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



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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 physi- cal 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 begin- ning 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 stochas- tic 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 red- shift will provide unique information about this era, complementing other probes. Another puzzle is that of the laws of nature in the environments that harbor 379

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Panel rePorts—new worlds, new HorIzons 380 the most extreme gravitational fields. Supermassive black holes inhabit the centers of galaxies, and they somehow—following the laws of gravity—generate tremen- dous 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 astro- physics issues. Much of the precursor work of the past decade was motivated by the scientific imperative of understanding gravitation, and an intense period of technol- ogy 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 tech- niques for pulsar observations that are promising probes of the gravitational waves associated with inflation and with supermassive black holes. Recognizing these de- velopments, 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 detec- tors continue their ongoing program of operation, upgrade, and further operation; and that the detection of gravitational waves through the timing of millisecond 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 electromagnetic 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-

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rePort Panel PartIcle astroPHysIcs G r av I tat I o n 381 of tHe on and 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 dark- matter 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 blos- somed 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 pro- vide 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 uni- verse, 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 exper- tise 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 bal- loons 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 neu- trino 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 abil- ity to participate in research in astrophysics in the future, the panel recommends

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Panel rePorts—new worlds, new HorIzons 382 that the technology-development programs of all three funding agencies relevant to particle astrophysics and gravitation be augmented. To enable missions to test theo- ries 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 ulti- mate goal of all these activities is the advancement of knowledge, for the achieve- ment 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 recom- mends 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 detec- tors 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 inter- section 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 as- trophysical 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

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rePort Panel PartIcle astroPHysIcs G r av I tat I o n 383 of tHe on and 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 gravita- tional 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.

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Panel rePorts—new worlds, new HorIzons 384 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 detec- tion 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 red- shift. See Box 8.1. Gravitational waves are a distinctive cosmic messenger. They carry informa- tion 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 dif- ferent 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 gravita- tional 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)

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rePort Panel PartIcle astroPHysIcs G r av I tat I o n 385 of tHe on and BOX 8.2 Science from Gravitational Waves SFP Questions Addressed Measurements Addressing the Questions GCT 1 ow do cosmic structures form and H Tracing galaxy-merger events by detecting and evolve? recording the gravitational-wave signatures GCT 3 ow do black holes grow, radiate, and H Using gravitational-wave inspiral waveforms to influence their surroundings? map the gravitational fields of black holes GCT 4 hat were the first objects to light up W Identifying the first generation of star formation the universe, and when did they do it? through gravitational waves from core-collapse events SSE 2 W hat are the progenitors of Type Ia Detecting and recording the gravitational wave supernovae and how do they explode? signatures of massive-star supernovae, of the spindown of binary systems of compact objects, SSE 3 H ow do the lives of massive stars end? and of the spins of neutron stars SSE 4 W hat controls the mass, radius, and spin of compact stellar remnants? CFP 1 ow did the universe begin? H Detecting and studying very-low-frequency gravitational waves that originate during the CFP 2 hy is the universe accelerating? W inflationary era 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 remark- ably, 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 minus- cule: 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

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Panel rePorts—new worlds, new HorIzons 386 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 back- ground) 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 pul- sars 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 10–10 to 10–8 Hz, a band that con- tains 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 >108 M◉. 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 ac- cumulate 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

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rePort Panel PartIcle astroPHysIcs G r av I tat I o n 387 of tHe on and Wavelength (m) Pulsar Timing LISA Advanced LIGO 108+108 BBH, 1 Gpc 107+107 BBH, 1 Gpc Strain (1/?Hz) Relic GWs 106+106 BBH SMBH background 105+105 BBH Known binaries 104+104 BBH White dwarf galactic background 103+103 BBH 10+10 EMRI 6 102+102 BBH 102+102 BBH, 1 Gpc Frequency (Hertz) FIGURE 8.2 Strain amplitude sensitivity expected for pulsar timing (red), LISA (green), and Advanced LIGO 8-2 edit.eps (blue). The continuous curves show strain-noise-amplitude spectral density. The pulsar-timing sensitivity as- sumes the use of 20 pulsars with 100-ns timing residuals. The dashed magenta curves show the instantaneous bitmaps, masks, vector elements evolving in frequency to strain of gravitational waves emitted by binary black hole (BBH) systems 1 Gpc away, 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

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Panel rePorts—new worlds, new HorIzons 388 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

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rePort Panel PartIcle astroPHysIcs G r av I tat I o n 389 of tHe on and 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” com- pact 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 his- tory 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 num- ber 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 analy- sis 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”

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Panel rePorts—new worlds, new HorIzons 428 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 stel- lar 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 assess- ing 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 de- cade 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 astrophysi- cal 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 continu- ation 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 ques- tion, 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 pro- vide 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-

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rePort Panel PartIcle astroPHysIcs G r av I tat I o n 429 of tHe on and 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 ef- fective 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 re- markable successes. Yet the current program has been cut back to the point that the oversubscription rate is very large. To support its recommendation for opportuni- ties 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 detec- tion 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 bal- loon 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-

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Panel rePorts—new worlds, new HorIzons 430 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 prepara- tion 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 astrophys- ics 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—sup- porting new physics and astronomy topics that fall outside traditional boundar- ies, 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 900a 1,500b LISA (U.S. cost only) Other (unranked) Explorer augmentation — 900 300c NASA technology — development augmentation 250c Ultralong-duration balloon 305 R&D and augmentation 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.

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rePort Panel PartIcle astroPHysIcs G r av I tat I o n 431 of tHe on and TABLE 8.5 Panel’s Estimates of Total Funds Allocatable for New Initiatives for the Next Decade Under Two Assumed Budget Scenarios Optimistic Budget Pessimistic Budget Scenario ($M) Scenario ($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.

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Panel rePorts—new worlds, new HorIzons 432 TABLE 8.6 Panel’s Recommendations for Ground-Based Activities, Optimistic Budget Scenario Project’s Cost Panel’s Cost Activity Agency Estimate ($M) Estimate ($M) Large 100a AGIS-CTA (U.S. portion) DOE — 50a NSF/Astronomy — 50a NSF/Physics — Medium (ranked) 70b Pulsar timing array for gravitational wave detection NSF/Astronomy 70 Technology development augmentation DOE — 65 NSF/Astronomy — 42 NSF/Physics — 42 30c Auger North (U.S. portion) DOE 60 30c NSF/Physics 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 $5.4 million per year). TABLE 8.7 Recommendations for Ground-Based Activities, Pessimistic Budget Scenario Project’s Cost Panel’s Cost Activity Agency Estimate ($M) Estimate ($M) Large 90a AGIS-CTA (U.S. portion) DOE — 50a NSF/Astronomy — 50a NSF/Physics — Medium (ranked) 70b Pulsar timing array for gravitational wave detection NSF/Astronomy 70 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.

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TABLE 8.8 Activities in Particle and Gravitational Astrophysics That Address Astro2010 Science Frontiers Panel Questions Missions Pulsar Timing Lunar Laser Science Question LISA Array Ranging AGIS/CTA HAWC ULDB Auger N Planetary Systems and Star Formation PSF 1: How do stars — — — — — — — form? PSF 2: How do — — — — — — — circumstellar disks evolve and form planetary systems? PSF 3: How diverse — — — — — — — are planetary systems? PSF 4: Do habitable — — — — — — — worlds exist around other stars, and can we identify the telltale signs of life on an exoplanet? Discovery area: — — — — — — — Identification and characterization of nearby habitable exoplanets continued 433

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TABLE 8.8 Continued 434 Missions Pulsar Timing Lunar Laser Science Question LISA Array Ranging AGIS/CTA HAWC ULDB Auger N Stars and Stellar Evolution SSE 1: How do — — — Gamma rays Gamma rays — Ultrahigh- rotation and from stars, from stars, energy cosmic- magnetic fields affect binary systems, binary systems, ray probe stars? supernova supernova of galactic remnants… remnants… magnetic field relevant for star formation SSE 2: What are the White-dwarf/ Provides pulsar — — — — — progenitors of Type white-dwarf survey Ia supernovae? binaries in galaxy, which can be progenitors of Type Ia supernovae SSE 3: How do the Black holes Provides — Gamma rays Gamma rays — Cosmic rays and lives of massive from the first millisecond from gamma-ray from GRBs, neutrinos from stars end? generation of pulsar survey bursts (GRBs), supernova GRBs stars supernova remnants remnants SSE 4: What controls >104 compact Requires — — — — — the mass, radius, binaries in the discrete sources and spin of compact galaxy stellar remnants? Discovery area: Binary black — — — Gamma-ray — — Time-domain surveys hole mergers transients and extreme mass ratio inspirals

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Galactic Neighborhood GAN 1: What are — — — — — — — the flows of matter and energy in the circumgalactic medium? GAN 2: What — — — Local interstellar Local ISM Local ISM Ultrahigh-energy controls the mass- medium (ISM) cosmic-ray energy-chemical probe galactic cycles within magnetic field galaxies? GAN 3: What is Mergers of the Background of — — — — — the fossil record first black holes, gravitational of galaxy assembly relics of the first waves from very from the first stars generation of massive black to the present? stars; important hole mergers in determining mass of “seed” black holes GAN 4: What are the Distribution of — — Dark matter Dark matter Dark matter — connections between black hole mass indirect indirect indirect dark and luminous and spin via searches searches searches matter? mergers and extreme mass ratio inspirals Discovery area: Binary black Requires — High-energy High-energy High-energy Ultrahigh-energy Time-domain hole mergers discrete sources flares (e.g., flares (e.g., flares (neutrino) flares astronomy and extreme GRBs, blazars, GRBs, blazars, mass ratio magnetars) magnetars) inspirals continued 435

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TABLE 8.8 Continued 436 Missions Pulsar Timing Lunar Laser Science Question LISA Array Ranging AGIS/CTA HAWC ULDB Auger N Galaxies Across Cosmic Time GCT 1: How do Black hole Background of — — — — — cosmic structures mergers out gravitational form and evolve? to z > 10; waves from very distribution of massive black masses and holes constrains spins with z models GCT 2: How do — — — — — — — baryons cycle in and out of galaxies, and what do they do while they are there? GCT 3: How do black Black hole Provides pulsar — Gamma rays Gamma rays Neutrinos from Ultrahigh-energy holes grow, radiate, mergers at survey from active from AGN, AGN, GRBs cosmic rays and influence their redshifts up galactic nuclei GRBs from AGN, surroundings? to z > 10 and (AGN), GRBs GRBs extreme mass ratio inspirals out to z ~ 1 GCT 4: What were Seed black — — — — Neutrinos from Neutrinos from the first objects to holes from the GRBs GRBs light up the universe, first generation and when did they of stars do it? Discovery area: The Black hole — — — — Neutrinos from Neutrinos from epoch of reionization mergers at high GRBs GRBs z > 10

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Cosmology and Fundamental Physics CFP 1: How did the Gravitational Gravitational Tests of general — — — — universe begin? waves are direct waves are direct relativity probe of early probe of early universe universe CFP 2: Why is Strong, direct Gravitational Tests of general — — — — the universe tests of general wave relativity accelerating? relativity from background gravitational model wave sources dependent CFP 3: What is dark — — — Indirect dark Indirect dark Indirect dark — matter? matter searches matter searches matter searches (gamma rays (gamma rays (positrons, anti- from dark from dark nuclei, …) matter halo) matter halo) CFP 4: What are — — — — — Greisen- Ultrahigh- the properties of Zatsepin- energy GZK neutrinos? Kuzmin (GZK) cosmic rays and ultrahigh-energy neutrinos neutrinos Discovery area: Open low- Open very- — — — — — Gravitational wave frequency low-frequency astronomy window— window— very rich in possibly best astrophysical chance to detect sources cosmological background NOTE: Shaded entry, direct connection to science question. Unshaded entry, indirect or possible connection but not guaranteed. 437

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