Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011) / Chapter Skim
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8 Report of the Panel on Particle Astrophysics and Gravitation
8 Report of the Panel on Particle Astrophysics and Gravitation
Pages 379-438
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From page 379... ...
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 379
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From page 380... ...
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.
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From page 381... ...
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.
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From page 382... ...
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.
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From page 383... ...
. 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.
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From page 384... ...
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.
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From page 385... ...
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
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From page 386... ...
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.
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From page 387... ...
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.
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From page 388... ...
• 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.
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From page 389... ...
• Capture of stellar-mass compact objects by massive black holes. "Small" compact objects near galactic centers (black holes, neutron stars, and white dwarfs)
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From page 390... ...
; luminosity distances, Observables DDL/DL < 4% (typical detections) Science Measure MBH spins, which reflect their growth history; populations and dynamics of payoffs 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 ~100-fold increase in census of short-period galactic binaries; white dwarf-white payoffs dwarf binaries as possible supernova Ia progenitors; evolutionary pathways (e.g., outcomes of common-envelope evolution)
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From page 391... ...
Theorists generally agree that deviations from classical TABLE 8.2 Current Limits on Deviations from General Relativity Distance Weak Gravity Strong Gravity Scale Weak equivalence principle, 10–13 Laboratory Limits on fifth force and compact extra dimension size, 56 m Gravitational redshift, 10–8 Weak equivalence principle, 10–13 Gravitational radiation from binary pulsars, 10–3 Solar system Strong equivalence principle, 10–4 Black holes 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% 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.
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From page 392... ...
Some of these theories predict devia tions from general relativity, but while many models produce deviations from gen eral 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.
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From page 393... ...
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?
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From page 394... ...
. 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.
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From page 395... ...
. 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.
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From page 396... ...
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 pro cesses under extreme conditions throughout the universe. Gamma rays and cosmic rays (charged particles or particles whose nature is not known)
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From page 397... ...
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.
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From page 398... ...
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.
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From page 399... ...
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?
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From page 400... ...
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.
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From page 401... ...
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.
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From page 402... ...
Panel rePorts -- new worlds, new HorIzons 402 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 preces sion) 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)
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From page 403... ...
TABLE 8.3 Possible Tests of Relativistic Gravity in the Next Decade Distance Scale Weak Gravity Strong Gravity Laboratory Improved equivalence-principle limits and Better constraints on extra dimensions, for measurements of parametrized post-Newtonian example, from accelerator experiments (PPN) parameters from atom interferometry Solar system Improved strong- and weak-equivalence- Gravitational waves detected directly and their principle limits; better determination of PPN predicted speed and polarization confirmed; parameters and the rate of change of the properties of rotating black holes confirmed gravitational constant from next-generation quantitatively by gravitational waves and X-ray lunar laser ranging reverberation Galactic Measurement of PPN parameters by lensing and Predicted connections between sources and velocity dispersion gravitational waves confirmed quantitatively Cosmological Better bounds on variations of fundamental Gravitational waves detected by cosmic microwave constants, e.g., a, me/mp background polarization; relation between expansion history and growth of structure tested with supernova, baryon acoustic oscillations, and weak lensing
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From page 404... ...
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.
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From page 405... ...
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.
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From page 406... ...
The situation indicates that it is important to improve knowledge of secondary production of cosmic rays to distinguish be tween 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.
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From page 407... ...
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.
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From page 408... ...
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
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From page 409... ...
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.
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From page 410... ...
In addition, the panel considered whether an activity merits what it would cost, would produce results related to theoretical predictions or opens a new capabil ity 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.
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From page 411... ...
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.
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From page 412... ...
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.
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From page 413... ...
, 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.
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From page 414... ...
This could result in no detections of extreme-mass-ratio inspirals over the life of the mission, given the astrophysi cal 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)
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From page 415... ...
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.
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From page 416... ...
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 theo ries of gravitation in the strong-field regime and will provide the most data on the effects of gravitation on mostly unexplored galactic scales.
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From page 417... ...
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.
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From page 418... ...
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 pro vide essential information.
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From page 419... ...
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.
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From page 420... ...
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.
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From page 421... ...
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.
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From page 422... ...
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.
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From page 423... ...
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.
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From page 424... ...
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 be yond the scope of this panel.
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From page 425... ...
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.
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From page 426... ...
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.
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From page 427... ...
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.
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From page 428... ...
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.
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From page 429... ...
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.
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From page 430... ...
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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From page 431... ...
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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From page 432... ...
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)
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From page 433... ...
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?
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From page 434... ...
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)
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From page 435... ...
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)
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From page 436... ...
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?
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From page 437... ...
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.
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