gravitation

SUMMARY

The fertile scientific ground at the intersection of astrophysics, gravitation, and particle physics addresses some of the most fundamental questions in the physical sciences. For example, the unexplained acceleration of the expanding universe leads scientists to question their understanding of cosmology. There may be an as-yet-uncharacterized component of the mass-energy that drives the dynamics of the universe—a cosmological constant or a new type of field—called “dark energy.” Or, gravity may be described not by Einstein’s general theory of relativity but rather by a different theory altogether. Solving these puzzles will require new astrophysical observations.

Another unsolved mystery is the origin of the initial conditions at the beginning of the universe, the first density fluctuations that grew into the structures seen today. There is evidence that these initial conditions were set down during the period of inflation in the very early universe. That leaves open the question, What caused inflation? Again, gravity may provide the clue. Measurements of the stochastic background of gravitational waves that formed at the same time as the initial density perturbations provide an important tool that might probe the inflationary period. Connecting physics and astronomy, the initial density perturbations set the stage for structure formation: How and when did the first structures form in the universe? Observations of gravitational waves from black hole mergers at high redshift will provide unique information about this era, complementing other probes.

Another puzzle is that of the laws of nature in the environments that harbor

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

the most extreme gravitational fields. Supermassive black holes inhabit the centers of galaxies, and they somehow—following the laws of gravity—generate tremendous outflows of energetic particles and radiation, twisting magnetic fields into concentrated pockets of magnetism. Scientists cannot help but strive to understand these extreme environments and to take advantage of them as laboratories to put theories of gravitation to their most demanding tests.

Gravitation is a unifying theme in nearly all of today’s most pressing astrophysics issues. Much of the precursor work of the past decade was motivated by the scientific imperative of understanding gravitation, and an intense period of technology development to build the necessary tools is reaching fruition and must now be exploited. Scientists now have ground-based laser interferometric detectors that are on a path to reaching the level of sensitivity at which the detection of gravitational waves is virtually assured. They have a plan and a design for a network of spacecraft that will measure long-wavelength gravitational waves where astrophysical sources are predicted to be the most abundant. They have developed high-precision techniques for pulsar observations that are promising probes of the gravitational waves associated with inflation and with supermassive black holes. Recognizing these developments, the Panel on Particle Astrophysics and Gravitation presents a program of gravitational-wave astrophysics that will bring the investments in technology to fruition. The panel recommends that the Laser Interferometer Space Antenna (LISA) be given a new start immediately; that ground-based-laser gravitational-wave detectors continue their ongoing program of operation, upgrade, and further operation; and that the detection of gravitational waves through the timing of milli second pulsars move forward. To complement the use of gravitational waves as a beacon for astrophysics and fundamental physics, the panel recommends that the theoretical foundations of gravity themselves be put to stringent test, when such tests can be carried out in a cost-effective manner. These tests of gravitation will be provided by LISA’s observations of strong-field astrophysical systems, by electro magnetic surveys to characterize dark energy (considered by other Astro2010 Program Prioritization Panels), by precise monitoring of the dynamics of the Earth-Moon system, and by controlled tests of gravity theories done in the nearly noise-free environment of space. The time has arrived to explore the still-unknown regions of the universe with the new tool of gravitation.

Understanding the nature of three-quarters of the universe is an important goal, but the other one-quarter, which is known to be some form of matter, must not be overlooked. Scientists have identified one-sixth of this matter: it is in the form of stars, galaxies, and gas that have been studied extensively for centuries. However, the nature of the other five-sixths is still a mystery. Evidence exists that the unknown part is not made up of familiar materials but rather must be a diffuse substance that interacts only weakly with ordinary matter. The leading candidates for this so-called dark matter are new families of particles predicted by some theo-

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

ries of fundamental particle physics. There are three complementary approaches to attacking the dark-matter problem: direct detection in the laboratory, indirect detection by way of astronomical observations, and searches for candidate particles in human-made high-energy particle accelerators. The panel’s recommendations concern only indirect detection by astrophysics, although all three approaches will be important in ultimately resolving this mystery.

The indirect detection of dark matter involves searching not for the darkmatter particles themselves, but rather for products of the annihilation or decay of dark-matter particles. These may be gamma rays, cosmic rays, or neutrinos. The sources will be places in the cosmos where scientists believe that dark matter concentrates, such as in the gravitational potential wells of galaxies. Therefore, the panel recommends a program of gamma-ray and particle searches for dark matter.

The field of high-energy and very-high-energy particle astrophysics has blossomed in the past decade, with a wealth of results from spaceborne and ground-based gamma-ray telescopes and cosmic-ray detectors, and it is hoped that similar exciting results will come soon from neutrino telescopes. These instruments provide unique views of astronomical sources, exploring the extreme environments that give rise to particle acceleration near, for example, supermassive black holes and compact binary systems. The panel recommends continued involvement in high-energy particle astrophysics, with particular investment in new gamma-ray telescopes that will provide a much deeper and clearer view of the high-energy universe, as well as a better understanding of the astrophysical environment necessary to disentangle the dark-matter signatures from natural backgrounds. The panel’s highest-priority recommendation for ground-based instrumentation is significant U.S. involvement in a large international telescope array that will exploit the expertise gained in the past decade in atmospheric Čerenkov detection of gamma rays. Such a telescope array is expected to be an order-of-magnitude more sensitive than existing telescopes, and it would for the first time have the sensitivity to detect, in other galaxies, dark-matter features predicted by plausible models.

The panel also recommends a broad program for particle detectors to be flown above the atmosphere, making use of the cost-effective platforms provided by balloons and small satellites. Major developments in large ground-based detectors for neutrinos are in progress already. These programs are an important component of dark-matter and astrophysical particle characterization and should be continued, along with the research and development that will improve the sensitivity of neutrino detectors in decades to come.

The above recommendations are possible only because there is now available a suite of new instruments that have recently achieved technical readiness. In the program areas that the panel considered, a significant component of the technology development has been done outside the United States. To maintain the nation’s ability to participate in research in astrophysics in the future, the panel recommends

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

that the technology-development programs of all three funding agencies relevant to particle astrophysics and gravitation be augmented. To enable missions to test theories of gravitation and to carry out timely and cost-effective experiments in particle astrophysics, gravitation, and other areas of astrophysics, the panel recommends an augmentation of NASA’s Explorer program. It is expected that such missions will compete in a forum of peer review. To enable particle-detection experiments, the panel recommends an augmentation of NASA’s balloon program to support ultralong-duration ballooning.

Finally, on an even more fundamental level, the panel recognizes that the ultimate goal of all these activities is the advancement of knowledge, for the achievement of which the culminating activities are the interpretation and dissemination of results, and which in turn lead to new frameworks for subsequent exploration. Therefore, the panel supports a strong base program in all areas of astronomy and astrophysics. This base program must include theory as one of its components.

The program in particle astrophysics and gravitation that this panel recommends includes missions, projects, and activities that will result in new tools for attacking many of the outstanding problems of astronomy and astrophysics, both in this decade and in the future. The recommended program will launch the new discipline of gravitational-wave astrophysics. It will develop new detectors for cosmic rays, gamma rays, and neutrinos that—working in tandem with gravitational-wave and longer-wavelength electromagnetic detectors—will enable multi-messenger astrophysics. It will confront theories of gravitation with new data, in the context of understanding the strong fields around black holes and the nature of dark energy on cosmological scales. It will seek to identify the elusive dark matter. It will elucidate the remarkable dynamics of black holes and their fields and outflows. All in the astronomy and astrophysics community look forward to the discoveries of the next decade.

THE SCIENCE CASE

The scope of this panel’s deliberations is defined by those areas of astronomy and astrophysics that use experimental and observational techniques at the intersection of astronomy and physics. In particular, the panel addresses the windows on the universe offered by high-energy particles (including gamma rays) and gravitational waves. Each provides a radically new and, especially in the cases of gravitational waves and high-energy neutrinos, so-far-unexplored window on astrophysical objects and processes. The panel also considers tests of general relativity and other theories of gravity. The scientific questions explored address regimes of fundamental physics not accessible to laboratory experiment, and they span the largest cosmological scales. They also probe the highest energies accessible to science, and they probe the smallest scales that may characterize a unified theory

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

of the basic interactions. The answers to these questions reveal clues to the most fundamental origins—of space and time, of matter and energy, of the universe itself. This quest will stretch our best tools and most cherished ideas about physics; it will require robust investment in both theory and observational infrastructure in the coming decade; and it will allow us literally to see the universe in new ways.

Gravitational-Wave Astrophysics

One of the remarkable predictions of Einstein’s theory of general relativity—and, more generally, any theory that describes non-instantaneous gravitational forces—is the existence of gravitational waves. The direct observation of gravitational waves will provide much more than just another confirmation of Einstein’s theory: it will open the study of the astrophysical sources in an entirely new way (Figure 8.1). When gravitational-wave astrophysics progresses to the point of

FIGURE 8.1 Gravitational waves computed from a numerical simulation of the merger of two black holes. The orange and red contours indicate the amplitude of the waves; the peak amplitude is reached around the time of the merger, when a single black hole is formed. This figure shows the configuration just after the black holes have merged. SOURCE: Courtesy of C. Henze, NASA.

FIGURE 8.1 Gravitational waves computed from a numerical simulation of the merger of two black holes. The orange and red contours indicate the amplitude of the waves; the peak amplitude is reached around the time of the merger, when a single black hole is formed. This figure shows the configuration just after the black holes have merged. SOURCE: Courtesy of C. Henze, NASA.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

detecting sources at a significant rate—a milestone expected to be reached in the next decade—it will create a vision of the universe as different and revolutionary as those that radio, X-ray, and gamma-ray telescopes provided in past decades.

Gravitational wave astrophysics specifically addresses many of the key science questions laid out by the Astro2010 Science Frontiers Panels (SFPs). Direct detection of gravitational waves was called out as an area of great discovery potential in astrophysics that would initiate a new era of gravitational wave astrophysics. Gravitational wave astrophysics will also inform two other SFP discovery areas: time-domain astronomy, by elucidating the process of gravitational collapse; and the epoch of reionization, by uniquely recording black hole mergers at high redshift. See Box 8.1.

Gravitational waves are a distinctive cosmic messenger. They carry information about the universe that cannot be obtained from electromagnetic radiation or particle detectors. They are generated by the motions of massive objects, rather than by the glow of plasma, and thus give information about the dynamics of black holes and other massive compact objects. The possibility of obtaining new information about cosmic sources, and of combining information from the different windows (electromagnetic, particle, gravitational), can advance astronomy into a new era (see Box 8.2).

Currently, indirect measurements of gravitational radiation as seen in the orbital dynamics of pulsars are available: the Hulse-Taylor pulsar B1913+16 and the double pulsar J0730-3039B, both binary systems in our galaxy, have provided exquisite tests of the theory of general relativity, including the energy emitted in gravitational waves shrinking their orbits. Although the weak coupling of gravitational waves makes direct detection very difficult, high-precision instruments have been under development for more than a decade in the quest to capture signals from these elusive waves. Today, the prognosis for direct detection is excellent.

BOX 8.1

Science from Gravitation and Gravitational Waves

Science Frontiers Panel (SFP) Discovery Areas

Relevant SFP Panel

Gravitational-wave astronomy

Panel on Cosmology and Fundamental Physics (CFP)

Time-domain astronomy

Panel on Galactic Neighborhood (GAN)

Panel on Stars and Stellar Evolution (SSE)

The epoch of reionization

Panel on Galaxies Across Cosmic Time (GCT)

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

BOX 8.2

Science from Gravitational Waves

SFP Questions Addressed

Measurements Addressing the Questions

GCT 1

How do cosmic structures form and evolve?

Tracing galaxy-merger events by detecting and recording the gravitational-wave signatures

GCT 3

How do black holes grow, radiate, and influence their surroundings?

Using gravitational-wave inspiral waveforms to map the gravitational fields of black holes

GCT 4

What were the first objects to light up the universe, and when did they do it?

Identifying the first generation of star formation through gravitational waves from core-collapse events

SSE 2

What are the progenitors of Type Ia supernovae and how do they explode?

Detecting and recording the gravitational wave signatures of massive-star supernovae, of the spindown of binary systems of compact objects, and of the spins of neutron stars

SSE 3

How do the lives of massive stars end?

SSE 4

What controls the mass, radius, and spin of compact stellar remnants?

CFP 1

How did the universe begin?

Detecting and studying very-low-frequency gravitational waves that originate during the inflationary era

CFP 2

Why is the universe accelerating?

 

 

Testing of general relativity—a deviation from general relativity could masquerade as an apparent acceleration—by studying strong-field gravity using gravitational waves in black hole systems, and by conducting space-based experiments that directly test general relativity

The universe is bright in gravitational waves, because many highly energetic sources exist. For example, the Hulse-Taylor binary pulsar emits the same power in gravitational waves as the Sun does in electromagnetic waves. Even more remarkably, the gravitational-wave luminosity during the final coalescence of a black hole binary is about 1023 solar luminosities; for a stellar black hole binary (10 solar masses) this luminosity lasts for about 5 milliseconds, whereas for a massive black hole binary (about 106 solar masses) it lasts about 10 minutes. This is far more energy than is emitted electromagnetically by a gamma-ray burst and in fact exceeds the total energy emitted as electromagnetic waves by all the stars in the observable universe during these same time periods! However, the physical effect of gravitational waves in stretching space-time or accelerating other masses is minuscule: the universe is almost transparent to gravitational waves. Hence, by detecting gravitational waves we can hope to see to black holes through the obscuring matter

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

surrounding them and possibly even to the earliest instants in the history of our universe through the plasma that obscures this epoch for electromagnetic radiation.

The gravitational-wave spectrum extends from the lowest frequencies (about 10−16 Hz for the waves that alter the polarization of the cosmic microwave background) to very high frequencies (above 1 kHz for oscillations of stellar black holes). In terms of wave periods, these correspond to a range from a few percent of the Hubble time to fractions of milliseconds. This vast span of physical scales encompasses an impressive variety of astrophysical sources, including massive black hole binaries, quantum fluctuations in the early universe, neutron-star-binary mergers, the final spiral of a stellar black hole into a massive black hole in the center of a galaxy, and stellar explosions.

According to most cosmological models, gravitational waves were produced in the early universe through amplification of the vacuum fluctuations that occurred during inflation or through phase transitions of cosmological fields. These waves are accessible at the ultralow- and very-low-frequency end of the spectrum—10−16 to 10−8 Hz (Figure 8.2). Detection of these waves would identify the energy of symmetry breaking in the inflationary phase transition. It would provide crucial information to help us understand the physics that drove inflation and that set the stage for the subsequent evolution of the universe and its contents.

Pulsar surveys are in the process of identifying a sample of short-period pulsars that can be timed with 100-nanosecond precision. In the next decade, these pulsars—observed from the ground with radio telescopes—will emerge as a tool for detecting gravitational waves in the range of 1010 to 10−8 Hz, a band that contains signals from supermassive black hole binaries as well as signals from the early universe. Observing an array of these pulsars, each with 100-ns root mean square residual timing noise in each observation, is expected to yield a confusion-limited gravitational-wave background signal from binaries with masses . On top of this confusion-limited signal should be a handful of individually resolvable higher-mass binaries at distances out to z ~ 2. In addition, the pulsar-timing array can detect possible contributions to the stochastic background from cosmic strings, superstrings, and inflation. All of the contributions to the stochastic background, including that from massive black holes, have different spectral indices and so might be separately identified. In addition, pulsar-timing observations may detect violations of general relativity resulting from quantum-gravity corrections that accumulate over long distances, or from additional polarization states not predicted by Einstein’s theory.

Extending somewhat higher in frequency, the low-frequency band from 10−5 to 10−1 Hz can be only probed by instruments in space, due to the obscuration by seismic noise in Earth’s crust and by gravity-gradient noise in ground-based arrays. This part of the gravitational-wave spectrum is exceptionally rich in astrophysical sources, including mergers of massive and seed black hole binaries, of inspirals

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 8.2 Strain amplitude sensitivity expected for pulsar timing (red), LISA (green), and Advanced LIGO (blue). The continuous curves show strain-noise-amplitude spectral density. The pulsar-timing sensitivity assumes the use of 20 pulsars with 100-ns timing residuals. The dashed magenta curves show the instantaneous strain of gravitational waves emitted by binary black hole (BBH) systems 1 Gpc away, evolving in frequency to the final coalescence frequency. In the LISA band, the figure shows an estimate of the unresolved background from galactic white-dwarf binary systems in shaded light green; the amplitude of some of the known binary systems; and a representative amplitude of the coalescence of an extreme-mass-ratio inspiral (EMRI) system. In the pulsar-timing band, the expected background is shown from relic gravitational waves (GWs) and from the unresolved signals of supermassive binary black hole systems.

FIGURE 8.2 Strain amplitude sensitivity expected for pulsar timing (red), LISA (green), and Advanced LIGO (blue). The continuous curves show strain-noise-amplitude spectral density. The pulsar-timing sensitivity assumes the use of 20 pulsars with 100-ns timing residuals. The dashed magenta curves show the instantaneous strain of gravitational waves emitted by binary black hole (BBH) systems 1 Gpc away, evolving in frequency to the final coalescence frequency. In the LISA band, the figure shows an estimate of the unresolved background from galactic white-dwarf binary systems in shaded light green; the amplitude of some of the known binary systems; and a representative amplitude of the coalescence of an extreme-mass-ratio inspiral (EMRI) system. In the pulsar-timing band, the expected background is shown from relic gravitational waves (GWs) and from the unresolved signals of supermassive binary black hole systems.

of compact objects into massive central black holes, and of compact binary stars within the galaxy.

The Laser Interferometry Space Antenna (LISA) has the goal of placing a cluster of three spacecraft in solar orbit to detect gravitational waves in this low-frequency band (Figure 8.3). Precise measurements of the relative positions of the spacecraft will detect gravitational waves from this extremely rich collection of astrophysical sources, thereby providing unprecedented scientific opportunities as the result of

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 8.3 Schematic of the orbits of the three LISA spacecraft around the Sun. SOURCE: Courtesy of NASA/JPL.

FIGURE 8.3 Schematic of the orbits of the three LISA spacecraft around the Sun. SOURCE: Courtesy of NASA/JPL.

an entirely new technique. Table 8.1 summarizes key astrophysics sources for the LISA mission described below.

  • Mergers of massive black holes—black holes at the centers of galaxies with masses of 10,000 to 10 million times the Sun’s mass—provide the most promising sources. LISA’s sensitive frequency band captures the late-inspiral phases of these systems, lasting weeks to years, with very high signal-to-noise ratios. For many of these sources, it is likely that there will be predictions of the time and sky location of the future merger, providing opportunities for simultaneous electromagnetic observations. The detection of the merger waveforms will provide unprecedented tests of general relativity in the regime of very strong and highly dynamical fields: such a measurement is qualitatively different from all the current tests of the theory. The expected detection rates are about 1 per year for close systems with small

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

redshift (less than 2), and up to 30 per year for systems with higher redshifts (up to 15). The study of these sources will provide detailed information on the spins and masses of the massive black hole population at different redshifts, as well as the history of the growth of black holes and the mergers of galaxies.

  • Capture of stellar-mass compact objects by massive black holes. “Small” compact objects near galactic centers (black holes, neutron stars, and white dwarfs) will inspiral and be captured by the black holes at the centers of the galaxies. The orbital periods will be short (hundreds of seconds), but the signals may last for years. The rate of capture of the small (less than 10 solar masses) objects will be observed up to small redshifts but at a high rate (about 50 per year); intermediate-mass black holes can be detected up to a redshift of 10. Again, the signals with large signal-to-noise ratios will provide very precise tests of general relativity. The measured properties of the black holes captured will provide much-sought-after information about abundances, masses, and spins—contributing to understanding of the history of black hole growth.

  • Close binaries of stellar-mass compact objects in our galaxy. Tens of thousands of white dwarf binary systems in our galaxy have orbital periods of hundreds to thousands of seconds and produce gravitational waves in LISA’s band. Some of the known binaries are close enough to be used as verification systems. Most of the systems will produce signals forming a diffuse foreground, sometimes called confusion noise. The resolved systems will provide a 100-fold increase in the number of known binaries, as well as information on their evolutionary pathways. It is likely that conclusions will be possible regarding white-dwarf binaries as possible progenitors of Type Ia supernovae, and about the physics of tidal interactions and mass transfer before merger.

Finally, LISA may detect signals from cosmological backgrounds (for example, from an early-universe phase transition), bursts from cusps on cosmic (super-) strings, and unforeseen sources.

The LISA mission will release its science data products to enable use and analysis by the astronomy and astrophysics community. In addition, the mission will make available the algorithms, the software, and the models (including a physical model of the gravitational reference sensor) used for processing the data, and will ensure that the data-processing history for any data published by the LISA Science Data Center is traceable and retrievable. With its rich and abundant sources, LISA will usher in a new era in gravitational-wave and multi-messenger astronomy. Therefore, the case is excellent for giving LISA the highest priority for a new start in the next decade.

At the high-frequency end of the spectrum, compact stars of one to hundreds of solar masses (neutron stars and small black holes) near coalescence will produce gravitational waves with frequencies of a few hertz to a few kilohertz, with “chirp”

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

TABLE 8.1 Key Astrophysics Sources for LISA

Source Type and Details

Massive black hole (MBH) mergers

Characteristics

Mergers of binaries involving 2 MBHs, with masses in the range of 104 to , orbital periods of 102 to 105 s, signal durations of ~ weeks to years, amplitude signal-to-noise ratios up to several thousand

Detection rate

~1/yr at z < 2, ~30/yr out to z ~ 15

Observables

Masses, ∆M/M < 1%; spins, ∆S/S < 2% (typical detections); luminosity distances, ∆DL/DL ~ (5-20)% (typical detections), ∆DL/DL < 3% (at z = 1, limited by weak lensing)

Science payoffs

Nature of black hole seeds at high z; history of MBH growth and galaxy mergers as function of z; tests of general relativity in strong-field, highly dynamical regime

Capture of stellar-mass compact objects by MBHs

Characteristics

Compact object (black hole, neutron star, white dwarf) spirals into MBH; MBH mass of 104 to ; orbital period of 102 to 103 s; signal duration ~ years.

Detection rate

~50/yr, mostly captures of black holes at z ~ 1; captured intermediate-mass black holes detected to z > 10

Observables

Masses, ∆M/M < 0.1%; spins, ∆S/S < 0.1% (typical detections); luminosity distances, ∆DL/DL < 4% (typical detections)

Science payoffs

Measure MBH spins, which reflect their growth history; populations and dynamics of compact-object populations in galactic nuclei; precision tests of general relativity and Kerr nature of black holes

Close binaries of stellar-mass compact objects in the galaxy

Characteristics

Close binary systems of black holes, neutrino stars, and white dwarfs in Milky Way; primarily white-dwarf/white-dwarf binaries, mass-transferring or detatched; orbital periods of 102 to 104 s

Detections

~20,000 individual sources, including ~10 known “verification binaries”; diffuse galactic foreground at frequencies below ~2 mHz

Observables

Orbital frequency; sky location to approximately a few degrees; chirp mass and distance from df/dt for some high-f binaries

Science payoffs

~100-fold increase in census of short-period galactic binaries; white dwarf-white dwarf binaries as possible supernova Ia progenitors; evolutionary pathways (e.g., outcomes of common-envelope evolution); physics of tidal interactions and mass transfer

signals that span the spectrum up to a coalescence frequency inversely proportional to the total mass of the system. Pulsars—and spinning stars in general, if not perfectly axisymmetric—radiate gravitational waves at twice their spin frequency. This band also includes mergers of stellar and intermediate-mass black holes, as well as binary neutron stars and supernovae. All these sources can be probed by ground-based interferometric detectors.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
Tests of General Relativity and Other Theories of Gravity

General relativity is important both on the large distance scales of astronomy and cosmology and on the small scales that may characterize a unified theory of the basic interactions. Relativistic gravity is therefore a two-way bridge between astronomy and fundamental physics. The SFP science questions How did the universe begin? and Why is the universe accelerating? require an accurate understanding of relativistic gravity, as does the discovery area of the SFP on cosmology and fundamental physics—gravitational wave astronomy.

Table 8.2, which summarizes the current status of tests of general relativity, organized by length scale and by whether the tests probe weak or strong gravitational fields, shows clearly that general relativity has been well tested on solar system scales in the weak-field regime. In contrast, for stronger fields and larger scales, there are mostly qualitative tests. In cosmology, for instance, general relativity has been assumed, and data have been used to determine the cosmological parameters that characterize our universe. Now that these are known to a good accuracy, it is reasonable to ask whether it is possible to begin to test general relativity on cosmological scales.

There is no compelling classical alternative to general relativity that has survived the solar system tests. Theorists generally agree that deviations from classical

TABLE 8.2 Current Limits on Deviations from General Relativity

Distance Scale

Weak Gravity

Strong Gravity

Laboratory

Weak equivalence principle, 1013

Limits on fifth force and compact extra dimension size, 56 μ

Gravitational redshift, 10−8

 

Solar system

Weak equivalence principle, 10−13

Strong equivalence principle, 10−4

Gravitational redshift, 10−4

Bending of light, 10−4

Shapiro time delay, 10−5

Precession of perhihelion, 10−3

Lense-Thirring precession, 5 to 15%

Gravitational radiation from binary pulsars, 10−3

Black holes

Galactic

Lensing bending of light, 10%

Black holes

Cosmological

 

Observations fit to values of the Hubble constant and the densities of matter, radiation, and dark energy

NOTE: The numbers in this table are order-of-magnitude values for the current accuracies with which the measured effects currently agree with general relativity. For instance, for most solar-system tests, the accuracy to which the parametrized post-Newtonian parameters have the general-relativistic values is listed. Italics are used to indicate tests that are only qualitative at this time.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

general relativity can be expected at the Planck scales that characterize quantum gravity (10–33 cm, 1019 GeV), but these scales are very far from those that can be probed directly with today’s experiments. Rather, the question is whether physics at the Planck scale leaves imprints at low energies that can be tested by our observations and experiments (Figure 8.4).

Contemporary ideas of fundamental theory allow the construction of many different four-dimensional theories of gravitation which might govern the results determined by observation and experiment. Some of these theories predict deviations from general relativity, but while many models produce deviations from general relativity that might be tested by experiment, there are no secure predictions to provide targets for tests.

The science case for tests of general relativity rests generally on the importance of the theory for astronomy, fundamental physics, and the connection between them. The following aspects of the current experimental and theoretical situation strongly motivate tests of the theory in the next decade (Figure 8.5):

FIGURE 8.4 The 4-cm-high torsion pendulum at the University of Washington is at the heart of probably the most accurate test of principle in all of physics—the equivalence principle, which states that all masses fall with the same acceleration in a gravitational field regardless of their composition. This principle is central to general relativity, and any deviation would entail a significant revision of the ideas about gravitation. The pendulum is suspended from a rotating platform by a fiber barely visible at the top of the original figure. Any difference in the accelerations of the test masses would appear as a twisting of the pendulum. The experiment confirms the equivalence principle for beryllium and titanium masses in the gravitational field of Earth to an accuracy of less than 1 part in 1013 (S. Schlamminger, K.-Y. Choi, T.A. Wagner, J.H. Gundlach, and E.G. Adelberger, Test of the equivalence principle using a rotating torsion balance, Physical Review Letters 100:041101, 2008). Similar experiments provide accurate tests of the gravitational inverse square law and Lorentz invariance. SOURCE: Courtesy of E. Adelberger, University of Washington.

FIGURE 8.4 The 4-cm-high torsion pendulum at the University of Washington is at the heart of probably the most accurate test of principle in all of physics—the equivalence principle, which states that all masses fall with the same acceleration in a gravitational field regardless of their composition. This principle is central to general relativity, and any deviation would entail a significant revision of the ideas about gravitation. The pendulum is suspended from a rotating platform by a fiber barely visible at the top of the original figure. Any difference in the accelerations of the test masses would appear as a twisting of the pendulum. The experiment confirms the equivalence principle for beryllium and titanium masses in the gravitational field of Earth to an accuracy of less than 1 part in 1013 (S. Schlamminger, K.-Y. Choi, T.A. Wagner, J.H. Gundlach, and E.G. Adelberger, Test of the equivalence principle using a rotating torsion balance, Physical Review Letters 100:041101, 2008). Similar experiments provide accurate tests of the gravitational inverse square law and Lorentz invariance. SOURCE: Courtesy of E. Adelberger, University of Washington.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 8.5 Waveform for one polarization of gravitational waves produced by a test mass orbiting a million-solar-mass black hole that is spinning at 90 percent of the maximum rate allowed by general relativity. The two panels correspond to different configurations of the test-mass orbit. The top panel assumes a slightly eccentric and inclined retrograde orbit moderately far from the horizon. The bottom panel assumes a highly eccentric and prograde orbit much closer to the horizon. The amplitude modulation visible in the top panel is due mostly to Lense-Thirring precession of the orbital plane. The bottom panel’s more eccentric orbit produces sharp spikes at each pericenter passage. SOURCE: Steve Drasco, Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Potsdam, and Scott A. Hughes, Department of Physics and Kavli Institute for Astrophysics and Space Research, MIT.

FIGURE 8.5 Waveform for one polarization of gravitational waves produced by a test mass orbiting a million-solar-mass black hole that is spinning at 90 percent of the maximum rate allowed by general relativity. The two panels correspond to different configurations of the test-mass orbit. The top panel assumes a slightly eccentric and inclined retrograde orbit moderately far from the horizon. The bottom panel assumes a highly eccentric and prograde orbit much closer to the horizon. The amplitude modulation visible in the top panel is due mostly to Lense-Thirring precession of the orbital plane. The bottom panel’s more eccentric orbit produces sharp spikes at each pericenter passage. SOURCE: Steve Drasco, Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Potsdam, and Scott A. Hughes, Department of Physics and Kavli Institute for Astrophysics and Space Research, MIT.

  • Much remains untested at large scales and in strong fields. Detailed astronomical observations in the next decade will probe the regimes of strong field and large scale in which general relativity is quantitatively untested. This also provides strong motivation to complete the missing parts of Table 8.2 at the same time. The LISA mission will, for the first time, probe these gaps in current knowledge by characterizing gravitational waves across cosmological distances from sources governed by strong-field gravity.

  • Much remains unknown about general relativity. Is there a non-zero cosmological-constant term in the Einstein equation, or is the observed acceleration of the universe due to a new type of field? Electromagnetic surveys that measure cosmological distances as a function of redshift, and which map the growth of structure as a function of redshift, will test the hypothesis of a cosmological constant. Are the massive central objects in galaxies indeed black holes, described by the Kerr metric of general relativity? Is the “cosmic-censorship” conjecture true, so that black holes rather than naked singularities form in sufficiently advanced gravitational collapse? LISA will observe the inspirals of compact stellar remnants

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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spiraling for years into the massive dark objects at the centers of galaxies, tracking the last 100,000 cycles of gravitational radiation emitted in the strong-field region and producing a high-precision map of the space-time that will reveal any small deviations from the Kerr metric (see Figure 8.5). If the central massive object is not a black hole, but rather an object with no horizon, then the radiation will continue long after it would have turned off in the black hole space-time.

  • Much remains unknown about fundamental theory. Modifications of general relativity on accessible scales are not ruled out by today’s fundamental theories and observations. It makes sense to look for them by testing general relativity as accurately as possible. Cost-effective experiments that increase the precision of measurements of parametrized post-Newtonian (PPN) parameters, and which test the strong and weak equivalence principles, should be carried out. For example, improvements in Lunar Laser Ranging promise to advance this area.

Dark-Matter Detection and Characterization

The inferred existence of dark matter, the mysterious substance that makes up 83 percent of the matter in the universe, raises profound questions for both astronomy and physics. What is dark matter? Where does it come from? What are the connections between dark matter and “ordinary” matter? (See Box 8.3.) The detection of astrophysical signatures of dark-matter particles will play an essential role in the discovery of dark matter, along with direct detection and collider experiments. Astrophysical signatures are expected to include gamma rays, antiparticles (such as positrons and anti-protons), anti-nuclei, and neutrinos.

The interaction of dark matter with the ordinary universe is so tenuous that it is known only through gravitational interactions. In spite of the indirect nature of the evidence, galactic rotation curves, strong gravitational lensing, and large-scale-structure measurements, combined with big bang nucleosynthesis constraints, all continue to support the dark matter hypothesis with increasingly convincing data.

BOX 8.3

Science from Dark Matter Detection and Characterization

SFP Questions Addressed

Measurements Addressing the Questions

CFP 3

What is dark matter?

Indirect astrophysical searches for dark-matter annihilation and decay signatures

GAN 4

What are the connections between dark and luminous matter?

 

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Furthermore, the power spectrum of the cosmic microwave background agrees with the scenario that dark matter constitutes 83 percent of the matter and 20 percent of the energy density in the universe. The matter power spectrum is measured at smaller scales by galaxy surveys, yielding consistent results.

In spite of these accomplishments, the nature of the dark-matter particles is not yet known. The 2001 decadal survey emphasized three leading candidates: massive neutrinos, axions, and weakly interacting massive particles (WIMPs).1 Massive neutrinos now appear less likely, as the most massive neutrino flavor is thought to be in the 0.04- to 2-eV range. This mass range cannot explain large-scale structure formation, both because such neutrinos remain relativistic for too long in the early universe, and because the mass density of relic neutrinos is far too small. The axion was postulated to solve the “strong CP problem”—that is, to prevent the violation of charge-parity symmetry in strong interactions. If axions exist, they may constitute a significant fraction of the dark matter. Because axions are light they are unlikely to produce high-energy particles via annihilation or decay in astrophysical scenarios, and signatures of axions are the subject of a separate class of direct-search experiments targeted for the axion’s coupling to two photons. Thus in this report the panel concentrates on indirect searches for WIMPs.

A WIMP is a thermal relic of the big bang—a massive particle in thermal equilibrium in the early universe that decouples when its number density falls so low that the annihilation timescale equals the Hubble time. This establishes a relationship between annihilation cross section and mass density that gives the correct dark-matter density today if the annihilation cross section is appropriate for the scale of the weak interactions, and the mass of the WIMP is within about two orders of magnitude of the weak scale (say, 1 GeV to 10 TeV).

Because the existence of dark matter has been inferred only from its gravitational effects, the expected astrophysical signals can be estimated only in the context of some models. A leading WIMP candidate is the lightest supersymmetric particle in the minimal supersymmetric standard model (MSSM). For the parts of MSSM parameter space that yield the correct relic density, the lightest particle is the superpartner of the neutral gauge bosons, known as the neutralino. In such a model interaction cross sections and masses can be estimated (at least to within several orders of magnitude), yielding a starting point for searches. However, this basic scenario may be extended in many ways.

Although any thermal relic WIMP must annihilate at some level, it is not yet known whether such annihilation produces a signal that will be detectable in the coming decade. The signals generally expected from conventional WIMP models are small; however, recent reports of a possible excess (over those expected from

1

National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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combined astrophysical and particle physics models) of particles and photons in the ~10- to 1,000-GeV range triggered interest in models of dark matter with an annihilation cross section significantly higher than that of the “standard” thermal relic WIMP. These “Sommerfeld-enhanced” models, which were proposed as a way to understand otherwise contradictory results, involve a new force of nature that acts on dark matter, but only very weakly on ordinary matter. This force causes a significant (1 to 3 orders of magnitude) enhancement in the annihilation cross section and favors annihilation to leptons rather than to hadrons. These models are of interest because they can fit the recently observed particle and photon spectra, which is difficult to do with a more conventional WIMP. Such models were originally invented to explain the annual modulation of the scattering signal in the DAMA/LIBRA sodium-iodide direct-detection experiment. In this scenario, the new force mediates an inelastic scattering between two WIMP states, allowing DAMA/LIBRA to be marginally compatible with other experiments. Such ideas will be thoroughly tested by the current generation of direct-detection experiments with target mass at the 100-kilogram scale.

Improved capabilities in spectral measurements of cosmic rays and gamma rays and in background rejection can provide evidence for dark matter. In fact, the most distinctive signature of dark matter may be in the gamma-ray spectrum from the inner Milky Way, Milky Way satellite galaxies, or other nearby galaxies. Interpretation of both gamma-ray and particle signals will also require improved modeling of sources and propagation. If the apparent excesses described above result from dark matter, this is of fundamental importance to particle physics. If not, it is essential to understand them as astrophysical signals, both in their own right and so that future projects will be able to isolate the faint dark-matter signals from the much larger astrophysical background. Experiments that push the limits of detector mass and exposure time are called for. The next generation of these experiments motivates a strong program in support of balloon payloads and flights, particularly ultralong-duration ballooning, and opportunities for cost-effective experiments on small to midsize satellites.

High-Energy Particle Astrophysics

High-energy particles, including gamma rays, cosmic rays, and neutrinos, bring new and complementary views of astronomical sources and probe physical processes under extreme conditions throughout the universe. Gamma rays and cosmic rays (charged particles or particles whose nature is not known) are produced in cosmic accelerators over a vast range of scales, from the solar system to powerful extragalactic sources. Cosmic rays are a major contributor to the energization of the interstellar medium in our galaxy and in others, as well as in galaxies undergoing formation, where jets from active galactic nuclei play major roles in regulating star

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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formation. High-energy cosmic neutrinos have yet to be detected; their detection will add invaluable information to the study of cosmic accelerators, as the neutrinos arise deep inside the acceleration regions and travel undeflected from cosmologically distant sources. Advancing the physical understanding of cosmic accelerators through observation, simulation, and theory feeds directly into several major themes of galactic and extragalactic astrophysics and cosmology. (See Box 8.4.)

Particle-astrophysics observations have the important ability to probe physics beyond the standard model of particle physics. A striking example of this potential is the search for dark-matter annihilation and decay products, discussed above, which could in principle involve any of the species of particles or gamma rays studied using the tools of particle astrophysics. Particle astrophysics extends the high-energy frontier well above energies accessible to laboratory accelerators, through study of ultrahigh-energy cosmic rays, photons, and neutrinos. To use high-energy particles effectively for probing dark-matter signatures and physics beyond the Large Hadron Collider scale requires understanding their origin and their propagation to Earth.

Gamma rays are produced both in cosmic electron and in hadron accelerators; they provide essential constraints for the study of electron accelerators and will be key in unveiling the sites of hadronic acceleration. Recent advances in high-energy

BOX 8.4

Science from High-Energy Particle Astrophysics

SFP Questions Addressed

Measurements Addressing the Questions

GAN 2

What controls the mass-energy-chemical cycles within galaxies?

Gamma rays, cosmic rays, and neutrinos from active galactic nuclei and gamma-ray bursts

GCT 3

How do black holes grow, radiate, and influence their surroundings?

 

SSE 1

How do rotation and magnetic fields affect stars?

Gamma rays and neutrinos from supernovae, gamma rays from gamma-ray bursts, and gamma rays from stars and binary systems

SSE 3

How do the lives of massive stars end?

 

CFP 4

What are the properties of neutrinos?

Detection of neutrinos from cosmic accelerators

 

 

Detection of cosmogenic (GZK) neutrinos

 

 

Tests of physics above the TeV scale

How are ultrahigh-energy particles accelerated? (not an SFP question)

Studies of ultrahigh-energy cosmic rays, neutrinos, and gamma rays (composition, spectra, sources)

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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gamma-ray observations have revealed a plethora of gamma-ray sources, including gamma-ray pulsars, compact binaries, the galactic center, and extragalactic sources such as starburst galaxies and radio galaxies. During the next decade, improved sensitivity and spectral coverage of the new generation of gamma-ray observatories will provide detailed spatial and spectral information on known sources to determine how binaries and pulsars produce gamma rays, how supermassive black holes power jets, what powers gamma-ray bursts at low and high redshifts, what is the extragalactic background light, and what is the origin of cosmic rays. Gamma-ray observations may also address the nature of dark matter and may discover new, completely unexpected sources. Much progress has been made in the detection of high-energy gamma rays by way of atmospheric Čerenkov emission. There is a strong science case now for a factor-of-10 improvement in sensitivity and for instruments with a wider field of view. There is also a strong science case for improvement by a factor of 2 to 3 in angular resolution. These requirements motivate U.S. involvement in an international Čerenkov telescope array with an effective area of approximately 1 square kilometer. Finally, the importance of observations that cover a wide field of view with a large duty cycle is also recognized, as demonstrated by the Fermi Gamma-Ray Space Telescope (formerly GLAST; ranked first among medium space-based missions in the 2001 decadal survey). These observations are particularly important for transient and extended sources.

The origin of cosmic rays is still a mystery, although much has been learned in the past decade. The observations span more than 30 orders of magnitude in flux and track acceleration and propagation processes at scales from the solar system to powerful extragalactic sources. The standard paradigm for the origin of galactic cosmic rays involves Fermi acceleration in non-relativistic shock waves in supernova remnants. Low- and intermediate-energy cosmic rays are consistent with a galactic origin. At ultrahigh energies, cosmic rays show a sky distribution and spectral features consistent with an extragalactic origin. The transition between galactic and extragalactic origins and the locations of the principal sites of highly efficient acceleration (in excess of 10 percent) are poorly understood. The observed attenuation of the ultrahigh-energy spectrum is consistent with the predicted Greisen-Zatsepin-Kuzmin (GZK) cutoff due to the interaction of cosmic-ray protons with the cosmic microwave background, but it could alternatively be due to the limits of acceleration in sources, especially if heavier nuclei dominate. The indication of an anisotropic distribution of the highest-energy cosmic rays, as reported by the Auger South Observatory, suggests a dominantly proton flux. More-precise determination of the degree of anisotropy, which may be possible with improved statistics by adding Auger North in the next decade, will help determine the nature of the ultrahigh-energy cosmic rays and thereby help establish expected GZK fluxes of ultrahigh-energy neutrinos and photons.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Neutrinos were identified by the Astro2010 SFPs as a particularly fruitful area of research for the next decade. They are weakly interacting messengers that, unlike gamma rays and cosmic rays, can reach Earth undeflected from the edge of the observable universe, independent of energy. They should be produced at sites of hadronic acceleration or interaction and thus provide a unique tool to study high-energy accelerators and particle interactions. The main challenge is their detection.

The expectations for neutrino astronomy are deeply connected with gamma-ray and cosmic-ray observations, and all three types of particle must be considered together. Gamma- and cosmic-ray sources are likely also to produce neutrinos, and neutrinos are produced by the interaction of ultrahigh-energy cosmic rays with the cosmic background radiation. Cosmogenic neutrinos (the GZK neutrinos) are often considered to be a guaranteed source of ultrahigh-energy neutrinos, but the predicted fluxes depend on the proton fraction and the cosmological evolution of cosmic-ray sources. New physics above the TeV scale may be tested by comparing rates of horizontal neutrino air showers initiated in Earth’s atmosphere with Earth-skimming neutrinos.

Connections to Other Areas of Physics and Astrophysics

The science questions discussed in this panel report can also be approached by methods that complement those that this panel considered. For example, to address the question What are the particles that make up the dark matter? three approaches are needed: indirect evidence from observations of high-energy particles produced in dark-matter annihilation or scattering processes; direct searches with detectors sensitive to the rare scattering of dark-matter particles by normal matter; and studies of production and decay cascades in collider experiments. While only astrophysical detection is within the purview of this report, it plays a critical role in establishing that any new signals found by the other approaches are dark-matter related, as opposed to unrelated new physics. Similarly, two of the most fascinating fundamental science questions relevant to particle astrophysics and general relativity are the SFP questions, Why is the universe accelerating? and How did the universe begin?, yet many of the experiments that address these questions, such as large electromagnetic surveys, fall in the purview of other Astro2010 panels and hence are not discussed in any depth in this panel report. Also excluded are experiments addressing the physics of low- to intermediate-energy gamma rays, which were considered by another panel. These exclusions are not a statement about the interest in this science, but rather that it is not probed by the types of experiments that this panel is charged to consider. As the panel presents its recommendations, connections to the experiments and programs considered by other panels are indicated, as appropriate.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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THE PROGRAMMATIC CONTEXT

Gravitational-Wave Astrophysics

The 2001 decadal survey recognized gravitational-wave astrophysics as a promising area; LISA was given the highest priority by the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics (PNGA)2 and was ranked second overall among the space-based medium-class missions. The PNGA recommended a technology-development program to reduce the risk in the LISA mission. It also noted the major challenge to theory of calculating waveforms from black hole mergers, a key LISA source, using numerical relativity. Much progress has been made in both these areas in the past decade.

Ground-based interferometric gravitational-wave detectors have improved sensitivity and are now collecting data with some potential for discovering gravitational waves from binary systems as far away as the Virgo cluster of galaxies. In the United States, the Laser Interferometer Gravitational Wave Observatory (LIGO) consists of three detectors, 2 and 4 km long (Figure 8.6). Two instruments in Europe are VIRGO, 3 km long, and GEO, 0.6 km long. Even more importantly, advanced designs for the LIGO detectors have been funded and are being built by the United States, and a similar advanced design is being pursued by VIRGO. With these advanced detectors operating at their design sensitivities, the predicted rate for observations of neutron stars and stellar black holes is in the range of dozens per year. Even now the existing detectors are producing results of astrophysical interest. For example:

  • A search with LIGO and VIRGO in temporal and direction coincidence with 22 gamma-ray bursts found no statistically significant candidates. Thus, neutron-star/black-hole progenitors are excluded from our galaxy or neighboring galaxies.

  • A LIGO search for gravitational radiation from the Crab pulsar limits the power radiated in gravitational waves to less than 2 percent of the spindown power.

  • Null results in a search for a stochastic gravitational-wave background with LIGO places an upper limit on the density of such a background at less than 1 part in 100,000 of the critical cosmological density, ruling out some models of early-universe evolution.

The current program in gravitational-wave astrophysics includes technology development for other advanced and future detectors around the world. An underground cryogenic prototype in Japan (CLIO) is operating, and a large Japanese

2

National Research Council, Astronomy and Astrophysics in the New Millennium: Panel Reports, National Academy Press, Washington, D.C., 2001.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 8.6 The Laser Interferometer Gravitational Wave Observatory (LIGO) facility in Hanford, Washington. SOURCE: LIGO Laboratory.

FIGURE 8.6 The Laser Interferometer Gravitational Wave Observatory (LIGO) facility in Hanford, Washington. SOURCE: LIGO Laboratory.

cryogenic telescope (LCGT) is under consideration. There are also plans for a European third-generation underground detector known as the Einstein Telescope.

Over the past decade, technology for LISA has matured, and the European Space Agency (ESA) is planning a LISA Pathfinder mission that will provide critical tests of several of LISA’s subsystems in a space environment. Significant progress has also been made in planning for LISA data reduction. Numerical relativists have conquered the theory challenge with breakthroughs in simulating the merger of two black holes and in computing the resulting gravitational waveforms. This has enabled detailed modeling of source waveforms, development of analysis software, and several community-wide “mock data challenges.” Addressing detection at even lower frequencies, there has also been much progress in developing pulsar-timing techniques and in completing surveys to identify suitable pulsar-timing systems.

Tests of General Relativity and Other Theories of Gravity

Relativistic gravitation underlies modern astrophysics and cosmology and is a cornerstone of fundamental physics, yet a coherent program to test general relativity that would systematically explore the ranges of scale and strength shown in Table 8.2 has not been established. Tests of gravity can be carried out using ground-based instrumentation, where a high degree of control is available, or in space where quiet conditions prevail. The targets of tests may involve either weak gravity, as in laboratory or solar-system sources, or strong gravity, as in some astrophysical sources and cosmology. As a consequence, the testing of general relativity is spread across different science-support agencies with little coordination among them. Many tests have exploited opportunities presented by missions developed primarily for other purposes.

Despite this fragmentation, significant progress in testing general relativity has been made in the past decade. To give a flavor of what has been achieved, the panel presents the following highlights:

  • Measurements of the parameterized post-Newtonian parameter γ on solar system scales using the differential Shapiro time delay, as a by-product of the Cas-

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

sini mission, and on kiloparsec scales from a comparison of gravitational-lensing and velocity-dispersion measurements;

  • The direct detection of gravitomagnetic effects (the Lense-Thirring precession) from Lageos/Grace, Gravity Probe B, and lunar laser ranging;

  • The measurement of geodetic spin precession in the double pulsar system J0737-3039A/B (see Figure 8.7 for an illustration of five tests of general relativity in that system);

  • The ongoing monitoring of the binary-pulsar orbital decay, consistent with the emission of gravitational radiation predicted by general relativity to within a fraction of a percent;

FIGURE 8.7 Five tests of general relativity using the double pulsar J0737-3039A/B. This figure illustrates both the variety and the precision of tests of general relativity that are possible today in one system. The post-Keplerian parameters are fitted along with the masses of the two pulsars. The parameter is the periastron advance, is the period decrease due to gravitational radiation, and s and r are related to the precession of pulsar B. The white area corresponds to allowed inclination angles of the orbit. The intersection of the allowed ranges in the tiny box at the center of the inset provides five tests of general relativity at the 10−4 to 10−5 level. SOURCE: From R.P. Breton, V.M. Kaspi, M. Kramer, M.A. McLaughlin, M. Lyutikov, S.M. Ransom, I.H. Stairs, R.D. Ferdman, F. Camilo, and A. Possenti, Relativistic spin precession in the double pulsar, Science 321(5885):104-107, 2008. Reprinted with permission from the AAAS.

FIGURE 8.7 Five tests of general relativity using the double pulsar J0737-3039A/B. This figure illustrates both the variety and the precision of tests of general relativity that are possible today in one system. The post-Keplerian parameters are fitted along with the masses of the two pulsars. The parameter is the periastron advance, is the period decrease due to gravitational radiation, and s and r are related to the precession of pulsar B. The white area corresponds to allowed inclination angles of the orbit. The intersection of the allowed ranges in the tiny box at the center of the inset provides five tests of general relativity at the 10−4 to 10−5 level. SOURCE: From R.P. Breton, V.M. Kaspi, M. Kramer, M.A. McLaughlin, M. Lyutikov, S.M. Ransom, I.H. Stairs, R.D. Ferdman, F. Camilo, and A. Possenti, Relativistic spin precession in the double pulsar, Science 321(5885):104-107, 2008. Reprinted with permission from the AAAS.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
  • The verification of the gravitational inverse square law down to ranges of 56 microns in laboratory experiments;

  • The lunar-laser-ranging verification of the strong equivalence principle to 10−4, meaning that the triple-graviton vertex is now known to better accuracy than is the triple-gluon vertex;

  • Limits on the fractional rate of change of the gravitational constant G (<10−12/yr) from lunar laser ranging;

  • Atomic experiments limiting time variation of the fine structure constant to 10−16/yr over periods of several years;

  • The supernova and cosmic microwave background measurements of the acceleration of the universe; and

  • Experiments in progress that include the MICROSCOPE equivalence principle experiment, the APOLLO lunar-laser-ranging observations, and tests of general relativity using torsion balances and atom interferometry.

The above list shows that much has been done to test general relativity, and Einstein’s theory is consistent with all experimental tests to date! However, a glance at Table 8.2 shows that much remains to be done to test the theory in the domains of strong gravity and on scales larger than the solar system. The accomplishments of the past decade set the stage for the next decade; in Table 8.3 the panel presents goals for the next decade. In making its final recommendations, the panel returns to those that fall within its purview.

TABLE 8.3 Possible Tests of Relativistic Gravity in the Next Decade

Distance Scale

Weak Gravity

Strong Gravity

Laboratory

Improved equivalence-principle limits and measurements of parametrized post-Newtonian (PPN) parameters from atom interferometry

Better constraints on extra dimensions, for example, from accelerator experiments

Solar system

Improved strong- and weak-equivalence-principle limits; better determination of PPN parameters and the rate of change of the gravitational constant from next-generation lunar laser ranging

Gravitational waves detected directly and their predicted speed and polarization confirmed; properties of rotating black holes confirmed quantitatively by gravitational waves and X-ray reverberation

Galactic

Measurement of PPN parameters by lensing and velocity dispersion

Predicted connections between sources and gravitational waves confirmed quantitatively

Cosmological

Better bounds on variations of fundamental constants, e.g., α, me/mp

Gravitational waves detected by cosmic microwave background polarization; relation between expansion history and growth of structure tested with supernova, baryon acoustic oscillations, and weak lensing

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
Dark Matter Detection and Characterization

The possibility that a new class of fundamental particles could make up the dark matter gives the search for WIMPs in the galactic halo a very high scientific priority. Direct detection of dark matter would be the most definitive way to determine that WIMPs make up the missing mass. The study of WIMP candidates in accelerator experiments is also critical for determining the relic density of these particles. The indirect detection of astrophysical signals due to WIMP-WIMP self-annihilation may also provide important clues, but in many cases such signals may be difficult to separate unambiguously from more mundane astrophysical sources. That leaves direct detection as playing a central role in establishing the presence of WIMPs in the universe today. Also, given both the technical challenge and the fundamental importance of direct detection of WIMPs, it is vital to have the means to confirm a detection in more than one type of detector. While evaluation of future direct-detection experiments is outside the charge of this panel, current and possible future experiments are summarized as part of the context for indirect astrophysical detection experiments.

Experiments based on cryogenic noble liquids are scaling up rapidly to provide large detector mass with very low background (for example, the U.S.-led XENON100 experiment and LUX (Figure 8.8); the Japanese-led XMASS experiment; and the Italian-led WArP experiment. The current generation is using 100-kg-scale detectors, and there are realistic prospects for a ton-scale experiment by the middle of the next decade (XENON1T). Experiments using silicon and germanium crystals cooled to millikelvin temperatures continue to scale up (the U.S.-led CDMS and the French-led EDELWEISS). The bubble chamber technique pioneered by the U.S.-led COUPP has carved out a niche in the spin-dependent model space and may be scalable to ton-scales in the future. As experiments using these various techniques increase substantially in size, it is quite possible that one or more will make a convincing detection of WIMP-nucleon scattering. If this happens, it will be possible to search for the annual modulation of the signal as Earth goes around the Sun. Direction-sensitive detectors would seek the changing direction of the particle “wind” as Earth moves in its orbit. Furthermore, by comparing nuclear-recoil spectra from multiple experiments, information about the WIMP mass could be derived.

These direct-detection experiments provide an essential step in establishing the existence of a WIMP candidate. However, whatever particle they may discover need not constitute the majority of dark matter. There may be many species of WIMPs, and because the annihilation cross section and scattering cross section are parametrically related (in tree-level interactions), the scattering probability may scale as the inverse of the thermal relic freeze-out density. This raises the possibility that there may be many kinds of WIMPs with densities and cross sections span-

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 8.8 The XENON100 experiment. One-inch-square photomultipliers, highly sensitive to the vacuum-ultraviolet scintillation of liquid xenon, detect the faint signals expected from a WIMP interaction in the liquid xenon time-projection chamber (TPC). The photo shows the bottom array of 80 such photomultipliers and the reflected image of the top array of 98 photomultipliers. Also visible is the hexagonal wire mesh, which serves as the TPC’s cathode for drifting electrons through the 30-cm-deep liquid-xenon target. SOURCE: XENON100 Collaboration.

FIGURE 8.8 The XENON100 experiment. One-inch-square photomultipliers, highly sensitive to the vacuum-ultraviolet scintillation of liquid xenon, detect the faint signals expected from a WIMP interaction in the liquid xenon time-projection chamber (TPC). The photo shows the bottom array of 80 such photomultipliers and the reflected image of the top array of 98 photomultipliers. Also visible is the hexagonal wire mesh, which serves as the TPC’s cathode for drifting electrons through the 30-cm-deep liquid-xenon target. SOURCE: XENON100 Collaboration.

ning several orders of magnitude. Therefore, proof that the first WIMP detected is indeed the dark-matter particle will require astrophysical detection of annihilation or decay signals compatible with the direct-detection signals. The most obvious such signals are cosmic rays, neutrinos, and gamma rays.

Recent high-energy particle results, while tantalizing, have failed to paint a coherent picture consistent with a plausible dark matter candidate. The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) spacecraft has measured the e+ fraction as 10 percent at 100 GeV, twice what it is at 10 GeV and much larger than that expected from models of secondary particles produced by proton cosmic rays interacting with the interstellar medium.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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However a similar effect is not observed for anti-protons. At still higher energies, the Advanced Thin Ionization Calorimeter (ATIC) balloon-borne experiment claimed a peak in the electron spectrum (ATIC cannot distinguish positrons from electrons). Fermi also measured the electron-plus-positron spectrum up to 1 TeV, finding a broad-spectrum excess above the expected spectrum, but not the ATIC peak. These signals are much larger than expected for a thermal WIMP annihilating through a hadronic cascade, but they are possible in more exotic models. Models requiring leptophilic dark matter are required to justify the lack of an excess in the PAMELA anti-proton measurements. The situation indicates that it is important to improve knowledge of secondary production of cosmic rays to distinguish between dark-matter models or alternative scenarios, such as cosmic rays produced by local sources.

Anti-nuclei are also a target for indirect dark-matter detection experiments. The Alpha Magnetic Spectrometer (AMS) experiment is designed to characterize anti-nuclei using an orbiting magnetic spectrometer. The AMS01 prototype detector provided a limit on the existence of anti-helium, in addition to measuring the proton and electron-positron spectra in near-Earth orbit. Scheduled for the International Space Station, the full AMS experiment, with a magnetic spectrometer, will provide spectra of electrons, positrons, protons, anti-protons, and various nuclei up to the TeV region. The panel notes that the best limits on anti-helium to date are those due to the BESS-polar balloon experiments, illustrating the potential of this relatively low-cost platform. AMS and balloon experiments under development will also have the potential to detect anti-deuterons, which may be produced in dark-matter annihilations but are not made by any other known process.

High-energy photons can also provide valuable information, and, since they are not deflected by magnetic fields, they have the advantage of pointing straight back to their sources. Dark-matter annihilation can produce gamma rays in three ways: prompt photons, produced directly or via pion decays in hadronic cascades; final-state radiation, in which electron-positron pairs produce bremsstrahlung as they are created; and inverse Compton scattering, in which high-energy electrons or positrons scatter ambient starlight photons to high energies. The first two trace the dark-matter annihilation, while the third involves particles that have diffused significant distances over millions of years. Such signals are sought in the galactic center, in Milky Way subhalos, and in other galaxies. Fermi is mapping the sky from 100 MeV to 300 GeV and will either detect such signals or place interesting limits. Ground-based data from HESS, VERITAS, and other imaging atmospheric Čerenkov telescopes provide useful constraints in the high-energy range, above 100 GeV.

Finally, neutrinos from WIMP annihilations in the Sun are being sought with IceCube, a gigaton neutrino detector currently under construction at the South Pole. For a particular mass range, WIMPs that happen to pass through the Sun do

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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so at sufficiently high speeds that they can lose enough energy in a nuclear scattering to become bound. The resulting WIMP over-density in the Sun leads to a substantial annihilation rate, which could be detectable.

High-Energy Particle Astrophysics

In high-energy particle astrophysics the past decade has brought into operation major observatories of gamma rays and cosmic rays that have provided a new view of the high-energy universe, probing the astrophysics of particle acceleration and non-thermal processes in a wide variety of physical regimes. Neutrino observatories are reaching the sensitivity level necessary to make the first detections of high-energy neutrinos from astrophysical sources, which would directly signal the acceleration of hadrons. The theory of astrophysical particle-acceleration processes, exploiting advances in computational plasma physics, has advanced to the point where useful contact with observations has become possible. As a result of the successes of the current instruments, there is great worldwide interest in initiating new, much more capable, high-energy observatories in the upcoming decade.

Over the past decade, ground-based arrays of imaging atmospheric Čerenkov telescopes have discovered very-high-energy (E > 100 GeV) gamma-ray emission from a wide variety of astrophysical sources, both galactic and extragalactic, increasing the source catalog by more than an order of magnitude (to almost 100 established sources). As important as the increase in number of detected sources, the quality of the data from Čerenkov telescope arrays has provided spatially resolved images of galactic sources such as supernova remnants, pulsar-wind nebulae, and the galactic center region, as well as spectral measurements of high-energy emission from active galactic nuclei with excellent (minute-scale) time resolution. The supernova-remnant observations suggest hadronic acceleration and subsequent emission of gamma rays, possibly confirming these as the main sites of proton acceleration in the galaxy, complementing their role as electron accelerators. The major Čerenkov telescopes currently operating are the U.S.-led VERITAS (a recommendation of the 2001 decadal survey) on Mt. Hopkins in Arizona (Figure 8.9), and the European-led instruments HESS in Namibia and MAGIC at La Palma.

The air-shower technique, as exemplified by the Milagro telescope, which operated until 2007 in New Mexico, is complementary to Čerenkov telescopes for detecting very-high-energy gamma rays. The latter achieve better angular and energy resolution, and hadron-background discrimination, than did Milagro. However, the air-shower technique permits a much wider field of view and continuous sky monitoring due to the essentially 100 percent duty cycle. Milagro carried out a survey of the Northern Hemisphere sky, detecting a number of sources in the galactic plane at energies above 10 TeV as well as diffuse emission from the plane at these energies. The Milagro sources were unexpected because of their relatively

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 8.9 The VERITAS atmospheric Čerenkov telescope array for gamma-ray astronomy on Mt. Hopkins in southern Arizona. SOURCE: Steve Criswell, Smithsonian Astrophysical Observatory.

FIGURE 8.9 The VERITAS atmospheric Čerenkov telescope array for gamma-ray astronomy on Mt. Hopkins in southern Arizona. SOURCE: Steve Criswell, Smithsonian Astrophysical Observatory.

high flux and hard spectra; they may signal the acceleration of hadrons. Another interesting result from Milagro, not yet fully understood, is the detection of anisotropy in the cosmic-ray arrival directions on the scale of 10 to 30 degrees (also reported by IceCube, the Tibet Array, and other experiments).

High-energy gamma-ray astronomy at GeV energies can be carried out very effectively by satellite telescopes. Fermi has worked flawlessly, providing continuous all-sky coverage of the gamma-ray sky, and is expected to continue operation through at least 2013. Early exciting results from Fermi include the discovery of many new pulsars not known from other wavebands, the detection of many new high-energy blazars, multi-GeV emission from gamma-ray bursts, and new measurements of the high-energy isotropic diffuse radiation and of the spectrum of cosmic-ray electrons up to 1 TeV. It is important to note that the Large Area Telescope (LAT), the main instrument of Fermi, was built by a successful international partnership that included astrophysics groups supported by NASA and particle-physics groups supported by DOE.

The spectrum of high-energy cosmic rays extends from GeV energies up to 1020 eV and perhaps beyond. Spaceborne instruments such as PAMELA and AMS-01—and balloon-borne experiments such as ATIC, CREAM, and TIGER—have played essential roles in measuring the composition of the cosmic rays up to energies of ~ 1014 eV, just below the knee in the energy spectrum, and efforts are underway to extend the reach of such instruments to even higher energies. The existence of ultrahigh-energy (E > 1017 eV) cosmic rays has been known for several decades, but their origin remains a deep mystery. At these energies, cosmic rays are detected by ground arrays of detectors or fluorescence telescopes, such as Auger South (described below) and the Telescope Array, that observe the air showers generated

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

by cosmic-ray interactions in the atmosphere. Modern observatories using both techniques allow for hybrid detection of events. Recent worldwide efforts have begun to develop alternative techniques for detecting cosmic rays using radio signals.

In the previous decade, the AGASA ground array in Japan found surprising evidence suggesting a continuation of the energy spectrum past the GZK cutoff near 6 × 1019 eV that is expected from the interaction of protons with the cosmic microwave background. In contrast, the Fly’s Eye HiRes fluorescence experiment in Utah reported a rollover in the spectrum at the highest energies. This confusing situation has now been clarified. Auger South, a hybrid array in Argentina with unprecedented collection area for ultrahigh-energy cosmic rays, started full operations in 2008 (Figure 8.10). It has reported several important new results: (1) a clear confirmation of the rollover of the energy spectrum; (2) an indication that the

FIGURE 8.10 Pierre Auger Observatory of ultrahigh-energy cosmic rays in Mendoza Province, Argentina. Shown is one of the particle detector tanks of the 1,660 units covering 3,000 square kilometers and one of the four fluorescence telescopes that overlook the array. SOURCE: Courtesy of the Pierre Auger Observatory.

FIGURE 8.10 Pierre Auger Observatory of ultrahigh-energy cosmic rays in Mendoza Province, Argentina. Shown is one of the particle detector tanks of the 1,660 units covering 3,000 square kilometers and one of the four fluorescence telescopes that overlook the array. SOURCE: Courtesy of the Pierre Auger Observatory.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

highest-energy cosmic rays are distributed anisotropically; (3) a strong limit on the photon fraction that rules out, in large part, top-down models for the production of ultrahigh-energy cosmic rays; and (4) the strongest limit to date on the flux of cosmogenic neutrinos between 1017 and 1019 eV.

Neutrino telescopes offer a compelling avenue for understanding high-energy astrophysics and to probe for dark-matter annihilations. The large IceCube detector under construction at the South Pole will be fully operational in 2012 and will search for TeV and PeV astrophysical sources of neutrinos with an expected capability that should permit the first high-energy-neutrino source detections. The detection of ultrahigh-energy neutrinos is challenging for existing detectors. New techniques are needed to establish their existence firmly and to extract useful physics and astrophysics. For cosmogenic neutrinos, the best energy range is around 1018 to 1019 eV where the flux is maximal. Various R&D activities are ongoing, led by radio-detection techniques such as radio antennas in ice (e.g., RICE and prototypes in the IceCube holes); antennas on balloons (e.g., ANITA) and on the ground to measure the Čerenkov signal produced by the Askaryan effect; and antennas at extensive-air-shower arrays (e.g., Auger and LOPES) for the detection of radio emission from neutrino-initiated atmospheric showers. R&D programs on acoustic detections of pulses due to heat produced by particle cascades in sea water, ice, and salt are ongoing.

FUTURE PROGRAM

The activities considered by the panel receive federal support from four sources: the NASA Astrophysics Division, the NSF Division of Astronomical Sciences, the NSF Division of Physics, and the High Energy Physics program within the DOE Office of Science. Therefore, the funding environment is complex. The panel presents a science program that cuts across all four agency funding units and constitutes a complete program with complementary contributions from the units. It also presents recommendations on infrastructure issues. The panel’s recommendations are based primarily on considerations of whether a particular project, mission, or activity addresses a high priority and a compelling science question or questions. In addition, the panel considered whether an activity merits what it would cost, would produce results related to theoretical predictions or opens a new capability with high discovery potential, is technically feasible or incorporates verifiable technology development, and is of interest in a worldwide context and does not unnecessarily duplicate efforts outside the United States. Table 8.8 at the end of this report indicates the relationship between the panel’s recommended activities and the science priorities identified by the Astro2010 Science Frontiers Panels.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
Science Program

In gravitational-wave astrophysics the panel recommends complementary ground- and space-based programs supported by NSF and NASA, respectively. Ongoing and expected improvements in ground-based detectors may well give us the first detection of gravitational waves in the next decade. The progression from detection to astrophysical insight will require subsequent exploitation of the higher-frequency portion of the spectrum accessible from the ground. In addition, opening the lower-frequency portion of the spectrum is predicted to provide access to many detectable astrophysical sources, different from those relevant to ground-based detectors. The key frequency range for many of the most exciting investigations is 10−4 to 10−1 Hz, which requires laser-interferometric measurements over extremely large baselines, which are possible only in space. The only option for opening this window is LISA. The LISA technology to enable these measurements has advanced considerably over the past decade, and the scientific case is compelling for a gravitational-wave mission to be the flagship space mission for astrophysics in the coming decade. While all of the risk cannot be eliminated from such a mission (or any space mission), even with thorough ground testing and a space precursor, the scientific case is strong enough to warrant moving forward even at a medium level of technical risk. To achieve that level of residual risk, however, requires successful completion of the LISA Pathfinder (LPF) precursor. Given the great astrophysical importance of opening up the low-frequency gravitational wave band, the panel recommends that the LISA mission be given the highest priority for a new start in the next decade. Furthermore, given the extensive technology development that has already been completed, the expected short time until LPF launch, and the need to maintain momentum in the U.S. community and guarantee a smooth transition to a joint NASA-ESA mission, the panel recommends that NASA funding of LISA begin immediately, with continuation beyond LPF contingent on the success of that mission.

The LPF is an ESA technology-demonstration mission, scheduled for launch in 2012, that will test several of the LISA subsystems in the space environment. This includes the Gravitational Reference Sensor (GRS), the Interferometry Measurement System (IMS), and two micro-Newton thruster designs. The tests to be performed are end-to-end performance of a shortened version of one LISA arm, a complete test-mass-to-local-spacecraft measurement, and modeling of physical disturbances. As described in more detail below, the spacecraft environment for LPF will not be the same as that for LISA; LPF will be placed at the Earth-Sun L1 Lagrangian point rather than in an Earth-trailing orbit. LPF development is complete, and flight hardware is under construction. It is intended that analysis of the LPF results be finished before the beginning of LISA Phase B.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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The panel identified four major areas of technical risk within the LISA project: (1) the Disturbance Reduction System (DRS), consisting of the GRS, micro-Newton thrusters, and accompanying control system; (2) the IMS; (3) the spacecraft environment; and (4) long-range interferometry. Risks in the first two areas are either entirely or partially mitigated by a successful LPF. Although the DRS must demonstrate a 5-orders-of-magnitude reduction in system noise compared to previous similar implementations, extrapolation from current ground-based tests indicate that it will meet requirements. The GRS and thrusters (noise and minimum-impulse bit) will be tested to LISA levels on LPF. However, the DRS must perform as a system, and the LPF architecture and space environment are different from those of LISA. Also, on LPF the second mass will be slaved to the first, rather than acting as a free test mass. With these differences, the overall DRS performance on LPF is predicted by models of the system to be a factor of 10 worse than on LISA, even if the subsystems perform as designed (i.e., at LISA levels). The performance of the LISA system must also be evaluated through modeled extrapolation from LPF performance of the subsystems. The panel also notes that two technologies for the thrusters are under study, and both will be tested on the LPF, although the necessary long lifetime will not be verified.

The second critical area is the interferometric phase measurement system. This has been extensively studied on the ground, including efforts at the Jet Propulsion Laboratory, and will be verified at LISA performance levels on LPF (Figure 8.11). The main residual risks after LPF relate to the differing thermal, magnetic, and gravitational environment and the extent to which LPF results can be used to extrapolate to LISA performance. Likewise, while the LPF will test the operation of the lasers, it will not demonstrate the enormous path lengths planned for LISA and the relevant spacecraft subsystems and software for control. The current program calls for ground testing with space performance evaluated by analysis and extrapolation.

There are a number of programmatic issues related to LISA that remain to be resolved. Given the cost of LISA, the concentration of technology development in Europe, and the fact that LPF is being executed entirely as a European program, a collaboration with ESA is appropriate and recommended. The nature of the collaboration, in particular the division of responsibilities, needs to be established. The panel recommends commitments based on statements of work, and not cost caps (as is currently proposed by ESA), for each partner. Its review of costs and schedule led the panel to believe that those provided by an independent evaluation are more plausible than those provided by the project, and the panel adopts the former numbers in its budget analysis below.

A major consideration in the panel’s deliberations has been the level and nature of risk associated with the LISA mission. The LISA components are integrated to a degree unprecedented in an astrophysics space mission, making the risk of total mission failure relatively high compared to typical science missions. A significant

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 8.11 Fully bonded optical bench. SOURCE: H. Ward, Department of Physics and Astronomy, University of Glasgow.

FIGURE 8.11 Fully bonded optical bench. SOURCE: H. Ward, Department of Physics and Astronomy, University of Glasgow.

investment in systems engineering early in the program, with clear responsibilities assigned to NASA and ESA, is essential. In addition, even with a successful LPF test of the critical subsystems at the LISA level (the GRS, the IMS, and the micro-Newton thrusters), certain risks will remain associated with the different environment and the lack of multiple spacecraft in LPF to test the long-range interferometry. Nevertheless, if LISA fails to meet the required strain sensitivity, the degradation in science is graceful. Consider, for instance, a situation in which LPF succeeds and all LISA subsystems perform at the required level for LPF, but the low-frequency acceleration is only as extrapolated from LPF performance. Here, the number of detectable galactic binaries below 0.7 mHz would decrease (which is a small loss), and a few verification binaries would be eliminated. The main effect is that the angular positional accuracy for black hole mergers would decrease substantially, and the likelihood of finding electromagnetic counterparts to massive black hole mergers would worsen substantially. However, most of the other science

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

is retained, including accurate measurements of the masses, spins, and luminosity distances of massive black hole mergers. Now consider a situation in which LISA sensitivity is significantly worse than the extrapolation from LPF (by a factor of 3), perhaps due to errors in modeling, failure to control the spacecraft environment, or unexpected problems in the GRS or IMS. In this case, the main effect would be on the expected rate of detection for extreme-mass-ratio inspirals, which would decrease from ~50 per year to ~2 per year or less. This could result in no detections of extreme-mass-ratio inspirals over the life of the mission, given the astrophysical modeling uncertainty. However, while detections of massive black holes would be reduced by a factor of ~2 and detections of galactic white dwarf binaries by a factor of 10, the science from those detected (such as accurate massive black hole masses and spins) would be unchanged.

LISA science is rather robust against failure of one of the gravitational-reference sensors, which would result in the loss of a single arm of the interferometer; in fact, there is no science requirement that LISA maintain six working links throughout the mission. Such a failure would decrease the detection rates by a factor of only 1.5 to 3, because the system is designed for two redundant laser links. The most significant impact would be degradation in position measurements of merging black holes, resulting in the loss of positional information critical for electromagnetic follow-up observations. Another situation might be that the thruster lifetime is not as long as predicted; the mission lifetime would be correspondingly curtailed, with a reduction in the number of events detected. Nevertheless, in any of these scenarios, gravitational waves from coalescing black holes and the centers of galaxies will still be observed, greatly advancing astrophysics studies and tests of strong-field general relativity.

For completeness, the panel considered a more extreme failure scenario: the complete loss of one of the three spacecraft would result in virtually no remaining science. All three spacecraft must operate, with two DRSs operational on at least two of them, for a successful mission.

The panel is of the opinion that the enormous potential science return of the LISA mission, and its robustness against degradations in performance, justify accepting these risks. The panel also thinks that, after a successful LPF mission, further testing and analysis would have limited return, and therefore that LISA should be funded now without further delay but with a gate to continuation through the final project phases to launch based on the performance of LPF. The spending rate during the initial years of the LISA project is low, and support in the pre-LPF phase will maintain the project teams. The panel also notes that LPF may be neither a complete success nor an outright failure. Its expectation is that an evaluation of expected LISA performance will have to be evaluated in light of LPF test results, enabling a re-evaluation of costs and scientific benefits. If for some reason LPF were a complete failure, then the panel would not support a continuation of the LISA mission unless a decision were made to repeat the pathfinder.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Despite LISA’s impressive capabilities, LISA will not reach the very lowest gravitational-wave frequencies that probe the stochastic background produced in the early universe via relic gravitational waves. A promising approach for detecting this cosmological background is provided by high-precision timing of millisecond pulsars, using a network of pulsars distributed in the galaxy as a gravitational-wave detector. As shown in Figure 8.2, a network consisting of 20 pulsars, each with a variance of 100 nanoseconds and observed for 5 years, can achieve sensitivities comparable to the stochastic background produced by the radiation from binary systems of merging supermassive black holes. As for LISA, this astrophysical source of “noise” would be a discovery in itself. The amplitude and shape of such a spectrum depend on a hierarchical galaxy-formation model, and so the astrophysical information derived from positive results, or the upper limits in the absence of discovery, will advance knowledge of the evolution of the universe.

To explore these exciting new areas, the panel recommends that NSF support a coherent program in gravitational-wave detection through the timing of a sample of millisecond pulsars. Such a program must begin with searches (which are already underway) for the additional millisecond pulsars with small timing noise that are necessary to complete the pulsar “array” required for detection. Technological challenges include compensation for the effects of the interstellar medium (especially critical for extending the pulsar sample to larger distances) and refining and extending algorithms to improve sensitivity and reduce pulsar-timing residuals. It will also be necessary in the long term to develop techniques to use effectively future array telescopes, which are attractive because of their planned large collecting areas, for pulsar timing. Progress toward these technical goals should be monitored over the course of the decade. The panel was provided with an estimate of $66 million as the cost of such a program over this period. The program is a collection of activities that include observing with radio telescopes, computing and signal processing hardware, salaries, and travel. Such an estimate is difficult to verify but probably is indicative of the level of support needed. The panel notes that the pulsar-timing program requires that Arecibo, or a future telescope with similar capabilities for pulsar timing, be available to the program.

Finally, the panel also recognizes that while the ground-based gravitational-wave detectors are likely to produce their first detections in the coming decade with expansions already underway, exploitation of their full scientific potential will require improvements in sensitivity and extension of the sensitive band to lower frequencies. The ground-based detectors are sensitive to a unique mass range for sources in our galaxy. The panel therefore recommends broad agency support for continued improvement of technologies for ground-based gravitational-wave detectors, including instrument technologies and data analysis and interpretation.

As complements to a vigorous program in gravitational-wave astrophysics, the panel believes that other activities that test general relativity and theories of gravity are scientifically valuable. However, as stated in the summary of the science case

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

above, theoretical guidance is lacking as to how one might expect general relativity to break down. Therefore, the panel recommends that these activities receive support based on the following principles:

  • Favor tests of general relativity on scales and domains where it has not been well tested so far. That means emphasizing (1) quantitative tests of its strong-field predictions, for example, black holes, gravitational waves, and cosmology, and (2) tests in either strong- or weak-field regimes that are on scales larger than the solar system.

  • Favor clean tests in which the physics of the testing system is simple and calculable. That is, emphasize observations that test general relativity with a minimum of astrophysical parameters to be modeled and determined. Examples are binary pulsars and extreme-mass-ratio black hole inspirals.

  • Favor tests that substantially improve the precision of basic parameters such as the parametrized post-Newtonian parameters, the rate of change of G, and increasing the accuracy of tests of the strong and weak equivalence principles, provided that results can be obtained at moderate cost so that there is high rate of return of science for the cost.

LISA is the panel’s top priority for testing relativistic gravity. The direct detection of gravitational waves, the verification of their propagation speed, and the nature of their polarization would in itself provide a significant test of general relativity. Advanced LIGO is likely to provide the first tests of strong-field gravity by measuring waveforms of black-hole and neutron-star mergers. However, LISA’s unprecedented sensitivity to a wide range of astrophysical sources will extend these tests to many objects, including black holes at the centers of galaxies, and to much higher signal-to-noise measurements. LISA is the most direct route to testing theories of gravitation in the strong-field regime and will provide the most data on the effects of gravitation on mostly unexplored galactic scales.

High-precision measurements of distance and growth of structure as a function of redshift offer exciting opportunities to test general relativity on cosmological scales. Data deviating from the precise relationship between distance and growth predicted by general relativity would signal its breakdown. Tests of general relativity on cosmic scales are deeply entwined with fundamental questions related to cosmic acceleration and dark energy. A strong electromagnetic-survey program providing distance/growth tests of general relativity as well as high sensitivity to the dark-energy equation of state would have a profound impact on our understanding of general relativity. These survey programs are being considered by other Astro2010 panels.

A new lunar laser ranging (LLR) program, if conducted as a low-cost robotic mission or an add-on to a manned mission to the Moon, offers a promising and

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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cost-effective way to test general relativity and other theories of gravity (Figure 8.12). So far, LLR has provided the most accurate tests of the weak equivalence principle, the strong equivalence principle, and the constancy in time of Newton’s gravitational constant. These are tests of the core foundational principles of general relativity. Any detected violation would require a major revision of current theoretical understanding. As yet, there are no reliable predictions of violations. However, because of the importance of these principles, the panel favors pushing their limits when it can be done at a reasonable cost. The installation of new LLR retroreflectors to replace the 40-year-old ones might provide such an opportunity.

The panel emphasizes again its opinion that experiments to improve measurements of basic parameters of gravitation theory are justified only if they are of moderate cost. Therefore, it recommends that NASA’s existing program of small-and medium-scale astrophysics missions address this science area by considering, through peer review, experiments to test general relativity and other theories of gravity. The panel notes that a robotic placement of improved reflectors for LLR is likely to be consistent with the constraints of such a program. It returns to this recommendation below in the context of a recommendation to augment the Explorer program.

A balanced program in particle astrophysics and gravitation must include experiments designed to identify the particle or particles that make up the dark

FIGURE 8.12 Left: Retroreflector placed on the lunar surface by Apollo astronauts. Right: A 10-centimeter solid corner cube developed for the next generation of lunar-laser-ranging experiments, shown next to a 3.8-centimeter engineering model. SOURCE: Left: NASA. Right: Douglas Currie, University of Maryland, College Park.

FIGURE 8.12 Left: Retroreflector placed on the lunar surface by Apollo astronauts. Right: A 10-centimeter solid corner cube developed for the next generation of lunar-laser-ranging experiments, shown next to a 3.8-centimeter engineering model. SOURCE: Left: NASA. Right: Douglas Currie, University of Maryland, College Park.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

matter. The panel’s recommendations address only indirect astrophysical searches for dark matter, although these recommendations reflect cognizance of the context provided by direct laboratory experiments and accelerator experiments. Astrophysical indirect searches for dark matter involve both particle and gamma-ray experiments. To distinguish between a dark-matter signature and a previously unknown, local astrophysical source of high-energy particles, it is important to measure the positron spectrum to higher energies—if possible up to 1 TeV, where the expected (combined electron and positron) spectrum rolls off—to see whether the positron excess persists through that entire energy range. Because of the high cost of putting a larger magnet into orbit, positron measurements up to 1 TeV are most likely to be done best with ultralong-duration balloon flights. Improved sensitivity to anti-protons and heavier anti-nuclei, up to helium, is also desirable. Again, the masses of the needed detectors argue for the value of the ultralong-duration balloon option. Improved modeling of particle and anti-particle production from astrophysical sources is also needed.

Much can also be done by extending the electron spectrum to higher energies and better sensitivities. In the 1- to 10-TeV electron-energy range, atmospheric Čerenkov telescopes have so far yielded the best results. With an experiment having an order of magnitude more sensitivity than VERITAS or HESS, the secondary tail of local e+e production should be visible at energies above a few TeV. This would provide helpful information about very local production and propagation and would give a baseline for measurements at 100 to 1,000 GeV to help interpret any excess gamma rays there from dark matter or pulsars.

The gamma-ray measurements of atmospheric Čerenkov telescopes also provide essential information. Current constraints on dark matter annihilation models by HESS (and at lower energies by Fermi) are within an order of magnitude of either detecting or ruling out most dark-matter annihilation scenarios that can produce the PAMELA positrons. The two plots in Figure 8.13 demonstrate the potential of a next-generation Čerenkov array. A significant region of parameter space could potentially be excluded (or the effort might result in a detection!) through observations of nearby dwarf galaxies. Therefore, increasing the sensitivity of atmospheric Čerenkov telescopes by another order of magnitude is the panel’s top priority for exploring the nature of dark matter.

The future gamma-ray astronomy instruments under development are expected to improve the sensitivity by about an order of magnitude over the current generation of experiments and to cover a wider energy range. These improvements are well motivated scientifically, as the success of VERITAS, HESS, MAGIC, and Milagro has led to important progress in TeV gamma-ray astronomy over the past decade. These instruments have greatly increased the number of known sources and have studied the high-energy astrophysical processes in these objects in great detail and with good temporal, spectral, and angular resolution. The performance

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 8.13 Left: Predicted gamma-ray signal from the dwarf spheroidal galaxy Ursa Major for a dark-matter neutralino with mass of 330 GeV. Right: Plot showing the ability of the next-generation Čerenkov array to exclude predicted dark matter candidates. Each point represents a prediction of a model that is a supersymmetric extension of the standard model of particle physics. SOURCE: Left: F. Aharonian, J. Buckley, T. Kifune, and G. Sinnis, High energy astrophysics with ground-based gamma ray detectors, Reports on Progress in Physics 71:096901, 2008. Right: Courtesy of Matthew Wood, University of California, Los Angeles.

FIGURE 8.13 Left: Predicted gamma-ray signal from the dwarf spheroidal galaxy Ursa Major for a dark-matter neutralino with mass of 330 GeV. Right: Plot showing the ability of the next-generation Čerenkov array to exclude predicted dark matter candidates. Each point represents a prediction of a model that is a supersymmetric extension of the standard model of particle physics. SOURCE: Left: F. Aharonian, J. Buckley, T. Kifune, and G. Sinnis, High energy astrophysics with ground-based gamma ray detectors, Reports on Progress in Physics 71:096901, 2008. Right: Courtesy of Matthew Wood, University of California, Los Angeles.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

of current detectors can be improved by one or two (in opposite hemispheres) atmospheric Čerenkov telescope arrays with a collection area of the order of 1 square kilometer, a larger field of view, and improvement by a factor of two to three in angular resolution. Europe is moving forward with the Čerenkov Telescope Array (CTA), and compelling concepts for future improvements in capability are being studied in the United States. Proposed for development in the United States is the Advanced Gamma Ray Imaging System (AGIS; Figure 8.14), a large atmospheric Čerenkov array comprising 36 new-concept telescopes in a regular grid, separated by 120 to 150 meters. The collecting area of AGIS as proposed is about a factor of 10 larger than that of VERITAS. The sensitivity of this array is improved by a factor of 10 to 20 with respect to the previous generation. The telescope concept is a double-mirror Schwarzschild-Couder design that has the potential to widen the field of view of a single telescope while reducing aberrations that limit the angular resolution.

FIGURE 8.14 The AGIS telescope concept, based on a Schwarzschild-Couder optical design, which has the advantages over traditional designs of a wider field and shorter focal length. This design enables compact, multi-pixel, wide-area detector arrays, reducing cost and increasing reliability. SOURCE: Jim Buckley, Washington University.

FIGURE 8.14 The AGIS telescope concept, based on a Schwarzschild-Couder optical design, which has the advantages over traditional designs of a wider field and shorter focal length. This design enables compact, multi-pixel, wide-area detector arrays, reducing cost and increasing reliability. SOURCE: Jim Buckley, Washington University.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Although the panel strongly endorses the goals of increasing the field of view and improving the angular resolution, the AGIS proposal raised concerns about the project schedule and cost. The R&D phase required by such a new telescope, with its tight constraints on the roughness of the mirror surfaces, will inevitably introduce a time lag with respect to the European schedule. CTA has adopted the well-established Davies-Cotton telescope concept to ensure timely readiness of the array and its science outcome. Moreover, the CTA strategy, which combines different mirror sizes and varying baselines, enables lowering the energy threshold and covering a wider energy range than the AGIS array as proposed.

The panel is concerned about the total cost of a stand-alone U.S. effort. It is also concerned that AGIS will not be competitive with the European CTA experiment, which is on a considerably shorter time schedule. Given these considerations, the panel recommends that there be U.S. involvement, jointly supported at a significant level by DOE and NSF, in an international Čerenkov array with a square-kilometer effective area. An international collaboration seems necessary for the full potential of the next-generation array to be realized. In addition, R&D by the U.S. group on new telescope technologies should be encouraged. This could lead to the possibility of adding or replacing telescopes with a new design in the future. The panel also encourages R&D on the cameras and photodetectors. Finally, the planning of the future AGIS-CTA science program should include provisions for broad access to the data. This should most certainly include a public data archive and might also include observing time being made available to the community on a competitive basis.

Unfortunately, the planning and development of the next-generation atmospheric Čerenkov arrays are at such an early stage that an accurate cost appraisal is not possible. The European CTA consortium has provided the panel with an estimate of the cost of the CTA of €180 million. However, a joint European-U.S. array might involve technologies substantially different from those now being considered for the CTA. Furthermore, European cost accounting typically does not include significant in-kind contributions, largely in the form of salaries, that quite possibly could double the true cost. The independent cost evaluation carried out as part of the decadal survey review activities concluded that a cost appraisal made at this time would be very uncertain. Therefore, the panel doubled the CTA estimate to account for in-kind contributions, and then took half that as the U.S. contribution, arriving at approximately $200 million for the U.S. share. Although the panel strongly supports the goals of an international Čerenkov array, it is quite clear that a thorough review of cost and technical feasibility will have to be carried out by the funding agencies before a decision can be made to proceed.

In addition to a strong program in atmospheric Čerenkov detection, the panel recognizes the promise of HAWC, a higher-energy, wide-field-of-view, and high-duty-cycle observatory. Based on a compact array of water Čerenkov detectors

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

at high altitude, HAWC would operate at a lower energy threshold and would be an order of magnitude more sensitive than the previous generation of wide-field TeV gamma-ray telescopes, allowing it to carry out deeper surveys and to catch weaker high-energy transient events. This kind of capability will be best utilized in conjunction with the operation of Fermi to guarantee the ability of observing transient sources over a wide energy range. A new approach to atmospheric Čerenkov detection, aimed at the very highest energies, is space-based observation of air showers that enables the use of a very large volume of the atmosphere as a detector. Participation in international efforts to develop this technique is a promising path for the future. The panel concluded that a balanced program must include support for these smaller, peer-reviewed programs.

Recent cosmic-ray observations have challenged the current understanding of local cosmic ray sources and their propagation to Earth. Dark-matter models have been proposed to explain these findings. A continuing ability to observe galactic cosmic rays is important in order to disentangle dark-matter signatures from nearby astrophysical sources. As described above, experiments carried by balloons have made an important contribution to dark-matter and cosmic-ray studies. The panel expects that the capability of balloons to transport the large masses required for achieving interesting event rates—enabling a number of different measurements that allow cross-checking of results with different systematic errors—will be critical to future progress. One example of a promising experiment is a search for anti-deuterons, with a very low expected background; it is anticipated that an ultralong-duration balloon (ULDB) experiment would improve limits to be set by AMS by an order of magnitude. Another example is the radio detection of ultrahigh-energy neutrinos, with expected improvements in sensitivity of an order of magnitude provided by the long exposures of ULDB flights, particularly if trajectory control enables the directing of flights away from populated areas with higher backgrounds. Because of its promise, the panel recommends that NASA maintain support for the ULDB program to provide the capability for a strong program in cosmic-ray and neutrino detection.

At ultrahigh energies, the ability to point back to the most powerful cosmic accelerators has been reported recently by the Pierre Auger Observatory. The next decade may see the identification of the sources of ultrahigh-energy cosmic rays and the ability to test hadronic interactions at energies above those at which laboratory accelerators operate. The limited statistics of the events observed at the highest energies have led the international Pierre Auger collaboration to propose a much larger northern observatory in Colorado to augment the results obtainable from continued operation of Auger South. Funding for the U.S. fraction of this international effort could be accommodated in a high-budget scenario for the decade, as discussed in the panel’s budget analysis below, in a program that preserves the balance between small and large programs.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

The construction of the IceCube detector at the South Pole is currently underway. IceCube may open the window of neutrino astronomy early in the next decade. In addition to the continuation of this effort in TeV/PeV neutrino astronomy, the panel supports the development of complementary techniques for neutrino detection at the highest energies in order to study neutrino interactions at energies above those achieved by laboratory accelerators.

Infrastructure Issues
The Base Program

All four government funding-agency units reserve a fraction of their budgets for a “base” program that supports the operation of existing projects and facilities, technology development, laboratory programs, data analysis, computation, and theory. At NASA and NSF, a substantial fraction of the support to university groups is provided through peer-reviewed programs open to individual investigators. In the DOE program, the individual-investigator component is relatively small, with support for particle astrophysics targeted chiefly at established efforts at DOE laboratories and university groups engaged in related project development and analysis. In all cases, the panel considers the base program to be critical to the realization of the scientific goals of the projects and facilities and to carrying out the precursor studies and prototyping that enable future activities. The panel supports continued investment in these areas with interagency coordination that fosters peer-reviewed competition. Under no plausible budget scenario would the panel recommend a reduction in the fraction of the program devoted to the base program; in fact, it recommends augmentations in certain areas as discussed below.

Technology Development

In all areas of astronomy and astrophysics research, the invention and the development of innovative technology have been key to progress. For the field to remain vital, it is essential that adequate funding be made available to encourage the birth of new technical concepts and to allow promising concepts to be brought to sufficient maturity so they can be incorporated into new experiments with minimal risk. However, enabling technology development has been a challenge for all federal agencies, given competition for resources and the uncertain payoffs of investing in technology the application of which is not guaranteed. Technologies are evaluated according to their technology readiness level (TRL): TRLs 1 to 4 are associated with the validation of basic concepts, analytical evaluations of expected performance, and breadboard testing of key components; TRLs 5 to 7 are associated with the fabrication and testing of prototypes in a representative environment; and TRLs 8

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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to 9 apply to fully representative systems that have been successfully deployed. The cost of technology development grows steeply with increasing TRL. NASA, NSF, and DOE have all struggled with diverse mechanisms to meet those costs in their own way, with mixed success. In the view of the panel, the situation has not been optimal at any of the three agencies.

Over the years, NASA has had various grant-funding mechanisms to seed development of new technologies at TRLs 1 to 3. However, a significant gap has existed for mid-TRLs (4 to 7). The costs of development of representative prototypes typically requires funding at a level of multimillion dollars per year, even for a single subsystem. There has been no mechanism to provide that level of funding for projects that have not yet been awarded Phase A approval. Yet “technical immaturity” has been cited as a rationale for the denial of Phase A approval for projects that were otherwise ranked very highly on scientific grounds. In the President’s Budget for FY2011, significant funding was made available for far-term investment in space technology. This is a very promising development; however, it remains to be seen how responsive that new funding line will be to the needs of astrophysics missions, both in the Explorer and major mission lines. In the panel’s view, an augmentation of $300 million over the decade to the base program at NASA is required for technology development to support missions addressing the science areas of particle astrophysics and gravitation. The panel believes that a further augmentation for other science areas is also justified, but a detailed analysis is beyond the scope of this panel. This would represent a reasonably small fraction of the total level of funding identified for the new space technology investment and so may not be incompatible with current NASA plans.

At NSF, the gap in technology-development funding parallels the well-known gap in project funding between the Major Research Instrumentation (MRI) awards and the Major Research Equipment and Facility Construction (MREFC) awards. MRIs can be approved at relatively low TRL values, and so the cost of technology development is appropriately absorbed into the cost of the project. However, given increasing attention to cost and schedule validation in the MREFC approval process, all future MREFCs will be required to demonstrate a high level of technical maturity in all enabling subsystems before they can proceed into development.

Because costs for MREFC construction do not come out of the divisional budgets, they are not, in principle, in competition with grants to university-based investigators. But in the present system pre-project-approval technology-development costs must come from the sponsoring division. It is not unusual for costs associated with bringing key technologies to final design readiness to amount to 30 percent or more of the construction costs. The panel therefore recommends that the division’s budgets include an augmentation for technology development.

Funding for particle astrophysics projects at DOE has come mostly from the Office of High Energy Physics (OHEP) within the Office of Science. Until recently, funding for technology development for future projects was handled naturally

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

through the university grants programs and through base funding at the national laboratories, and this system worked reasonably well. In the last few years, however, this system has evolved in a more conservative direction, and funding for technology development is very limited until a given project reaches a relatively advanced state of approval. The rationale has been to avoid spending significant amounts of money on projects that may never be constructed. That has led to a state of affairs similar to that described above for NASA and NSF: there is a noticeable gap in the availability of funding for mid-TRL technologies. This panel recommends that OHEP should also increase the funding available for candidate projects that are still awaiting approval. Therefore, the panel recommends an augmentation in the base program at OHEP to support technology development for experiments addressing the science areas of particle astrophysics and gravitation.

NASA’s Explorer Program

For several decades, NASA’s Explorer program has supported relatively small missions with focused scientific goals. In the current implementation of the program, proposals for small-scale missions (SMEX; capped at around $150 million, excluding launch), mid-scale missions (MIDEX; capped at around $250 million, excluding launch), and Missions of Opportunity (instruments to be flown on non-NASA missions) are selected according to peer review. The missions are led by a principal investigator who has the ultimate responsibility for scientific leadership, management, and the overall success of the mission. In the past, the goal of the Explorer program—in astrophysics and heliospheric science combined—has been a launch rate of about one mission per year, responding to new scientific opportunities in a timely way. The program has produced some remarkable recent successes, such as the Wilkinson Microwave Anisotropy Probe, the Swift high-energy transient mission, and the Galaxy Evolution Explorer. In the past decade, however, cost overruns in large missions and overall NASA budget constraints led to severe cutbacks in the Explorer program, and only three new missions in astrophysics were approved. The current oversubscription rate is very large, with a 6 percent success rate in the most recent Explorer competition. The panel recommends that NASA’s Explorer program be restored to its previous funding level and launch rate. Within the particle astrophysics and gravitation science area, the panel suggests that these funds be used to carry out—if justified by peer review—innovative missions that address tests of general relativity and other theories of gravity. It might also be possible to carry out indirect searches for dark matter on the Explorer platform. One of the strengths of the Explorer program is that missions are chosen based on the strength of the science case and on technical feasibility, independent of the specific science topic. The panel thus recommends an augmentation to the Explorer program in the next decade. Adding three Explorer opportunities over this period, at an average cost of $300 million per opportunity (including an allowance for

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Missions of Opportunity and launch costs), would require an augmentation of $900 million. The panel intends that this recommendation pertain to all areas of astrophysics, exclusive of the heliospheric science portion of the Explorer program, which is outside the scope of this study.

NASA’s Balloon Program

NASA’s balloon program supports the astrophysics community in several unique ways. It provides a testing environment that qualifies new technology for spaceflight missions, a critical step in the development of a payload. It provides a platform for certain experiments that can be conducted more cheaply and quickly on suborbital balloon flights than in space. It provides a means to conduct experiments whose large masses would make an orbital implementation prohibitively expensive. Finally, it provides an environment in which students and experimentalists can be trained for future leadership in space missions and suborbital experiments. In the discussion above, the panel recommends that ULDB development be completed and that ULDB flights be supported for the purposes of conducting experiments in the detection and characterization of dark-matter and cosmic-rays. The balloon program cuts across many scientific areas and is also being considered by other Program Prioritization Panels. For this panel’s purposes, it recommends that the development of technologies needed for ULDB flights be completed and that a ULDB program of one or two flights per year be supported, including their payloads, possibly replacing some long-duration balloon flights. The panel estimates that the cost for this capability requires an augmentation to the balloon program of about $250 million over the next decade.

Theory

The panel recognizes the important role that theoretical investigations play in the advancement of knowledge in astronomy and astrophysics. “Theory” encompasses a wide range of activities and includes “blue sky” theory, pencil-and-paper analysis, simulation, and investigation of data analysis techniques. The recent success of NASA-supported theoretical investigations of astrophysical gravitational-wave sources also highlights the importance of strategic theory in enabling new avenues of investigation. The panel supports a strong base program at the funding agencies that includes theory as one of its components and that is flexible enough to include new interdisciplinary topics. In addition, the panel draws attention to two particular issues. The first is the difficulty of properly supporting areas that straddle the traditional boundary between physics and astronomy. The second is the lack of support at universities and research centers for cluster computing, which is necessary as a testbed for parallel-computing programs at the national supercomputing centers.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Most of modern astronomy leans on physics and chemistry for basic conceptual and technical tools needed to interpret the universe and its contents. Astronomy also initiates new fields of inquiry in physics and chemistry. For example, astronomical discoveries such as dark matter and dark energy have opened new directions in fundamental physics beyond the standard model of particle physics. Similarly, studies of cosmic rays and of compact objects have driven new directions in plasma physics. The development of new concepts and tools in these and other areas of physics and chemistry now proceed with astronomical observation and modeling as an integral part of the studies of the basic physics. There is a concern that the mechanisms that funding agencies use to allocate resources to theoretical activities are not always effective in funding new areas of intersection in physics and astronomy, and the panel urges that the agencies structure their reviews so that these new areas receive full consideration.

In theoretical astrophysics, computation has become essential for progress, with analytic estimates and models still important as first steps in formulating concepts and for developing computational models of observed phenomena. Much effort has gone into developing computing resources at the national scale that are open to all users and that are located in a variety of supercomputer centers sponsored by NSF, NASA, and DOE. However, resources for intermediate-scale, massively parallel computing are also needed to provide the rapid turnaround essential for algorithm and code development and for student education in computational techniques. Access to such resources has been difficult at the national centers. They tend to be provided through various local funding strategies, such as startup packages for new faculty with and interest in computational techniques, but such strategies are clearly inadequate to keep up with replacements of and improvements in equipment, which are needed on a 3- to 5-year cycle. Funding of such medium-scale facilities needs to be stabilized. Recognizing that development programs on medium-scale cluster computers are essential for the effective use of supercomputing centers, the panel is of the opinion that a reasonable fraction of the resources devoted to supercomputing should be targeted to medium-scale facilities suitable for algorithm development and rapid exploration of concepts. Such funds should be competed following normal peer-review procedures. The panel also thinks it would be beneficial for an organization such as the American Astronomical Society to provide a forum that encourages efforts in student training in computational astrophysics.

Laboratory Astrophysics

An important component of research in astronomy and astrophysics is experimentation that is not an end in itself, but that provides data necessary for the interpretation of experiments and observations aimed at scientific issues. (Thus, experiments whose main motivation is to probe fundamental physics would not

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

fall under the category of laboratory astrophysics.) Traditional areas include, for example, laboratory measurements of atomic cross sections and rates. The panel notes that the experimental techniques of particle astrophysics and gravitation are used in some measurements critical to the interpretation of experiments and observations under consideration by this panel and by other panels. Measurements of nuclear cross sections are needed to inform studies of nucleosynthesis and stellar evolution. Laboratory experiments in plasma and fluid physics are needed to inform quantitative modeling of astronomical systems in, for example, the areas of fluid and magnetohydrodynamic turbulence, high-energy particle acceleration in turbulent plasmas, magnetic reconnection, and collisionless shocks. When assessing their programs the funding agencies must include consideration of the indirect yet important benefit of such experiments.

RECOMMENDATIONS

The first detection of gravitational waves is likely to take place in the next decade with ground-based detectors. Substantial progress in testing general relativity and studying astrophysical sources requires exploiting the lower-frequency part of the spectrum, which can be done only from space. The potential scientific benefit is enormous, because quantitative strong-field tests of general relativity will be possible for the first time, and a qualitatively new window for studying astrophysical systems at a broad range of redshifts will be opened. A great deal of technical progress has been made in the past decade, and a successful LISA pathfinder will eliminate much of the remaining risk. The panel recommends that the LISA mission be given the highest priority for a new start in the next decade, given the extensive technology development that has already been completed, the expected short time until the LISA Pathfinder (LPF) mission launch, and the need to maintain momentum in the U.S. community and guarantee a smooth transition to a joint NASA-ESA mission. The panel recommends that NASA funding of LISA begin immediately, with continuation beyond LPF contingent on the success of that mission.

LISA will not be sensitive to the lowest frequencies of gravitational waves that are predicted. This portion of the spectrum probes the fundamental question, How did the universe begin? It might also provide signals from merging supermassive black holes. Pulsar timing is a promising technique for detecting very-low-frequency gravitational waves, and the panel recommends that NSF provide support for a coherent program in gravitational-wave detection through timing of millisecond pulsars.

Although much progress has been made in the past decade in testing general relativity in the weak-field limit and on scales of the solar system, little has been done to test strong-field general relativity and gravitation on large (cosmological) scales. The discovery that the universe is apparently accelerating may be a manifes-

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

tation of a breakdown of general relativity. As yet, theory provides little guidance as to where best to search for deviations from general relativity. Therefore, it makes sense to carry out experiments that favor scales and domains where the theory has not yet been tested, tests that are relatively unambiguous in their predictions, and experiments that improve the precision of measurement of basic parameters in a cost-effective way. Therefore, the panel recommends that NASA’s existing program for small- and medium-scale astrophysics missions (the Explorer program) include consideration, through peer review, of experiments to test general relativity and other theories of gravity.

Over the past decade, ground-based arrays of imaging Čerenkov telescopes have discovered gamma-ray emission from a wide variety of astrophysical sources. There is a strong case for moving forward with studies of the extreme physics of these sources, probing the highest-energy particles and photons, the strongest magnetic fields, and the strongest gravitational fields. Furthermore, gamma-ray signatures of dark-matter annihilation and decay provide a promising signal for dark-matter searches. The panel recommends U.S. involvement, supported jointly at a significant level by DOE and NSF, in an international Čerenkov array with an effective area of a square kilometer. The future science program should include broad access to the data by the community.

For several decades, NASA’s Explorer program has supported small missions with focused and timely science goals, and the program has produced some remarkable successes. Yet the current program has been cut back to the point that the oversubscription rate is very large. To support its recommendation for opportunities for space experiments focused on tests of gravitation, and possibly dark-matter searches and particle astrophysics, the panel recommends that the Explorer program be restored to its previous launch rate. To support reestablishing the launch rate for astrophysics missions in particular, the panel recommends that the Explorer program be augmented by $900 million for astrophysics missions over the next decade.

Recent measurements of certain particle excesses in the spectra of cosmic rays may be the first indirect evidence for dark-matter particles. Or these results may signify new, nearby sources of cosmic rays. Future progress in indirect detection of dark matter requires a better understanding of cosmic-ray acceleration mechanisms and propagation processes, and further improvements in sensitivity to dark-matter annihilation and decay products. Ultralong-duration ballooning could enable new experiments by providing a combination of long integration times and the transport of massive detectors. The panel recommends that NASA support ultralong-duration balloon technology development and augment the balloon program to support ULDB missions for indirect detection of dark matter and for cosmic-ray physics and astrophysics.

All three funding agencies that support activities related to particle astrophysics and gravitation maintain base programs that are critical to the realization of sci-

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

ence goals and that enable future projects, missions, and facilities. It is important that investment continue in these areas. The panel recommends that the funding agencies maintain their levels of base funding, and that, in addition, certain specific components of the base programs be augmented as identified below.

In all areas of astronomy and astrophysics, the invention and development of innovative technology are critical to success. It is the view of the panel that in the past decade the level of funding for technology development has been lower than optimal, with a particular shortfall in the development that addresses preparation of mission-specific technologies. At NSF, there is no good mechanism for supporting the technology development necessary to fully design and cost major facilities. The panel recommends that the base programs in astronomy and astrophysics be augmented for technology development at all three agencies—NASA, NSF, and DOE—across divisional boundaries.

The panel recognizes the important role that theoretical investigations play in the advancement of knowledge in astronomy and astrophysics, and that theory encompasses a wide range of activities. The panel supports a strong base program that includes theory as one of its components. It draws attention to two issues—supporting new physics and astronomy topics that fall outside traditional boundaries, and supporting mid-range computing facilities that provide for training and development.

In Table 8.4 the panel presents its recommendations, including costs, for space-based activities in particle astrophysics and gravitation (except for the Explorer

TABLE 8.4 Panel’s Recommendations for Space-Based Activities

Activity

Project’s Cost Estimate ($M)

Panel’s Cost Estimate ($M)

Large

 

 

LISA (U.S. cost only)

900a

1,500b

Other (unranked)

 

 

Explorer augmentation

900

NASA technology development augmentation

300c

Ultralong-duration balloon R&D and augmentation

305

250c

TOTAL

 

2,950

NOTE: All costs are in fixed-year 2009 dollars.

aBased on an assumption of a €650 million contribution from Europe in a joint U.S.-European project.

bCost appraisal by an independent assessment, assuming 50 percent U.S. participation in a joint U.S.-European project and including launch costs and 5 years of operations.

cThese augmentations are the panel’s recommendations for particle astrophysics and gravitation-related activities only, and not for the entire astrophysics program.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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TABLE 8.5 Panel’s Estimates of Total Funds Allocatable for New Initiatives for the Next Decade Under Two Assumed Budget Scenarios

 

Optimistic Budget Scenario ($M)

Pessimistic Budget Scenario ($M)

NSF/Astronomy

324

324

DOE Particle Astrophysics

390

221

NOTE: All costs are in fixed-year 2009 dollars.

augmentation, which includes all astrophysics Explorer missions). For the “other” category, the panel recognizes that all are important components of a balanced program, and the list is not a ranked list.

For the ground-based program, the panel makes its recommendations within the context of two assumed budget scenarios for the Department of Energy. For both budget scenarios the panel assumes a “budget doubling” profile for NSF’s Division of Astronomical Sciences; without such an assumption, no new activities are possible for that division. Budget projections for NSF’s Physics Division were not available to the panel. For the DOE Office of Science (particle astrophysics only) budgets the panel assumed an “optimistic” scenario and a “pessimistic” scenario. Table 8.5 shows the total funds that can be allocated over the next decade for new initiatives under the two different budget assumptions.

It is not realistic to assume that all the funds projected in Table 8.5 will be allocated only to the activities under consideration by the panel, which therefore makes the assumption that 50 percent of the funds will be available for activities in particle astrophysics and gravitation.

The costs for the recommended ground-based activities under the two budget scenarios are tabulated in Tables 8.6 and 8.7. The list in the “medium” category is a ranked list. Table 8.6 does not include small projects.

SUMMARY TABLE

Table 8.8 shows the relationship between the capabilities of activities endorsed by the Panel on Particle Astrophysics and Gravitation and the scientific priorities identified by the Astro2010 Science Frontiers Panels.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

TABLE 8.6 Panel’s Recommendations for Ground-Based Activities, Optimistic Budget Scenario

Activity

Agency

Project’s Cost Estimate ($M)

Panel’s Cost Estimate ($M)

Large

 

 

 

AGIS-CTA (U.S. portion)

DOE

100a

 

NSF/Astronomy

50a

 

NSF/Physics

50a

Medium (ranked)

 

 

 

Pulsar timing array for gravitational wave detection

NSF/Astronomy

70

70b

Technology development augmentation

DOE

65

 

NSF/Astronomy

42

 

NSF/Physics

42

Auger North (U.S. portion)

DOE

60

30c

 

NSF/Physics

 

30c

TOTAL

 

 

479

DOE

 

 

195

NSF/Astronomy

 

 

162

NSF/Physics

 

 

122

NOTE: All costs are in fixed-year 2009 dollars.

aThe AGIS and CTA projects are at a preliminary stage, and the cost appraisals are not reliable. Therefore, these numbers should be taken to be illustrative only, although they are consistent with the independent cost appraisal carried out as part of the survey.

bThe cost is the project’s estimate for costs associated with technique development, observing, and data analysis. No independent cost appraisal is available.

cNo independent cost appraisal for Auger North was carried out, and the panel adopted the project’s cost appraisal. The cost listed in this table assumes $42 million for the U.S. share of construction costs ($40 million increased by 6.1 percent for inflation), and 10 years of operations at $1.8 million per year (one-third of the total project operations cost of $5.4 million per year).

TABLE 8.7 Recommendations for Ground-Based Activities, Pessimistic Budget Scenario

Activity

Agency

Project’s Cost Estimate ($M)

Panel’s Cost Estimate ($M)

Large

 

 

 

AGIS-CTA (U.S. portion)

DOE

90a

 

NSF/Astronomy

50a

 

NSF/Physics

50a

Medium (ranked)

 

 

 

Pulsar timing array for gravitational wave detection

NSF/Astronomy

70

70b

Technology development augmentation

DOE

20

 

NSF/Astronomy

42

 

NSF/Physics

42

Auger North (U.S. portion)

DOE

60

0

 

NSF/Physics

 

0

TOTAL

 

 

394

DOE

 

 

110

NSF/Astronomy

 

 

162

NSF/Physics

 

 

92

NOTE: All costs are in fixed-year 2009 dollars.

aThe AGIS and CTA projects are at a preliminary stage, and the cost appraisals are not reliable. Therefore, these numbers should be taken to be illustrative only, but they are consistent with the independent cost appraisal carried out as part of the survey.

bThe cost is the project’s estimate of costs associated with technique development, observing, and data analysis. No independent cost appraisal is available.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

TABLE 8.8 Activities in Particle and Gravitational Astrophysics That Address Astro2010 Science Frontiers Panel Questions

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

NOTE: Shaded entry, direct connection to science question. Unshaded entry, indirect or possible connection but not guaranteed.

Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

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Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Suggested Citation:"8 Report of the Panel on Particle Astrophysics and Gravitation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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×
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Every 10 years the National Research Council releases a survey of astronomy and astrophysics outlining priorities for the coming decade. The most recent survey, titled New Worlds, New Horizons in Astronomy and Astrophysics, provides overall priorities and recommendations for the field as a whole based on a broad and comprehensive examination of scientific opportunities, infrastructure, and organization in a national and international context.

Panel Reports--New Worlds, New Horizons in Astronomy and Astrophysics is a collection of reports, each of which addresses a key sub-area of the field, prepared by specialists in that subarea, and each of which played an important role in setting overall priorities for the field. The collection, published in a single volume, includes the reports of the following panels:

  • Cosmology and Fundamental Physics
  • Galaxies Across Cosmic Time
  • The Galactic Neighborhood
  • Stars and Stellar Evolution
  • Planetary Systems and Star Formation
  • Electromagnetic Observations from Space
  • Optical and Infrared Astronomy from the Ground
  • Particle Astrophysics and Gravitation
  • Radio, Millimeter, and Submillimeter Astronomy from the Ground

The Committee for a Decadal Survey of Astronomy and Astrophysics synthesized these reports in the preparation of its prioritized recommendations for the field as a whole. These reports provide additional depth and detail in each of their respective areas. Taken together, they form an essential companion volume to New Worlds, New Horizons: A Decadal Survey of Astronomy and Astrophysics. The book of panel reports will be useful to managers of programs of research in the field of astronomy and astrophysics, the Congressional committees with jurisdiction over the agencies supporting this research, the scientific community, and the public.

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