NASA's Beyond Einstein Program An Architecture for Implementation (2007) / Chapter Skim
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2 Science Impact
2 Science Impact
Pages 15-65
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What is the mysterious dark energy pulling the universe apart? These fundamental questions lie at the heart of NASA's Beyond Einstein Program.
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BLACK HOLE FINDER PROBE Introduction The Black Hole Finder Probe is one of the three Einstein Probes discussed in the Beyond Einstein roadmap. BHFP is designed to find black holes on all scales, from one to billions of solar masses.
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Table 2.3 summarizes some of the key science questions that will be investigated by BHFP as part of these Research Focus Areas. Perform a Census of Black Holes Throughout the Universe. For Beyond Einstein, the most directly relevant science goal of the BHFP is to perform a census of black holes throughout the universe.
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Determine How Black Holes Are Formed and How They Evolve. The formation and evolution of black holes can be studied by two means. The census of x-ray sources described in the preceding subsection will provide x-ray luminosities for massive black holes, which are related to the accretion rates and hence to the black hole growth rates.
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range black holes evolve (see science question below) by providing a characterization of the accretion rates of Quantities X-ray flux at low and high massive black holes.
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However, the bulk of the energy in the cosmic x-ray background resides in the higher energy regime, and the nature of the objects emitting between 10 and 600 keV still is not determined. By providing a census of extragalactic hard x-ray sources, BHFP can help determine whether the background is due to massive black holes or to some other set of point sources.
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Supermassive black holes play a central role in this process through mechanisms not yet fully understood. In order to study this connection, the number, size, and evolution of black holes must first be determined.
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Inferences about black hole masses and their evolution frequently make use of the assumptions that the massive black holes are accreting at or near their Eddington limit and that approximately 10 percent of the mass-energy accreted is turned into radiation. In fact, both assumptions are known to be incorrect in many circumstances.
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Since massive black holes already are known in many galaxies, finding more such objects would not constitute a revolutionary contribution to Beyond Einstein science. However, detecting the formation of black holes through gamma-ray bursts in the early universe would be a revolutionary new discovery of relevance for Beyond Einstein.
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However, BHFP certainly will measure difficult to infer the evolution of black hole masses or hard x-ray variability on a variety of time scales x-ray emission over time; (2) the conversion from x-ray that are associated with the evolution of accretion luminosity to black hole growth rate is uncertain by at disks and relativistic jets near massive black holes.
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17 9:439. The Beyond Einstein Great Observatories are major, facility-class missions with broad applications to problems throughout astrophysics 18 and physics, similar in their expected impact to the Hubble Space Telescope and the Chandra X-ray Observatory (National Aeronautics and Space Administration, Beyond Einstein: From the Big Bang to Black Holes, Washington, D.C., January 2003, p.
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Microcalorimeter 0.3-10 15 2,400 at 6 keV 15,000 at 1.25 keV 7 spectrometer -- 6,000 at 6 keV core array Microcalorimeter 0.3-10 15 300 15,000 at 1.25 keV 21 spectrometer -- 6,000 at 6 keV outer array Grating 0.3-1 15 1,250 1,000 Not spectrometer applicable Hard x-ray 6-40 30 10 150 25 telescope NOTE: HPD, half power diameter. Connecting Quarks with the Cosmos cited Con-X and LISA as holding "great promise for studying black holes and for testing Einstein's theory in new regimes."20 Mission Science Goals Contribution of the Mission Directly to Beyond Einstein Science Goals Con-X will test general relativity in the strong field limit by time-resolved spectroscopy of material being accreted just outside the horizon of black holes.
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A concern with the Con-X test of strong gravity around black holes is that the physics of accretion may turn out to be quite complex, and hotspot accretion disks may not move ballistically. However, Con-X should provide very detailed information on the behavior of accreting matter, thus addressing the last part of the Beyond Einstein Program key question: How do black holes manipulate space, time, and matter?
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dark matter Detection of energy inputs from dark matter in Measurements Line emission in galaxy clusters could help determine its nature. clusters Quantities Line energies, luminosities, determined and widths Contribution of the Mission to Other Science Because Con-X is a facility-class Great Observatory, it is likely that many of the most important scientific contributions of Con-X will be in areas outside those key projects and other Beyond Einstein science mentioned above.
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Quantities determined Magnetic-field strength Cosmic Science question How do supermassive black holes The formation of galaxies is strongly affected by feedback from affect galaxies? energy inputs from the supermassive black holes supermassive in galaxy centers.
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These are x-ray sources that, although not located at the centers of galaxies, have luminosities which are too large to be due to simple accretion by neutron stars or stellar-mass black holes. One theory is that these are binary stars with intermediate-mass (100 to 10,000 solar masses)
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It can also constrain the nature of dark matter and clarify how heavy elements are formed. It can determine the nature of the ULXs that have recently been detected in nearby galaxies; the presence of relativistic iron K-shell lines and the variability time scales could solidify the interpretation of these observations as intermediate-mass black holes (IMBHs)
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Some of the science questions that Con-X will address (e.g., the nature and evolution of dark energy, or the structure of spacetime near black holes) can be addressed by other missions.
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Science Assessment Summary The primary strength of Con-X, one of the two Great Observatories included in the Beyond Einstein Program, is the ability to carry out x-ray astronomy with very high spectral resolution and high throughput, representing
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However, other missions address the measurement of dark energy parameters and tests of strong-field general relativity in a more focused and definitive manner. A summary of the committee's evaluation of the scientific merit of Con-X within the Beyond Einstein Program is given in Table 2.10.
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Its objectives are challenging, since direct observational connections with this early era are difficult to find. Four proposed missions in the Beyond Einstein Program fall under the "Inflation Probe" title.
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In addition, understanding the accelerating expansion during the inflationary era might help to understand the dark energy that is causing the current acceleration of the expansion. The more detailed statement of the Beyond Einstein goal on what powered the big bang sets forth the goal of searching for gravitational waves from inflation and phase transitions in the big bang.
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Science question Dark energy properties NOTE: See Appendix G in this report for definitions of acronyms. Contributions of the Mission to Other Science Two kinds of mission have been proposed as Inflation Probes, and their contributions to other science differ.
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Quantities determined Not applicable Neutrino mass Science question What are the masses of the three kinds of Neutrinos are now known to have (CIP) neutrinos?
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The system has not yet been used in an astronomical measurement but a balloonborne system using this technology is in development.42 Cosmic Inflation Probe CIP will generate a 140 square degree survey of galaxies at redshift from 3 to 6.5 in Hα emission. The goal is to measure the primordial power spectrum at spatial scales smaller than is possible with CMB anisotropy.
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The Inflation Probe is the next step along the path to that understanding. The accelerating expansion that occurred during inflation may have a connection to the accelerating expansion occurring today because of the presence of dark energy.
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Science Risk One concern about the B-mode polarization experiments is based on the fact that the B-mode power varies as the fourth power of the energy scale during inflation, so there is only a factor of 3 range in energy scale between the current limits on the B-mode power and the likely detection limits of the Inflation Probe. 43 Possibly mitigating this concern is the fact that -- for the current best estimates for the spectral index of the primordial power spectrum -- the energy scale for inflation may be in this range for typical inflation models.
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TABLE 2.15 Inflation Probe (IP) : Summary of Scientific Evaluation Potential Contributions to Science Beyond Einstein Broader Science Factors Revolutionary Knowing the energy scale is crucial for understanding Interstellar dust and galactic magnetic-field properties discovery inflation (CMB polarization)
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A brief description of the mission and a listing of the instrument properties are provided in Tables 2.16 and 2.17. TABLE 2.16 Joint Dark Energy Mission (JDEM)
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JDEM will address one of the central questions of the Beyond Einstein Program: What is the mysterious dark energy pulling the universe apart? Little is known at present about dark energy.
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Contribution of the Mission Directly to Beyond Einstein Goals JDEM will probe the nature of dark energy by measuring its effects on the expansion history of the universe and on the history of the growth of structure. Several observational techniques exist for the exploration of dark energy.
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A brief summary of the Beyond Einstein science goals of each of the proposed JDEM concepts is provided in Table 2.18. Contribution of the Mission to Other Science Dark energy manifests itself only on large scales; consequently, any JDEM mission will probe large volumes of space, which will naturally lead to a substantial observational data set that can be used to address a significant range of astrophysics questions.
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The significant potential impact of JDEM imaging studies would derive from the very wide and deep fields that they image. Requirements for dark energy studies using weak lensing demand a very-wide-field survey, typically at least 1,000 square degrees, and requirements for dark energy studies using supernovas demand multiple images of a wide field, typically 15 square degrees, that provide a very-deep-field survey.
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A JDEM infrared imaging or spectroscopic large-format telescope could also prove invaluable in locating infrared transients associated with LISA signals indicating imminent supermassive black hole mergers. Thus, as a secondary but potentially equally important contribution to science, JDEM will produce an extraordinary database that, properly archived and made available to the community in a timely manner after acquisition, would provide the basis for a broad archival research program leading to opportunities for unexpected discoveries in many areas of astrophysics.
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Using its figure of merit to characterize sensitivity to dark energy parameters, the Dark Energy Task Force projected that near-term projects taken in combination may improve the figure of merit by a factor in the range of approximately 3 to 5 beyond the ultimate results of ongoing experiments, whereas DETF projected that JDEM could be capable of improving the figure of merit by at least a factor in the range of
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By virtue of being space-based, JDEM will be able to reduce significantly systematic uncertainties with better angular resolution and using a wider spectrum of diagnostic data for supernova, weak lensing, and/or galaxy cluster surveys than is possible from the ground. Furthermore, JDEM capabilities in the NIR could strengthen constraints on dark energy parameters by studying supernovas and weak lensing of galaxies at higher redshifts than possible from the ground.
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The principal science risk to JDEM arises from the challenge to control systematic uncertainties to the sub-percent level required to achieve at least the factor-of-10 improvement in sensitivity called for by the Dark Energy Task Force. None of the observational techniques that may be employed by JDEM has yet demonstrated the ability to reach this level of control.
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Ongoing analysis will be important for determining which combination of techniques can best achieve the Beyond Einstein goal. Science Assessment Summary Understanding the nature of dark energy is one of the most important scientific endeavors of the present era.
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By contrast, gravitational waves are extremely weakly absorbed, and thus they propagate directly to us from the region of accelerated bulk motions of massive objects. Gravitational waves are a uniquely powerful means of peering into those regions of the universe where the spacetime curvature is greatest and most rapidly changing and of seeing to the most distant reaches of the universe in space and time.
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: Mission Instrument Properties Instrument Spectral Range Spatial Resolution Spectral Resolution Collecting Area Field of View Gravitational 3 × 10 Hz to −5 1° angular Measures waveform directly 10 sq. km 13 All sky, all the time wave antenna 0.1 Hz resolution for to fractions of a cycle over MBH mergers hundreds to thousands of cycles waves produced by a rich variety of known and exotic sources.
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Mission Science Goals Contribution to Beyond Einstein Science LISA will contribute directly to Beyond Einstein goals by studying the properties of cosmic black holes, testing general relativity in new regimes, and making interesting cosmological measurements (see Table 2.23)
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evolution of massive black hole black hole masses as a function of distance or co-evolution Measurements Gravitational waveform shape as a function of time time, and will shed light on how from massive black hole binary inspiral and merger black hole growth and galactic evolution may be linked. Quantities Mass as a function of distance determined Additional Map Science Are black holes correctly described by general Observations will yield maps of the Beyond black hole question relativity?
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If the basic ideas of massive black hole growth are qualitatively correct, LISA may expect to see tens to hundreds of events per year for inspirals at the high-mass end. For inspirals at the low-mass end, the rates are highly uncertain.
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Contributions to Other Science Because of the apparent close connection between galactic center black holes and the structure of their host galaxies, information on the formation and growth of massive black holes over cosmic time will feed into models of galactic formation and evolution. The study of EMRIs using coordinated gravitational-wave and electromagnetic observations will improve the understanding of the stars and gas in the close vicinity of galactic black holes.
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LISA will uncover how massive black holes formed and interacted, and it will yield for the first time precise measurements of their masses and spins. It will test how well general relativity accounts for extreme gravity, will verify the dragging of inertial frames in extreme situations, and will check whether black holes are indeed those described by general relativity, tests that cannot be done by any other means or that are prone to uncertainties owing to complex nongravitational physics phenomena.
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, with the ringdown signal, which is determined from perturbation theory of black holes.68 These new methods are now being applied to the more complex and interesting case of mergers of rapidly spinning black holes, and substantial progress is likely during the next few years, well in advance of LISA. The EMRI problem is somewhat different: there the small compact object can be viewed as a perturbation of the background spacetime of the large black hole, but one must take into account the "backreaction" of the small body's gravitational field on itself, including the damping of the orbit due to the emission of gravitational waves.
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SCIENCE SUMMARY This section summarizes the committee's assessment of the contribution that the candidate missions would make to the Beyond Einstein science questions. The summary captures the strengths, scientific uncertainties, readiness, and uniqueness of the associated scientific programs.
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As small bodies spiral into massive black holes, they trace tens of thousands of orbits and emit waves that encode details of the spacetime structure around the massive black hole. By detecting these waves, LISA will provide a rigorous and clean test of whether spacetime is described by the Kerr geometry predicted by general relativity for rotating holes and measure black hole masses and spins to a fraction of a percent.
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In addition to understanding how black holes distort spacetime, the Beyond Einstein Program seeks to understand how they are formed and evolve and how they interact with galaxies and clusters. LISA, Con-X, BHFP, and JDEM will all make significant contributions to different aspects of this important problem.
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One risk to the success of cosmography with gravitational waves from merging supermassive black holes is the uncertain merger rate. Also at the present time researchers do not know if it will be possible to determine optical counterparts in order to measure redshifts.
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The Black Hole Finder Probe will contribute to a black hole census, but it provides less direct measurements of black hole properties than LISA measurements. It was the committee's judgment that for a focused program like Beyond Einstein, it is most important to provide the definitive measurement against at least one of the questions.
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