7
Realizing the Opportunities

Based upon the science reviewed in the previous chapters, the committee has identified special opportunities at the intersection of astronomy and physics in the form of eleven key questions that are of deep interest and are ripe for answering.

Some of the critical work needed to address these 11 questions is part of ongoing programs in astronomy and in nuclear, particle, and gravitational physics. Other needed work is spelled out in the most recent astronomy decadal survey1 or has been recommended by the DOE/NSF High Energy Physics Advisory Panel or their Nuclear Science Advisory Committee.

The committee’s recommendations, which are presented at the end of this chapter, are meant to complement and supplement the programs in astronomy and physics already in place or recommended, to ensure that the great opportunities before us are realized. They are in no way intended to override the advice of the groups mentioned above.

In the section entitled “The Eleven Questions,” the committee presents the 11 questions and summarizes the type of work needed to answer each of them. The detailed strategy for realizing these scientific opportunities that the committee was charged to develop is laid out in seven recommendations contained in section “Recommendations.” The remaining sections provide the justifications for the recommendations and tie the seven recommendations to the science questions.

The committee’s seven recommendations do not correspond simply to the questions; the interconnectedness of the science precluded such a mapping. Some of the projects it recommends address more than one science question, while some of the questions have no clear connection to the recommendations, although programs already in place or recommended by

1  

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



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7 Realizing the Opportunities Based upon the science reviewed in the previous chapters, the committee has identified special opportunities at the intersection of astronomy and physics in the form of eleven key questions that are of deep interest and are ripe for answering. Some of the critical work needed to address these 11 questions is part of ongoing programs in astronomy and in nuclear, particle, and gravitational physics. Other needed work is spelled out in the most recent astronomy decadal survey1 or has been recommended by the DOE/NSF High Energy Physics Advisory Panel or their Nuclear Science Advisory Committee. The committee’s recommendations, which are presented at the end of this chapter, are meant to complement and supplement the programs in astronomy and physics already in place or recommended, to ensure that the great opportunities before us are realized. They are in no way intended to override the advice of the groups mentioned above. In the section entitled “The Eleven Questions,” the committee presents the 11 questions and summarizes the type of work needed to answer each of them. The detailed strategy for realizing these scientific opportunities that the committee was charged to develop is laid out in seven recommendations contained in section “Recommendations.” The remaining sections provide the justifications for the recommendations and tie the seven recommendations to the science questions. The committee’s seven recommendations do not correspond simply to the questions; the interconnectedness of the science precluded such a mapping. Some of the projects it recommends address more than one science question, while some of the questions have no clear connection to the recommendations, although programs already in place or recommended by 1   National Research Council, Astronomy and Astrophysics in the New Millennium, Washington, D.C., National Academy Press, 2001.

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other NRC committees or advisory groups will address them. The committee calls for three new initiatives—an experiment to map the polarization of the cosmic microwave background, a wide-field space telescope, and a deep underground laboratory. It adds its support to several other initiatives that were previously put forth or set in place and addresses structural issues. THE ELEVEN QUESTIONS What Is Dark Matter? Dark matter dominates the matter in the universe, but questions remain: How much dark matter is there? Where is it? What is it? Of these, the last is the most fundamental. The questions concerning dark matter can be answered by a combination of astronomical and physical experiments. On small astronomical scales, the quantity and location of dark matter can be studied by utilizing its strong gravitational lensing effects on light from distant bright objects and from the distribution and motions of galaxies and hot gas under its gravitational influence. These can be studied using ground-based and space-based optical and infrared telescopes and space-based x-ray telescopes. On larger scales, optical and infrared wide-field survey telescopes can trace the matter distribution via weak gravitational lensing. (Strong gravitational lensing produces multiple images of the lensed objects, while weak gravitational lensing simply distorts the image of the lensed object; see Figure 5.6.) The distribution of dark matter on large scales can be measured by studying motions of galaxies relative to the cosmic expansion. While these observations will measure the quantity and location of dark matter, the ultimate determination of its nature will almost certainly depend on the direct detection of dark matter particles. Ongoing experiments to detect dark matter particles in our Milky Way such as Cold Dark Matter Search II and the US Axion Experiment, future dark-matter experiments in underground laboratories, and accelerator searches for supersymmetric particles at the Fermilab Tevatron or the CERN LHC are all critical. Elements of the program live in the purview of each of three funding agencies, DOE, NASA, and NSF; coordination will be needed to ensure the most effective overall program. What Is the Nature of the Dark Energy? There is strong evidence from the study of high-redshift supernovae that the expansion of the universe is accelerating. Fluctuations in the

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temperature of the CMB indicate that the universe is flat, but the amount of matter is insufficient (by about a factor of 3) to be in accord with this. All this points to the presence of a significant dark energy component, perhaps in the form of a cosmological constant, both to make up the deficit and, through its repulsive gravitational effects, to cause a universal acceleration. This mysterious energy form controls the destiny of the universe and could shed light on the quantum nature of gravity. Because of its diffuse nature, the best methods to probe its properties rely upon its effect on the expansion rate of the universe and the growth of structure in the universe. The use of high-redshift (z ~ 0.5 to 1.8) supernovae as cosmic mileposts led to the discovery of cosmic speed up. They have great promise for shedding light on the nature of the dark energy. To do so will require a new class of wide-field telescopes to discover and follow up thousands of supernovae as well as a better understanding of type Ia supernovae to establish that they really are standard candles. In addition, clusters of galaxies can be detected out to redshifts as large as 2 or 3 through x-ray surveys, through large-area radio and millimeter-wave surveys using the Sunyaev-Zel’dovich effect and through gravitational lensing. Future x-ray missions will be able to determine the redshifts and masses of these clusters. High-redshift supernovae, counts of galaxy clusters, weak-gravitational lensing, and the microwave background all provide complementary information about the existence and properties of dark energy. Already NASA and NSF have programs and special expertise in parts of this science with their traditional roles in space- and ground-based astronomy, while DOE has made contributions in areas such as CCD detector development. Again, interagency cooperation and coordination will be needed to define and manage this research optimally. How Did the Universe Begin? The inflationary paradigm, that the very early universe underwent a very large and rapid expansion, is now supported by observations of tiny fluctuations in the intensity of the CMB. The exact cause of inflation is still unknown. Inflation leaves a telltale signature of gravitational waves, which can be used to test the theory and distinguish between different models of inflation. Direct detection of the gravitational radiation from inflation might be possible in the future with very-long-baseline, space-based laser interferometer gravitational-wave detectors. A promising shorter-term approach is to search for the signature of these gravitational waves in the polarized radiation from the CMB. If the relevant polarization signals are strong

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enough, they may be detected by the current generation of balloon-borne and satellite experiments, such as MAP, which is now taking data, and the European Planck satellite, planned for launch late in this decade. However, it is likely that a more sensitive satellite mission devoted to polarization measurements will be required. Support for detector development is critical to realizing such a mission. NSF, NASA, and DOE have already played important roles in CMB science, and their cooperation in the future will be essential. Did Einstein Have the Last Word on Gravity? Although general relativity has been tested over a range of length scales and physical conditions, it has not been tested in the extreme conditions near black holes. Its predicted gravitational waves have been indirectly observed, but not directly detected and studied in detail. Gravity has not been unified with the other forces, nor has Einstein’s theory been generalized to include quantum effects. A host of experiments are now probing possible effects arising from the unification of general relativity with other forces, from laboratory-scale precision experiments to test the principle of equivalence and the force law of gravity to the search for the production of black holes at accelerators. Space experiments envisioned in NASA’s Beyond Einstein plan will further test general relativity. Constellation-X, a high-resolution x-ray spectroscopic mission, will be able to probe the regions near the event horizons of black holes by measuring the red- and blueshifts of spectral lines emitted by gas accreting onto the black holes. LISA, a space-based laser interferometer gravitational-wave observatory, will be able to probe the space-time around black holes by detecting the gravitational radiation from merging massive black holes. DOE, NASA, and NSF all have roles to play in establishing a better understanding of gravity. What Are the Masses of the Neutrinos and How Have They Shaped the Evolution of the Universe? The discovery that neutrinos have mass and can oscillate among their different types has implications for both the universe and the laws that govern it. Further progress in understanding the masses and oscillations of neutrinos will require an ongoing program of large-scale detectors to study neutrinos from atmospheric and solar sources, striving eventually for sensitivity to the low-energy neutrinos from the proton-proton sequence of

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nuclear reactions. Experiments that send beams of neutrinos from accelerators to remote detectors (e.g., MINOS) will also provide critical information on neutrino masses and mixing. Detectors will need to be stable and to run for extended periods if they are to provide a window for the observation and timing of neutrinos from any nearby supernova event. Finally, the absolute scale of neutrino masses can be probed by end-point studies of beta decay and high-sensitivity searches for neutrinoless double beta decay. If neutrino masses are large enough, they may play a small but detectable role in the development of large-scale structure in the universe. Elements of this program will require a deep underground laboratory. Such an underground laboratory would perform experiments at the intersection of particle and nuclear physics. It is likely that scientists supported by both DOE and NSF will be involved in its programs. How Do Cosmic Accelerators Work and What Are They Accelerating? Cosmic rays and photons with energies far in excess of anything we can produce in laboratories have been detected. We do not yet know the sources of these particles and thus cannot understand their production mechanism. Neutrinos may also be produced in association with them. Identifying the sources of ultrahigh-energy cosmic rays requires several kinds of large-scale experiments to collect sufficiently large data samples and determine the particle directions and energies precisely. Dedicated neutrino telescopes of cubic kilometer size in deep water or ice can be used to search for cosmic sources of high-energy neutrinos. Further study of the sources of high-energy gamma-ray bursts will also be relevant. DOE, NASA, and NSF are all involved in studying the highest-energy cosmic particles. To understand the acceleration mechanisms of these particles, a better understanding of relativistic plasmas is needed. Laboratory experiments that use high-energy-density pulses to probe relativistic plasma effects can provide important tests of our ability to model the phenomena in astrophysical environments that are the likely sources of intense high-energy particles and radiation. Laboratory work thus will help to guide the development of a theory of cosmic accelerators, as well as to refine our understanding of other astrophysical phenomena that involve relativistic plasmas. This work will require significant interagency and interdisciplinary coordination. The facilities that can produce intense high-energy pulses in plasmas are laser or accelerator facilities funded by DOE. The expertise needed to bring these resources to bear on astrophysical phenomena crosses both disciplinary and agency boundaries.

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Are Protons Unstable? The observed preponderance of matter over antimatter in the universe may be tied to interactions that change baryon number and that violate matter-antimatter (CP) symmetry. Further, baryon number violation is a signature of theories that unify the forces and particles of nature. Two possible directions to search for baryon-number-changing interactions are direct searches for proton decay and searches for evidence of neutron-antineutron oscillations. To attack proton decay at an order of magnitude increase in sensitivity over current limits will require a large detector in a deep underground location. It will also be desirable to achieve improved sensitivity to decay modes involving a kaon and a neutrino, as well as to modes involving a pion and a positron. These searches are complemented by the program in CP violation physics involving kaon and B-meson decays, which is a central part of the ongoing high-energy physics agenda. In the future, it may be feasible to determine CP violation in neutrino interactions in an underground lab via long-baseline experiments with intense neutrino beams from accelerators. As in the case of the related neutrino experiments mentioned above, this work will require coordinated planning among all agencies supporting any underground laboratory. What Are the New States of Matter at Exceedingly High Density and Temperature? Computer simulations of quantum chromodynamics (QCD) have provided evidence that at high temperature and density, matter undergoes a transition to a state known as the quark-gluon plasma. The existence and properties of this new phase of matter have important cosmological implications. Quark-gluon plasmas may also play a role in the interiors of neutron stars. Some, but not all, aspects of the transition from ordinary matter to a quark-gluon plasma can be probed with accelerators (see Figure 6.4). Experiments at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory may probe the transition to a quark-gluon plasma in the fireball formed when two massive nuclei collide at high energy. If this phase existed in the early universe, it may have left its signature in a gravitational-wave signal. The LISA space gravitational-wave interferometer will begin a search for this signal, but a follow-on experiment with higher sensitivity may be needed in order to observe it. Transitions to other new phases of matter may have occurred in the early universe and left detectable gravitational-wave signatures (possibilities include transitions to states where the

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forces of nature are unified). X-ray observations of neutron stars can shed light on how matter behaves at nuclear and higher densities, providing insights about the physics of nuclear matter and possibly even of new states of matter. Are There Additional Space-Time Dimensions? Theories containing more than four dimensions (with at least some of the additional dimensions having macroscopic scale) have been suggested as the explanation for why the observed gravitational force is so small compared with the other fundamental forces. Such theories have two types of possible experimental signature. Small-scale precision experiments can search for deviations from standard predictions for the strength of gravity on the submillimeter scale. High-energy accelerator searches can test for events with missing energy, signaling the production of gravitons, evidence for the excitations of a compact additional dimension. Accelerator searches for new particles and/or missing energy are typically not done with a dedicated experiment but by additional analyses of data collected in high-energy collision experiments. It is important for the agencies to recognize the value of these analyses, even if they do not find the desired effect but instead set new limits. This science falls into the realms of both NSF and DOE. How Were the Elements from Iron to Uranium Made? While we have a relatively complete understanding of the origin of elements lighter than iron, important details in the production of elements from iron to uranium remain a puzzle. A sequence of rapid neutron captures by nuclei, known as the r-process, is clearly involved, as may be seen from the observed abundances of the various elements. Supernova explosions, neutron-star mergers, or gamma-ray bursters are possible locales for this process, but our incomplete understanding of these events leaves the question open. Progress requires work on a number of fronts. More realistic simulations of supernova explosions and neutron star mergers are essential; they will require access to large-scale computing facilities. In addition, better measurements are needed for both the inputs and the outputs of these calculations. The masses and other properties of neutrinos are crucial parts of the input. The masses and lifetimes of many nuclei that cannot be reached with existing technology are also important input parameters; however, a complete theoretical description of such nuclei remains out of reach. Almost all

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the relevant r-process nuclei could be accessible for study in a suitably designed two-stage acceleration facility (such as RIA) that produces isotopes and reaccelerates them. Such a facility has been recommended by NSAC as a high-priority project for nuclear physics. For the outputs, sensitive high-energy x-ray and gamma-ray space experiments will allow us to observe the decays of newly formed elements soon after supernova explosions and other major astrophysical events. Comparison of these observations with the outputs of simulations can constrain the theoretical models for the explosions. Better measurements of abundances of certain heavy elements in cosmic rays may also provide useful constraints. The program suggested above spans nuclear physics, astrophysics, and particle physics and will require coordination between all three agencies. Is a New Theory of Matter and Light Needed at the Highest Energies? While few scientists expect that the theory of QED will fail in any astrophysical environment, checking the consistency of observations with predictions of this theory does provide a way to test the self-consistency of astrophysical models and mechanisms. The predictions of QED have been tested with great precision in regimes accessible to laboratory study, such as in static magnetic fields as large as roughly 105 gauss. However, magnetic fields as large as 1012 gauss are commonly found on the surfaces of neutron stars (pulsars), and a subset of neutron stars, called magnetars, have magnetic field strengths in the range 1014 to 1015 gauss, well above the QED critical field, where quantum effects produce polarized radiation. As magnetars rotate rather slowly, it may be possible to observe this polarization and map out the neutron star magnetic field. To carry out such observations will require x-ray instruments capable of measuring polarization. As can be seen from these brief summaries, important parts of the answers to the 11 questions lie squarely in the central plans of the core disciplines of high-energy physics, nuclear physics, plasma physics, or astrophysics, and much exciting science relevant to our questions is already being pursued. The fact that the recommendations made in this report do not speak directly to existing programs should not be construed as lack of support for those programs. Rather the committee has been charged to focus attention on projects or programs that because they lie between the traditional disciplines may have fallen through the cracks. When viewed from

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the broader perspective this science takes on a greater urgency and must be made a priority. It is also notable that many of the efforts described above have features that fall within the purview of more than one agency, or involve competing approaches that, in the present system, would be reviewed by different agencies. To ensure that the approach to these problems follows the most effective path, interagency cooperation is needed, not just at the stage of funding decisions but also at the level of project oversight when a project is funded through more than one agency. UNDERSTANDING THE BIRTH OF THE UNIVERSE The CMB is a relic from a very early time in the history of the universe. The spectrum and anisotropy of the CMB have already given us valuable information about the birth of the universe and provide some evidence that the universe went through an inflationary epoch. Future measurements of the polarization of the anisotropy of the CMB are the most promising way to definitively test inflation and to learn directly about the inflationary epoch. The photons of the CMB come to us from a time when their creation and destruction effectively stopped because the universe had expanded to relatively low densities. The spectrum of the CMB differs from that of a blackbody by less than 1 part in 104, showing that the energy of CMB photons has not been perturbed since about 2 months after the big bang. For the next 400,000 years, photon-electron scattering scrambled only the directions of the photons. When the universe cooled to about 3000 K, electrons and baryons combined to form neutral atoms. After this “recombination” or last-scattering epoch, the CMB photons traveled freely across the universe, allowing us to compare those coming from parts of the universe that are very distant from us with those coming from parts nearby. In this way the anisotropy of the CMB can reveal the distribution of matter in the universe as it was a half million years after the big bang, before the creation of stars and galaxies. Efforts to detect the anisotropy of the CMB started immediately after its discovery. Initially, only the anisotropy due to the motion of the solar system at 370 km/sec relative to the average velocity of the observable universe was found. Finally, in 1992, the Differential Microwave Radiometer (DMR) instrument on the Cosmic Background Explorer (COBE) satellite detected the intrinsic anisotropy of the CMB at a level of 10 parts per million. The DMR detected these 30 millionths of a degree temperature differences across the sky by integrating for a full year. The DMR beam size

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(about 7 degrees), when projected to the edge of the observable universe, spans a region a few times larger than the distance light could have traveled in the half million years between the big bang and the last scattering. As a result, the anisotropy seen by the DMR is truly primordial, unaffected by the interaction between the CMB and matter. But interactions between the CMB and matter are a critical part of the later formation of clusters and superclusters of galaxies, and these interactions can be studied by looking at the CMB with smaller beam sizes. These interactions produce a series of “acoustic” peaks in the plot of temperature difference vs. angular scale, with scales of a degree and smaller (see Figure 5.3). They are called acoustic because they record the effect of the interaction of the radiation with matter variations analogous to sound waves. When looking at small angular scales, the foreground interference from the atmosphere is less of a problem, and experiments on the ground and on stratospheric balloons have observed evidence for a series of acoustic peaks. These experiments have shown by the position of the first peak that the geometry of our universe is consistent with being uncurved, and by the height of the second peak have made an independent measurement of the amount of ordinary matter in the universe. The existence of acoustic peaks and the flatness of the geometry of the universe are consistent with the predictions of inflation and have given the theory its first significant tests. The determination of the amount of ordinary matter, about 4 percent of the critical density, agrees with the determination based on the amount of deuterium produced during the first seconds and strengthens the case for a new form of dark matter dominating the mass in the universe. For the future, the Microwave Anisotropy Probe (MAP), launched on June 30, 2001, will measure the entire sky with a 0.2-degree beam and a sensitivity 45 times better than that of the DMR. The European Space Agency’s Planck satellite, to be launched in 2007, will have a 0.08-degree beam and a sensitivity 20 times better than MAP. Since anisotropy signals on even smaller angular scales are suppressed by the finite thickness of the surface of last scattering, MAP and Planck will essentially complete the study of the temperature differences resulting from these primordial density fluctuations. We expect to learn much about the earliest moments of the universe from these two very important missions. There remains one critical feature of the microwave sky to be explored: its polarization. Polarization promises to reveal unique features of the early universe, but it will be difficult to measure. First, its anisotropies are expected to be an order of magnitude smaller than those for the temperature field. This means that more sensitive detectors and longer integration times are required.

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And second, it is likely that polarizing galactic foregrounds will be more troublesome than they are for determining the temperature field. At every point on the sky, the temperature of the radiation can be represented as a single number, while polarization is represented by a line segment (see Figure 4.6). For example, a given point may have a temperature of 2.725 degrees and its temperature may differ from the average by 30 millionths of a degree. But the signal measured by a polarized detector aligned toward the north galactic pole might exceed that measured by a detector aligned in the east-west direction by just two millionths of a degree. The polarization line segment in this example would point north-south, and its length would be related to the latter temperature difference. The science comes from a study of the pattern of these line segments on the sky and how they correlate with the temperature pattern. To reveal this polarization field, more sensitive detectors with polarization sensitivity are required. According to our understanding of the oscillations in the plasma of photons, electrons, and baryons that were under way before recombination, the inhomogeneities that developed lead naturally to a predictable level of polarization of the CMB photons. This polarization anisotropy is expected to be most prominent at even finer angular scales than those for the temperature, requiring instruments with beams that are smaller than 0.1 degrees. In the fall of 2002, the first detection of the polarization of the anisotropy of the CMB by the DASI experiment was announced (see Figure 4.6); the amplitude and variation with angular scale was as expected. Nearly two dozen efforts are under way to further characterize the polarization. While most are modifications of existing temperature anisotropy experiments, some are dedicated to detecting polarization. These ongoing efforts are also important in that they will allow accurate study of the foregrounds that are expected to contaminate the measurements. It is highly likely that experiments already in progress will systematically characterize the CMB polarization and the associated foregrounds. This will be an important confirmation of our understanding of the initial fluctuations that led to anisotropies and structure formation. However, these experiments (including MAP and Planck, which have polarization sensitivity) will not be able to fully characterize the polarization of the CMB anisotropy because their sensitivity is not adequate. Measuring CMB polarization in essence triples the information we can obtain about the earliest moments and exploits the full information available from this most important relic of the early universe.

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achieved by deriving universal scaling laws like those that are widely used in fluid dynamical investigations. The field of high-energy-density physics is in its infancy. In order to fulfill its potential, it must draw in expertise from astrophysics, laser physics, magnetic confinement and particle beam research, numerical simulation, and atomic physics. It should attract younger scientists and help rejuvenate conventional plasma physics. It will require skillfully designed experiments that elucidate fundamental properties of plasmas. This will, in turn, require the development of far more sophisticated diagnostic devices to measure particle densities and speeds and to map dynamical magnetic fields. It will also require improved numerical simulations. Many of the facilities that will be used reside in large national laboratories. It is therefore important that outside users have access to these facilities for the purpose of designing, conducting, and analyzing major experiments. STRIKING THE RIGHT BALANCE In discussing the physics of the universe, one is naturally led to the extremes of scale—to the largest scales of the universe as a whole and to the smallest scales of elementary particles. Associated with this is a natural tendency to focus on the most extreme scale of scientific projects: the largest space observatories, the most energetic particle accelerators. However, our study of the physics of the universe repeatedly found instances where the key advances of the past or the most promising opportunities for the future come from work on a very different scale. Examples include laboratory experiments to test gravitational interactions, theoretical work and computer simulations to understand complex astrophysical phenomena, and small-scale detector development for future experiments. These examples are not intended to be exhaustive but to illustrate the need for a balanced program of research on the physics of the universe that provides opportunities for efforts that address the scientific questions but that do not necessarily fit within major program themes and their related large projects. Two of our scientific questions—“Did Einstein have the last word on gravity?” and “Are there additional space-time dimensions?”—are being addressed by a number of laboratory and solar-system experiments to test the gravitational interaction. Tests of the principle of equivalence using laboratory torsion balances and lunar laser ranging could constrain hypothetical weakly coupled particles with long or intermediate range. These experiments have reached the level of parts in 1013 and could be improved by another order of magnitude. Improvement by a factor of around 105

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could come from an equivalence principle test in space. While there are no robust predictions of violations of the equivalence principle at these levels, null experimental results provide important constraints on existing theories, and a positive signal would make for a scientific revolution. Searches for deviations from the inverse square law of gravity at submillimeter scales could provide evidence that gravity leaks into macroscopic extra dimensions, or that additional light particles couple to matter with gravitational strength. Already, preliminary results rule out any violations greater than 1 percent of gravity down to half a millimeter and yield an upper limit of 0.2 mm on the size of the larger of two extra dimensions. Future experiments could improve the bounds in strength and distance scale by factors between 10 and 100. A balanced program should provide opportunities for such investigator-initiated projects. Theoretical and computational work will play integral roles in addressing several of the scientific questions. To test whether Einstein had the last word on black holes will require analytically and numerically generated gravitational waveforms from black hole mergers that can be compared with gravitational-wave data. A better theoretical understanding of Type Ia supernovae is key to exploiting them as cosmological mileposts in the search for evidence of dark energy. In addition, fully three-dimensional numerical simulations of explosive nucleosynthesis during supernovae, including neutrino transport and armed with improved input data on nuclear reaction rates away from the line of nuclear stability, will be needed to address the production of the elements from iron to uranium. These simulations will require terascale computing capabilities. To infer the distribution and possible nature of dark matter from measurements of the development of structure in the universe, large-scale numerical simulations of the predictions of the different cold dark matter models are crucial. Numerical modeling of high-density plasma behavior will be key to revealing the scalable physical principles that can be invoked to understand high-energy astrophysical phenomena, such as the acceleration mechanisms for the highest energy particles. Numerical simulations of the Standard Model of quarks and gluons using lattice gauge theory are critical to understanding the nature of the transition from a quark-gluon plasma to hadrons in the early universe and the possible signatures of that event in gravitational-wave signals. To realize fully the potential of high-performance computing to address these scientific questions will require a combination of access to the largest-scale computing facilities, resources for developing local, special-purpose computing clusters, development of simulation “collaboratories” that inter-

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connect researchers and computers worldwide, and a new cadre of trained computational scientists to make use of these computing capabilities. The theoretical research that both underpins the simulations and interprets their outputs is also essential. While the committee does not have the expertise to make specific recommendations related to computational infrastructure, it strongly recommends that a balanced program of research on the physics of the universe include appropriate support for theory and computation. A third example of small-scale effort with the potential to help realize many of our science objectives is research and development on advanced detectors. Examples include low-noise cryogenic instrumentation for future dark-matter searches, detectors for future microwave background polarization measurements, accelerometer development for future gravity and gravitational-wave measurements, advanced optical imagers, and x-ray polarimeters. Such modest investments could enable the next generation of discoveries. Yet oftentimes they are carried out by individual investigators or small teams not connected with a major ongoing program or mission, and they involve research based in one funding agency (the properties of certain solids) and application based in another (astronomical detectors). Such work sometimes fails to find a funding home. A balanced program on the physics of the universe should include mechanisms for detector R&D to support future experiments or observations. RECOMMENDATIONS The committee has identified timely opportunities for advancing our understanding of the universe and the laws that govern it. They range from understanding the birth and destiny of the universe to testing Einstein’s theory of gravity in black holes and understanding the fundamental nature of matter, space, and time. In this chapter the next steps that must be taken to realize the opportunities are discussed. No one agency currently has unique ownership of the science at the intersection of astronomy and physics; nor can one agency working alone mount the effort needed to realize the great opportunities. DOE, NASA, and NSF are all deeply interested in the science at this intersection, and each brings unique expertise to the enterprise. Only by working together can they take full advantage of the opportunities at this special time. Coordination and joint planning are essential. In some instances, two of the agencies, or even all three, will need to work together. In others, one agency may be able to close the gap between the disciplines of physics and astronomy.

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Finally, the committee’s charge was to focus on DOE, NASA, and NSF. Although it does view the science in the larger context of activities around the world, the committee did not address the issue of international collaboration. The committee believes that the charge was sensible; absent a national plan for addressing science at the intersection of physics and astronomy, it will be difficult for the United States to pursue a coherent program of international collaboration. Likewise, the DOE, NASA, and NSF are the primary federal funding agencies with an interest in this interfacial science. Some of the opportunities discussed involve international partners (e.g., LISA involves both NASA and the European Space Agency, Auger involves European and other American partners) or could involve them (e.g., an underground laboratory or a wide-field telescope in space). The strategy the committee has developed for DOE, NASA, and NSF should facilitate the participation of additional partners, be they international organizations, other agencies in the United States, or private foundations. Further, because U.S. scientists have been pioneers in recognizing the importance of this interdisciplinary science and in pursuing it, the committee believes it is likely that the United States can have a significant impact in this exciting science. Recommendation on Understanding the Birth of the Universe The cosmic microwave background radiation is a snapshot of the universe at a simpler time, some 400,000 years after its beginning. Important clues about the birth of the universe are encoded in the tiny variations of its intensity and its polarization. Already, CMB measurements have determined the shape of the universe, determined precisely the amount of ordinary matter, and given the first firm evidence for cosmic inflation. More discoveries will be made with the projects in place (e.g., MAP and Planck). The portion of the polarization of the CMB that is produced by primordial gravitational waves offers great promise in testing further and understanding the inflationary era that may have occurred shortly after the birth of the universe. It is the clearest signature that inflation took place and reveals when it took place. Measuring this signature of inflation is extremely challenging and will require a significant R&D program before serious experimental efforts can be mounted. NASA, NSF, and DOE have played important roles in CMB discoveries so far, and the talents of all three agencies will be critical to the successful detection of the polarization signature of inflation.

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Measure the polarization of the cosmic microwave background with the goal of detecting the signature of inflation. The committee recommends that NASA, NSF, and DOE undertake the research and development to bring the needed experiments to fruition. Recommendation on Understanding the Destiny of the Universe Measurements of distant supernovae that indicate that the expansion of the universe is speeding up and not slowing down rank as one of the most important discoveries of the past quarter century. This accelerated expansion has revealed a new and mysterious energy form—dark energy—which accounts for two thirds of the matter and energy content of the universe. It has also changed how we view the destiny of our universe. Without understanding the properties of dark energy we cannot rule out an eventual recollapse or continued acceleration and an almost complete darkening of the sky in 150 billion years. Dark energy also raises questions about the fundamental nature of matter, space, and time. Because of the diffuse nature of dark energy, the universe is the primary laboratory in which it can be studied. By controlling the expansion rate, dark energy determines cosmic distances and affects the growth of structure in the universe. A host of experiments is on the horizon to probe dark energy through these effects. To get at the nature of the dark energy will require a new class of large, wide-field (greater than 1 square degree) telescopes, both in space and on the ground. A wide-field, space-based telescope with a 2-meter mirror (such as SNAP) would provide crystal-clear images of large patches of the universe, ideal for deep gravitational lensing studies and for the discovery and follow-up of large numbers of supernovae out to high redshift (z ~ 1.5). A ground-based, wide-field telescope with an effective aperture of 6 to 8 meters (such as the LSST recommended by the Astronomy and Astrophysics Survey Committee) would rapidly image large portions of the sky, ideal for gravitational lensing studies and for the discovery of supernovae out to moderate redshift (z ~ 0.7). Both telescopes will also help to elucidate our understanding of the distribution of dark matter. In this quest to solve one of the great puzzles of physics and astronomy, NASA and NSF have their traditional roles to play in space-based and ground-based astronomy, respectively. DOE also has an important role to play because of its contributions to the discovery of cosmic speed-up and its contributions to CCD detector development. Determine the properties of dark energy. The committee supports the Large Synoptic Survey Telescope (LSST) project, which

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has significant promise for shedding light on dark energy. The committee further recommends that NASA and DOE work together to construct a wide-field telescope in space to determine the expansion history of the universe and fully probe the nature of dark energy. Recommendation on Exploring the Unification of the Forces from Underground Three of the committee’s 11 questions—the nature of the dark matter, the question of neutrino masses, and the possible instability of the proton—must be addressed by carrying out experiments in a deep underground laboratory that is isolated from the constant bombardment of cosmic-ray particles. One of the most important discoveries in the past 10 years, that neutrinos have mass, was made in an underground laboratory. This discovery has implications for both the universe and the laws that govern it. The mass scale implied by measurements to date suggests that neutrinos contribute as much mass to the universe as do stars; and neutrino mass points to a grander theory that brings together the forces and particles of nature and may even shed light on the origin of ordinary matter. The committee believes there are more opportunities for discovery at an underground laboratory. Experiments proposed for the near future to address the fundamental questions it has identified require depths up to 4,000 meters of water equivalent (mwe). More visionary experiments, as well as the long-term potential of such a laboratory to make discoveries, require even more shielding, to depths up to 6,000 mwe. A laboratory for underground research must do more than provide shielded space. Many of the envisaged experiments require large, technically sophisticated, and costly detectors. An underground laboratory must also provide appropriate infrastructure to enable such experiments. Equally important is planning, selecting, and coordinating the experiments that carry out the science in the laboratory. DOE and NSF have some of the mechanisms in place (e.g., the SAGENAP process), but additional ones may be needed. A North American laboratory with a depth significantly greater than 4,000 mwe and adequate infrastructure would be unique in the world and provide the opportunity for the United States to take a lead in “underground science” for decades. Such a laboratory might also be useful for carrying out important science in other disciplines, such as biology and geophysics.

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Determine the neutrino masses, the constituents of dark matter, and the lifetime of the proton. The committee recommends that DOE and NSF work together to plan for and to fund a new generation of experiments to achieve these goals. It further recommends that an underground laboratory with sufficient infrastructure and depth be built to house and operate the needed experiments. Recommendation on Exploring the Basic Laws of Physics from Space Our view of the universe has been transformed by the opening up of the whole electromagnetic spectrum, from low-frequency radio waves to high-energy gamma rays. The observable spectrum spans some 67 octaves, in contrast to the single octave that is visible to the eye. Most of the spectrum can only be observed from space. In recent years, NASA has recognized the potential of space observations to address questions that surround the basic laws of physics. The intellectual thrust of NASA’s Beyond Einstein initiative aligns well with the science opportunities at the intersection of physics and astronomy, although not every project in it is relevant to the science opportunities the committee has identified. The committee supports NASA’s strategic planning activity and the NRC’s decadal survey as procedures for determining the highest priority initiatives in space astronomy. The charge of this committee was complementary: to examine the science opportunities at the intersection of physics and astronomy and to recommend a prioritized program to realize them. Astronomy and Astrophysics in the New Millennium recently recommended two near-term missions described in the Beyond Einstein initiative, Constellation-X and LISA. Although it made these recommendations solely on the basis of the potential of the missions to address key questions in astronomy, these two missions also have great potential to address questions that lie at the boundary between physics and astronomy. Constellation-X is a sensitive, high-resolution x-ray spectroscopy mission. Among its many potential targets are the gas disks orbiting black holes and the surfaces of neutron stars at nearly the speed of light, which will enable it to test general relativity and measure how matter behaves at high density. LISA is a joint ESA-NASA project to measure low-frequency gravitational radiation from sources such as coalescing black holes and to undertake new tests of Einstein’s theory. LISA will also be able to make an initial search for gravitational waves from the early universe, paving the way for

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future, more sensitive detectors that could possibly detect the gravitational whispers from inflation and other early universe sources. NSF, with its experience in developing the ground-based LIGO detectors, and DOE, with its experience in optics and lasers, could play important roles in developing future gravitational-wave detectors in space. Use space to probe the basic laws of physics. The committee supports the Constellation-X and LISA missions, which have high promise for studying black holes and for testing Einstein’s theory in new regimes. The committee further recommends that the agencies proceed with an advanced technology program to develop instruments capable of detecting gravitational waves from the early universe. Recommendation on Understanding Nature’s Highest-Energy Particles The particles with the highest observed energies are not produced by terrestrial accelerators but come to us from space. How and where they were accelerated to such energies is unknown but may involve gamma-ray bursters, massive black holes, or the decay of exotic objects produced in the early universe. Such particles offer the opportunity to explore physics at the highest energies. Significantly more and better data on the particles themselves are needed, as well as observations of high-energy gamma rays and searches for neutrinos associated with the same sources. A coordinated attack on the problem is essential. The elements of this program are in operation or scheduled for construction. They include GLAST, STACEE, and VERITAS for observation of gamma rays, AMANDA and IceCube to open the neutrino astronomy window, and Hi-Res and Auger for study of the highest-energy cosmic rays. The Southern Auger detector, in Argentina, is in its early phases of construction by an international collaboration with strong U.S. involvement and leadership. Completion and multiyear operation of the hybrid Auger detector is crucial because it can look at the highest-energy cosmic rays using two independent techniques, thereby providing cross calibration for detectors operating in the northern hemisphere using only one or the other technique. In addition, the Auger array will extend coverage to the southern sky. On the horizon are a larger ground-based detector complex in the northern hemisphere and proposals for large field-of-view observations from space of giant cosmic-ray air showers. The data that will be collected from

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the projects now in place are crucial for defining the science questions that will underpin the newer projects. Determine the origin of the highest-energy gamma rays, neutrinos, and cosmic rays. The committee supports the broad approach already in place and recommends that the United States ensure the timely completion and operation of the Southern Auger array. Recommendation on Exploring Physics Under Extreme Conditions in the Laboratory Astronomical telescopes provide glimpses of extreme physical environments under conditions that can never be replicated or probed experimentally on Earth. They challenge laboratory physicists to devise and perform experiments that will uncover the physical principles that can be scaled up to understand the most powerful astronomical sources, like quasars, neutron stars, supernova explosions, gamma-ray bursts, and the big bang. Conversely, observation of these astronomical phenomena can provide remote data points to bolster our understanding of these principles and to suggest new insights directly relevant to terrestrial investigations—a service that astronomy has been providing to physics for centuries. Although the field of high-energy-density experimentation is in its infancy, the capability will soon be at hand to push our understanding of condensed matter and plasma physics into regimes unimaginable a decade ago.4 One immediate challenge is to improve our understanding of the generic, global properties of plasmas under a broad range of conditions not specialized to the program to achieve fusion. It is intended to carry out these physics experiments over a wide range of conditions, using powerful lasers, electron beams, and magnetic pinch facilities. Another use of these plasma research facilities is to expand our measurements of the important spectral lines and opacities needed to interpret observations with ultraviolet and x-ray telescopes and to model cosmic explosions. The key to taking advantage of these unique facilities is to bring together a diverse group of scientists working in different disciplines and supported by different agencies. Discern the physical principles that govern extreme astrophysical environments through the laboratory study of high-energy- 4   Another NRC committee, chaired by R. Davidson, has been charged with surveying this field, and this committee will defer to it for detailed programmatic advice.

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density physics. The committee recommends that the agencies cooperate in bringing together the different scientific communities that can foster this rapidly developing field. Recommendation on an Interagency Initiative on the Physics of the Universe The committee has identified opportunities for major advances in our understanding of the birth and destiny of the universe and the fundamental laws that govern matter, space, and time. The opportunities lie at the intersection of a number of physics and astronomy disciplines and span the responsibilities of DOE, NASA, and NSF. While many opportunities have evolved from the existing programs of these agencies, these opportunities now transcend those programs, requiring combinations of expertise (e.g., particle accelerators and detectors and space experimentation) that are currently maintained by different agencies or by different disciplines within one of them. If the opportunities before us are to be realized, the three agencies must work together both in planning and in implementation. There are already a number of examples where such cooperation has succeeded, such as the Sloan Digital Sky Survey (NSF, DOE, and NASA), the Cryogenic Dark Matter Search (NSF and DOE), the BOOMERanG and MAXIMA cosmic microwave background experiments (NASA and NSF), and new initiatives just under way, including the Large Hadron Collider (DOE and NSF) and GLAST (NASA and DOE). Valuable lessons in management, coordination, and funding have been learned from these projects. No program in DOE, NASA, or NSF provides ongoing stewardship for, or funding of, the full breadth of this new field. Further, the talents and unique capabilities of all three agencies are required for progress to be made. Realize the scientific opportunities at the intersection of physics and astronomy. The committee recommends establishment of an interagency initiative on the physics of the universe, with the participation of DOE, NASA, and NSF. This initiative should provide structures for joint planning and mechanisms for joint implementation of cross-agency projects. Such an initiative can realize many of the special scientific opportunities that this report has described, but not within the budgets of the three agencies as they stand. The answer is not simply to trim existing programs to make room for these new initiatives. Many of the existing programs in

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astronomy and in physics are also critical to answering the 11 questions, as outlined in the first section of this chapter. Other programs address exciting and timely questions within physics and astronomy separately. New funds will be needed to realize the grand opportunities before us. In addition, the committee believes that it is essential that an interagency initiative on the physics of the universe maintain a balanced approach that provides opportunities for investigator-initiated experiments, detector R&D, theoretical work, and computational efforts that address the committee’s scientific questions but that do not necessarily fit within major program themes and their related large projects. Our understanding of the physics of the universe is often advanced by large projects, such as space observatories, particle-physics laboratories, or ground-based observation efforts. Indeed, most of the committee’s recommendations involve large projects. However, because the physics of the universe is interdisciplinary in character, significant advances can emerge from work carried out at the interface between fields. Often this work involves small-scale efforts, such as table-top experiments and detector development, or computational science and theory. Unlike many large-scale projects, some small-scale efforts are able to respond on a short time scale to address specific but important scientific questions. Remarkable advances have been made in the past two decades in our understanding of the basic constituents of matter and the forces that shape them. These advances, as well as technological breakthroughs, now present an unprecedented opportunity to answer some of the most fundamental questions that mankind can ask. Progress in addressing the fundamentals of matter, space, and time and progress in understanding the birth of the universe are now inextricably linked, so that astronomers and physicists as well as the agencies that fund them must work together more closely than ever before. The Committee on the Physics of the Universe believes that this is possible, and further, that if its recommendations are implemented, the next two decades could see a significant transformation of our understanding of the origin and fate of the universe, of the laws that govern it, and even of our place within it.