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Report of the Panel on Cosmology and Fundamental Physics

SUMMARY

Astronomical observations have become a vital tool for studying fundamental physics, and advances in fundamental physics are now essential for addressing the key problems in astronomy and cosmology.

The past 15 years have been a period of tremendous progress in cosmology and particle physics:

  • There is now a simple cosmological model that fits a host of astronomical data. Fifteen years ago, cosmologists considered a wide range of possible models; their best estimates of the Hubble constant differed by nearly a factor of two, and estimates of the mass density of the universe differed by as much as a factor of five. Today, the Lambda cold dark matter model is remarkably successful in explaining current observations, and the key cosmological parameters in this model have been measured by multiple techniques to better than 10 percent.

  • Measurements of the cosmic microwave background (CMB), supplemented by observations of large-scale structure (LSS), suggest that the very early universe underwent a period of accelerated expansion that is likely to be attributable to a period of cosmological “inflation.” The inflationary model predicts that the universe is nearly flat and that the initial fluctuations were Gaussian, nearly scale-invariant, and adiabatic. Remarkably, all of these predictions have now been verified.

  • The astronomical evidence for the existence of dark matter has been improving for more than 60 years. Within the past decade, measurements of acoustic peaks



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1 Report of the Panel on Cosmology and Fundamental Physics SUMMARY Astronomical observations have become a vital tool for studying fundamental physics, and advances in fundamental physics are now essential for addressing the key problems in astronomy and cosmology. The past 15 years have been a period of tremendous progress in cosmology and particle physics: • There is now a simple cosmological model that fits a host of astronomical data. Fifteen years ago, cosmologists considered a wide range of possible models; their best estimates of the Hubble constant differed by nearly a factor of two, and estimates of the mass density of the universe differed by as much as a factor of five. Today, the Lambda cold dark matter model is remarkably successful in explaining current observations, and the key cosmological parameters in this model have been measured by multiple techniques to better than 10 percent. • Measurements of the cosmic microwave background (CMB), supplemented by observations of large-scale structure (LSS), suggest that the very early universe underwent a period of accelerated expansion that is likely to be attributable to a pe- riod of cosmological “inflation.” The inflationary model predicts that the universe is nearly flat and that the initial fluctuations were Gaussian, nearly scale-invariant, and adiabatic. Remarkably, all of these predictions have now been verified. • The astronomical evidence for the existence of dark matter has been improv- ing for more than 60 years. Within the past decade, measurements of acoustic peaks 3

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Panel rePorts—new worlds, new HorIzons 4 in the CMB have confirmed the predictions of big bang nucleosynthesis (BBN) that the dark matter must be nonbaryonic. Gravitational lensing measurements have directly mapped its large-scale distribution, and the combination of lensing and X-ray measurements has severely challenged many of the modified-gravity alternatives to dark matter. • Supernova data, along with other cosmological observations, imply that the expansion of the universe is accelerating. This surprising result suggests either a breakdown of general relativity on the scale of the observable universe or the existence of a novel form of “dark energy” that fills space, exerts repulsive gravity, and dominates the energy density of the cosmos. • The discovery that neutrinos oscillate between their electron, muon, and tau flavors as they travel, and hence that they have mass, provides evidence for new physics beyond the standard model of particle physics. The effects of oscillations were seen in the first experiment to measure solar neutrinos, and the interpretation was confirmed by measurements of atmospheric neutrinos produced by cosmic rays and by new solar neutrino experiments with flavor sensitivity. • In the past few years, a cutoff has been seen in the energy spectrum of ultrahigh-energy cosmic rays (UHECRs) consistent with that predicted to arise from interactions with the CMB. UHECRs have become a powerful tool for prob- ing the active galactic nuclei (AGN), galaxy clusters, or radio sources responsible for accelerating such particles. Looking forward to the coming decade, scientists anticipate further advances that build on these results. The Astro2010 Science Frontiers Panel on Cosmology and Fundamental Phys- ics was tasked to identify and articulate the scientific themes that will define the frontier in cosmology and fundamental physics (CFP) research in the 2010-2020 decade. The scope of this panel report encompasses cosmology and fundamental physics, including the early universe; the cosmic microwave background; linear probes of large-scale structure using galaxies, intergalactic gas, and gravitational lensing; the determination of cosmological parameters; dark matter; dark energy; tests of gravity; astronomical measurements of physical constants; and fundamen- tal physics derived from astronomical messengers such as neutrinos, gamma rays, and ultrahigh-energy cosmic rays. In response to its charge, the panel identified four central questions that are ripe for answering and one general area in which there is unusual discovery potential: • How did the universe begin? • Why is the universe accelerating? • What is dark matter? • What are the properties of neutrinos? • Discovery area: Gravitational wave astronomy.

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rePort Panel cosMoloGy fundaMental PHysIcs 5 of tHe on and How Did the Universe Begin? Did the universe undergo inflation, a rapid period of accelerating expansion within its first moments? If so, what drove this early acceleration, when exactly did it occur, and how did it end? When introduced in the early 1980s, the inflationary paradigm made a number of generic observational predictions: we live in a flat universe seeded by nearly scale-invariant, adiabatic, Gaussian scalar fluctuations. Over the past decade, cosmological observations have confirmed these predic- tions. Over the coming decade, it may be possible to detect the gravitational waves produced by inflation, and thereby infer the inflationary energy scale, through measurements of the polarization of the microwave background. It may also be possible to test the physics of inflation and distinguish among models by precisely measuring departures from the predictions of the simplest models. Why Is the Universe Accelerating? Is the acceleration of the universe the signature of a breakdown of general rela- tivity on the largest scales, or is it due to dark energy? The current evidence for the acceleration of the universe rests primarily on measurements of the relationship between distance and redshift based on observations of supernovae, the CMB, and LSS. Improved distance measurements can test whether the distance-redshift relationship follows the form expected for vacuum energy or whether the dark energy evolves with redshift. Measurements of the growth rate of LSS provide an independent probe of the effects of dark energy. The combination of these mea- surements tests the validity of general relativity on large scales. The evidence for cosmic acceleration provides further motivation for improving tests of general relativity on laboratory, interplanetary, and cosmic scales, and for searching for variations in fundamental parameters. What Is Dark Matter? Astronomical observations imply that the dark matter is nonbaryonic. Particle theory suggests several viable dark matter candidates, including weakly interacting massive particles (WIMPs)1 and axions. Over the coming decade, the combina- tion of accelerator experiments at the Large Hadron Collider (LHC), direct and indirect dark matter searches, and astrophysical probes are poised to test these and other leading candidates and may identify the particles that constitute dark matter. 1 WIMPs are hypothetical particles serving as one possible solution to the dark matter problem. These particles interact through the weak nuclear force and gravity, and possibly through other interactions no stronger than the weak force. Because they do not interact with electromagnetism, they cannot be seen directly, and because they do not interact with the strong nuclear force, they do not react strongly with atomic nuclei.

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Panel rePorts—new worlds, new HorIzons 6 Successful detections would mark the dawn of dark matter astronomy: the use of measurements of dark matter particles or their annihilation products to probe the dynamics of the galaxy and the physics of structure formation. What Are the Properties of Neutrinos? What are the masses of the neutrinos? What are their mixing angles and cou- plings to ordinary matter? Is the universe lepton-number symmetric? Solar neutrino astronomy determined the Sun’s central temperature to 1 percent and provided the first evidence for neutrino oscillations. Neutrino events from Supernova 1987A confirmed scientists’ basic ideas about stellar core collapse and placed important new constraints on neutrino properties. Owing to rapid advances in neutrino- detection technologies, over the coming decade astronomers will be able to use neutrinos as precise probes of solar and supernova interiors and of ultrahigh- energy cosmic accelerators. Cosmological measurements of structure growth offer the most sensitive probe of the neutrino mass scale, with the potential to reach the 0.05-eV lower limit already set by oscillation experiments. The next generation of neutrino detectors could detect the cosmic background of neutrinos produced over the history of star formation and collapse. Ultrahigh-energy neutrino detectors will record the neutrino by-products of the interactions of UHECRs with CMB photons, the same interactions that degrade the energy of the charged particles and cause the high-energy cutoff. These experiments offer a unique probe of physics at and beyond the TeV scale. Improved measurements of light-element abundances might relieve the current tension between the predictions of BBN and observa- tions, or they might amplify this tension and point the way to a revised model of neutrino physics or the early universe. Discovery Area: Gravitational Wave Astronomy With upcoming and prospective experiments about to open a new window on the universe, gravitational wave astronomy is an area of unusual discovery potential that may yield truly revolutionary results. Gravitational waves, on the verge of being detected, can be used both to study astrophysical objects of central importance to current astronomy and to perform precision tests of general relativity. The strongest known sources of gravitational waves involve extreme conditions—black holes and neutron stars (and especially the tight binary systems containing them), core-collapse supernovae, evolving cosmic strings, and early-universe fluctua- tions—and studies of these phenomena can advance the understanding of matter at high energy and density. General relativity predicts that gravitational waves propagate at the speed of light and produce a force pattern that is transverse and quadrupolar. Observations of black hole mergers with high signal-to-noise ratios

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rePort Panel cosMoloGy fundaMental PHysIcs 7 of tHe on and will make possible extremely precise tests of many predictions of general relativity in the strong-field regime, such as whether black holes really exist and whether the warped space-time that surrounds them obeys the theorems developed by Hawking, Penrose, and others. And because merging black hole binaries act as “standard sirens,” there is a well-understood relationship between their waveform and their intrinsic luminosity. If their optical counterparts can be detected, they will enable a novel approach to absolute distance measurements of high-redshift objects. A worldwide network of terrestrial laser interferometric gravitational wave observatories is currently in operation, covering the 10- to 1,000-Hz frequency range. This network may soon detect neutron star–black hole mergers and stellar mass black hole–black hole mergers. Operating in the much lower nanohertz (10–9 Hz) frequency range are pulsar timing arrays. The low-frequency range, between 10–5 and 10–1 Hz, is believed to be rich in gravitational wave sources of strong interest for astronomy, cosmology, and fundamental physics. This portion of the gravitational wave spectrum can be accessed only from space. Space-based detec- tions can achieve much higher precision measurements of black hole mergers and thus much stronger tests of general relativity. Theoretical and Computational Activities Theory and observation are so closely intertwined in investigations of cosmol- ogy and fundamental physics that it is often difficult to define the border between them. Many of the ideas that are central to the next decade’s empirical investiga- tions originated decades ago as theoretical speculations. Many of the tools now being used for these investigations grew out of theoretical studies that started long before the methods were technically feasible. Theory plays an important role in de- signing experiments, optimizing methods of signal extraction, and understanding and mitigating systematic errors. Theoretical advances often amplify the scientific return of a data set or experiment well beyond its initial design. More-speculative, exploratory theory may produce the breakthrough that leads to a natural explana- tion of observed phenomena or a prediction of extraordinary new phenomena. In all these areas, high-performance computing plays a critical and growing role. Robust development of a wide range of theoretical and computational activities is essential in order to reap the return from the large investments in observational facilities envisioned over the next decade. Key Activities Identified by the Panel The panel identified the following key activities as essential to realizing the scientific opportunities within the decade 2010-2020 (the list is unranked):

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Panel rePorts—new worlds, new HorIzons 8 Inflation and Acceleration • Measure the amplitude of the initial scalar fluctuations of both matter and space-time across all observationally accessible scales through measurements of CMB E-mode polarization, the LSS of galaxies, weak lensing of galaxies and the CMB, and fluctuations in the intergalactic medium. • Search for ultra-long-wavelength gravitational waves through measure- ments of CMB B-mode polarization, achieving sensitivities to the tensor-scalar ratio at the level set by astronomical foregrounds. Detection of these gravitational waves would determine the energy scale of inflation. • Search for isocurvature modes, non-Gaussian initial conditions, and other deviations from the fluctuations predicted by the simplest inflationary models. • Measure the curvature of the universe to a precision of 10−4, the limit set by horizon-scale fluctuations. • Determine the history of cosmic acceleration by measuring the distance- redshift relation and Hubble parameter to sub-percent accuracy over a wide range of redshifts. • Determine the history of structure growth by measuring the amplitude of matter clustering to sub-percent accuracy over a wide range of redshifts. • Improve measurements that test the constancy of various physical constants and the validity of general relativity. Dark Matter • Probe both spin-independent and spin-dependent dark matter scattering cross sections with searches that explore much of the parameter space of WIMP candidates, through both underground experiments and searches for dark matter annihilating to neutrinos. Although a review of laboratory dark matter detection methods is outside the scope of this panel’s charge, progress in this area (as well as progress at the Large Hadron Collider) is critical for determining the properties of dark matter. As noted in the NRC report Revealing the Hidden Nature of Space and Time,2 the proposed International Linear Collider may turn out to be an essential tool for studying dark matter. • Carry out indirect searches for dark matter that probe the annihilation cross sections of weakly interacting thermal relics. Identifying “smoking gun” signals is essential for detecting dark matter annihilation products above the astronomical backgrounds. • Improve astrophysical constraints on the local dark matter density and 2 National Research Council, Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics, The National Academies Press, Washington, D.C., 2006.

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rePort Panel cosMoloGy fundaMental PHysIcs 9 of tHe on and structure on subgalactic scales to test the paradigm of cold, collisionless, and stable dark matter and to look for evidence for alternative dark matter candidates. These astronomical observations, particularly of dwarf galaxies, help to optimize dark matter search strategies and will be critical for determining the implications of dark matter signals for the particle properties of dark matter. Neutrinos • Develop the sensitivity to detect and study the ultrahigh-energy (UHE) neutrinos that can be expected if the cosmic-ray energy cutoff is due to protons annihilating into neutrinos and other particles. UHE neutrino fluxes above those expected from the Greisen-Zatsepin-Kuzmin (GZK) mechanism would be the signature of new acceleration processes. • Measure the neutrino mass to a level of 0.05 eV, the lower limit implied by current neutrino mixing measurements, through its effects on the growth of structure. • Enable precision measurement of the multiflavor neutrino “light curves” from a galactic supernova. • Improve measurements of light-element abundances in combination with big bang nucleosynthesis theory to test neutrino properties and dark matter models. Gravitational Waves • Detect gravitational waves from mergers of neutron stars and stellar mass black holes. • Detect gravitational waves from inspiral and mergers of supermassive black holes at cosmological distances. • Achieve high signal-to-noise ratio measurements of black hole mergers to test general relativity in the strong-field, highly dynamical regime. • Identify electromagnetic counterparts to gravitational wave sources. • Open a radically new window on the universe, with the potential to reveal new phenomena in stellar-scale astrophysics, early-universe physics, or other un- anticipated areas. Theory • Advance theoretical work that provides the foundation for empirical ap- proaches, through the development of methods, design of experiments, calculation of systematic effects, and statistical analysis. • Advance theoretical work that provides interpretation of empirical results in terms of underlying physical models.

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Panel rePorts—new worlds, new HorIzons 10 • Push the frontiers of exploratory theory, which can lead to breakthrough ideas needed to address the deep mysteries of cosmology and fundamental physics. INTRODUCTION Since the dawn of modern science, advances in fundamental physics have elu- cidated the deepest mysteries of astronomy. As part of this symbiotic relationship, astronomical observations have stimulated new advances in fundamental physics. Kepler, Galileo, and Newton devised new theories of motion, force, and universal gravity to explain the wandering of the planets across the sky. Quantum mechan- ics enabled the understanding of stellar spectra and revealed that stars are made primarily of hydrogen and helium rather than of the oxygen, silicon, and iron that dominate Earth and meteorites. Advances in nuclear physics were essential for explaining the unknown energy source of stars. Today, astronomers confront new mysteries: dark matter, cosmic acceleration, and the origin of structure (Figure 1.1). Again, advances in fundamental physics are needed—and astronomy again offers a powerful laboratory for probing fundamental physics. Over the past three decades, astronomers and physicists have made remarkable progress toward a detailed scientific theory of the cosmos, a “standard model” of cosmology that explains observations that probe an enormous range of time and distance. But this theory is still incomplete, and it relies on three key physical ideas that are at best partly understood: inflation, cold dark matter, and vacuum energy. The inflation hypothesis, first proposed in the early 1980s, asserts that the universe grew by an enormous factor during its first moments. This accelerat- ing expansion not only erases any pre-existing fluctuations but also generates a nearly scale-invariant spectrum of Gaussian fluctuations that leave an imprint on the CMB and grow to form galaxies and clusters of galaxies. Cold dark matter, composed of weakly interacting particles with low thermal velocities in the early universe, explains the dynamics of galaxies and clusters and allows consistency be- tween CMB and LSS observations. Vacuum energy exerts repulsive gravity, driving the present-day acceleration of cosmic expansion (which is many, many orders of magnitude slower than the acceleration hypothesized to occur during inflation). Despite its observational successes, this standard model is unsatisfying in several ways. Scientists do not know the physics that caused inflation to happen or to end, nor do they know for sure that inflation is the mechanism that created a large, radiation-filled universe and seeded it with fluctuations. There are several plausible ideas for what the dark matter might be, but it is not known which of them, if any, is correct. By far the most surprising element of the model is the vacuum energy. While quantum physics does allow “empty” space to be filled with energy, the naively predicted value of this energy is 10120 times larger than the value allowed by ob-

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rePort Panel cosMoloGy fundaMental PHysIcs 11 of tHe on and FIGURE 1.1 The composition of the universe has evolved over the 13.7 billion years since the big bang. Today the universe is dominated by dark energy (upper), whereas after the big bang it was dominated by dark matter (lower). SOURCE: NASA/WMAP Science Team. servations. It may be that the true vacuum energy is zero and a pervasive, previ- ously unknown fundamental field, akin to the field that caused inflation in the early universe, is driving the present-day acceleration. Alternatively, the observed acceleration could be a sign that general relativity itself breaks down on the scale of the observable universe. The most-studied hypothesized dark matter particles are in many ways analo- gous to neutrinos in that they interact with baryonic matter only by way of gravity and the weak interaction (although neutrinos are much, much less massive). Over the past four decades, and especially over the past decade, progress in neutrino physics has been driven principally by astronomical observations. Most notably, observations of solar neutrinos and of atmospheric neutrinos produced by cosmic rays have demonstrated that the three neutrino species in the standard model of particle physics have non-zero mass and that they oscillate from one form to an-

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Panel rePorts—new worlds, new HorIzons 12 other as they propagate through matter or empty space. Cosmological observations set the strongest upper limits on the neutrino mass; they show that the standard- model neutrinos cannot be the main form of dark matter, but it remains possible that a fourth, sterile neutrino species could constitute the dark matter. These developments, and the successes and limitations of the current cosmo- logical model, suggest the following four questions to guide research in cosmology and fundamental physics over the next decade: • How did the universe begin? • Why is the universe accelerating? • What is dark matter? • What are the properties of neutrinos? The sections that follow elucidate these questions and describe the capabilities needed to answer them. The panel also identified gravitational wave astronomy as an emerging science area with unusual discovery potential. Scientists expect the next decade to see the first direct detection of gravitational waves, the propagating ripples of space-time predicted by Einstein nearly a century ago. The strongest expected sources of gravitational waves are violent events such as mergers of black holes and neutron stars; gravitational wave measurements will provide unique insights into the physics of these events and allow powerful tests of general relativity in a completely new regime. More enticing still are the prospects for sources that have not yet been imagined or have only been speculated about, perhaps new classes of stellar implo- sions or collisions, or backgrounds of gravitational waves produced in the early universe. If the history of radio astronomy and X-ray astronomy is any guide, then the dawn of gravitational wave astronomy will fundamentally change our view of the cosmos and the objects that it contains. The panel discusses the extraordinary discovery potential of gravitational wave astronomy and the technical capabilities needed to realize it in the section below titled “CFP Discovery Area—Gravitational Wave Astronomy: Listening to the Universe.” Three themes connect the observational approaches to all of these questions: • The first is the mapping of cosmological initial conditions over the widest possible dynamic range with measurements of CMB temperature and polarization fluctuations, observations of weak lensing, and optical and radio observations that use galaxies and intergalactic gas to map the distribution of matter at lower redshifts. The enormous increase in statistical precision and dynamic range now possible will enable new tests of models of inflation, precision measurements of the geometry of space, the determination of the masses of neutrinos by means of their cosmological effects, and tests of theories for the origin of cosmic acceleration.

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rePort Panel cosMoloGy fundaMental PHysIcs 13 of tHe on and Realizing these vast improvements in statistical power requires exquisitely careful control of systematic uncertainties, which often present the greatest challenge to these methods. • The second theme is the opening of new windows that allow scientists to view astrophysical phenomena in radically new ways. Gravitational waves are the most dramatic example of such a new window, but they are not the only one. Dark matter searches hinge on great advances in the sensitivity and sky coverage of high- energy gamma-ray and cosmic-ray experiments. New facilities should achieve the first detections of ultrahigh-energy neutrinos. Searches for highly redshifted 21-cm radiation will provide the first three-dimensional maps of structure at the epoch of cosmic reionization, and advances in these techniques should eventually allow maps of cosmic initial conditions over unprecedented volumes. • The third theme is the universe as a laboratory for fundamental physics. Studies of primordial fluctuations probe early-universe physics at energies that can never be achieved in terrestrial accelerators. The explanation of cosmic acceleration may radically reshape the understanding of gravity, the quantum vacuum, or both. Dark matter experiments provide windows on extensions of the standard model that beautifully complement the traditional tools of particle physics. Astrophysi- cal measurements provide the most powerful and varied constraints on neutrino properties. Gravitational waves probe general relativity in the strong-field regime, a test that can be done only in the extreme environment near black holes. CFP 1. HOW DID THE UNIVERSE BEGIN? Although little is understood about the origin of the universe, cosmologists have made significant progress in studying its very early history. In the early 1980s, they theorized that during its first moments, the universe underwent a rapid period of accelerated expansion called inflation. During inflation, microscopic, causally connected regions expanded exponentially, driving the spatial curvature to nearly zero and producing a homogeneous universe. This inflationary paradigm not only explained many of the open questions in cosmology but also predicted that quantum fluctuations of both matter and space-time curvature create nearly scale- invariant, adiabatic, Gaussian random phase fluctuations. During the past decade, observations have shown impressive agreement with these predictions. Although inflation is a successful paradigm, its underlying mechanism re- mains a mystery. Inflation may have something to do with grand unified theories that amalgamate the strong and electroweak interactions at an ultrahigh-energy scale. It may derive from a quantum-gravity theory such as string theory. Infla- tion may arise at some lower-energy phase transition, such as the breaking of the Peccei-Quinn symmetry posited to explain the lack of charge-parity violation in the strong interaction. It may be a consequence of the compactification of large

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Panel rePorts—new worlds, new HorIzons 42 BOX 1.3 Conclusions Regarding Neutrinos by the Science Frontiers Panel on Cosmology and Fundamental Physics Goals • Develop the sensitivity to detect and study ultrahigh-energy (UHE) neutrinos expected if the cosmic-ray energy cutoff is due to protons annihilating into neutrinos and other particles. The detection of UHE neutrino fluxes above those expected from the Greisen-Zatsepin-Kuzmin (GZK) mechanism would be the signature of new acceleration processes. • Measure the neutrino mass to a level of 0.05 eV, the lower limit implied by current neutrino mixing measurements, through its effects on the growth of structure. • Enable precision measurement of the multiflavor neutrino “light curves” from a galactic super- nova. • Improve measurements of light-element abundances in combination with big bang nucleosyn- thesis theory to test neutrino properties and dark matter models. Needed Capabilities • Measurements of small-scale structure from dwarf-galaxy dynamics, gravitational lensing, and the Lyman-a forest. • Precision measurements of the power-spectrum amplitude using a combination of cosmic mi- crowave background lensing, weak lensing, the galaxy power spectrum, and measurements of neutral hydrogen fluctuations. • Neutrino detectors that can measure the time, energy, and flavor distribution of neutrinos from a nearby supernova and detect the integrated supernova neutrino background. • Radio-frequency experiments for UHE neutrinos, with sensitivity to detect the expected events associated with the proton GZK cutoff. • Improved characterization of the energy spectrum and sources of cosmic rays near the GZK cutoff. • Improved measurements of light-element abundances in stellar atmospheres and the interstellar and intergalactic medium, principally through high-resolution spectroscopy on approximately 30-m telescopes. CFP DISCOVERY AREA—GRAVITATIONAL WAVE ASTRONOMY: LISTENING TO THE UNIVERSE In the past century, our ability to view the universe expanded to encompass a vast sweep of the electromagnetic spectrum from gamma rays to radio waves, bringing with it the discovery of many unexpected phenomena. In the coming decade, some of the most exciting discoveries may come from opening a new ob- servational window with the first direct detections of gravitational waves. In the same way that the sense of hearing complements the sense of sight, gravitational wave observations complement and enrich what can be learned elec-

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rePort Panel cosMoloGy fundaMental PHysIcs 43 of tHe on and tromagnetically. Gravitational waves are produced by the bulk motions of matter, and they propagate essentially unabsorbed through even the densest material to convey information about the overall dynamics of the source. In contrast, electro- magnetic waves tell only about the thermal and magnetic environment of the gas that surrounds a source, and they can be bent or absorbed along their propagation paths to telescopes. However, the weak coupling to matter that allows gravitational waves to travel unimpeded also makes them very hard to detect: the merger of two stellar rem- nant black holes at 10 Mpc would bathe Earth with a peak energy flux exceeding 10 percent of the solar constant, but so little of this energy is captured that the mirrors of a kilometer-scale detector are displaced by less than 10 percent of the width of a proton. Compelling though indirect evidence for their existence can be seen in the orbital decay of binary pulsar systems, whose discovery earned Hulse and Taylor the 1993 Nobel Prize in physics. Efforts at direct detection initially em- ployed massive bar detectors fitted with vibration sensors but are now focused on laser interferometers, which use interference of laser beams to detect the minute motions of mirrors suspended at the ends of kilometers-long evacuated cavities. A worldwide network of terrestrial interferometers is currently in operation, covering the frequency range 10 to 1,000 Hz with the sensitivity to detect relative displace- ments a thousand times smaller than the width of a proton. Operating in the much lower nanohertz (10–9 Hz) frequency range are pulsar-timing arrays, which seek to detect gravitational waves by the delays that the waves impart on the arrival times of pulses from radio pulsars. The low-frequency range, between 10–5 and 10–1 Hz, is believed to be rich in gravitational wave sources of strong interest for astronomy, cosmology, and fundamental physics. Because terrestrial sources of noise dominate at such low frequencies, this portion of the gravitational wave spectrum can be accessed only from space. Gravitational Wave Astrophysics All astrophysical objects emit gravitational radiation at some level, but the ex- treme stiffness of space-time implies that only systems that pack a large amount of rapidly moving material into a small volume will emit detectable signals. As a general rule, the more massive systems radiate deeper and louder signals. Accordingly, for example, high-frequency ground-based interferometers look for black hole mergers with total mass of 1 to 103 solar masses, whereas low-frequency space-based systems will look for mergers with total mass of 103 to 107 solar masses (Figure 1.9). Among the anticipated results from ground-based detectors are measurements of the merger rates of binary systems containing two black holes or two neutron stars (or one of each), measurements of the deviations from spherically symmetric

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Panel rePorts—new worlds, new HorIzons 44 (a) (b) FIGURE 1.9 (a) Recent advances in numerical relativity make it possible to simulate accurately the ripples in 1-9 b.eps the curvature of space-time stirred up by the merger of two black holes. The top image shows two spinning bitmap black holes that are spiraling together toward merger. The black hole horizons (gray surfaces) and spin direc- tions (green arrows) are shown. A cross section of the adaptive mesh refinement computational grid is shown, with a cut-away view of the strong dynamical gravitational fields underneath. (b) The simulated response to the late inspiral and merger of two 105 M◉ black holes at z = 15 is superimposed on the galactic foreground from white-dwarf binaries and the instrument noise from a space detector. In contrast to the ground-based detectors, these signals will be clearly visible in the raw data, with signal-to-noise ratios in the hundreds of thousands. The noise in the plot is the unresolved gravitational radiation from the ordinary binary stellar systems in our galaxy. SOURCE: (a) Image courtesy of Chris Henze, NASA Ames Research Center. (b) Figure reprinted with permission from J. Baker, S.T. McWilliams, J.R. van Meter, J. Centrella, D.-I. Choi, B.J. Kelly, and M. Koppitz, Binary black hole late inspiral: Simulations for gravitational wave observations, Physical Review (Section) D: Particles and Fields 75:124024, 2007, copyright 2007 by the American Physical Society.

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rePort Panel cosMoloGy fundaMental PHysIcs 45 of tHe on and or pure dipole signals6 that can be supported by millisecond pulsars, constraints on the equation of state of matter at nuclear densities through observations of tidally distorted neutron star mergers, and exploration of the dynamics of nearby core- collapse supernovae. By mid-decade it should known if compact binary mergers are indeed responsible for short-hard gamma-ray bursts, and if gravitational wave emission is indeed balancing the accretion torque in low-mass X-ray binaries. Even the most pessimistic estimates of rates and strengths of signals predict a detec- tion by the advanced ground-based interferometers scheduled to be operational by 2014. Beyond the anticipated discoveries, the opening of a new observational window will likely produce surprising, and perhaps even revolutionary, results. Pulsar-timing arrays have the potential to detect a stochastic background from the slow inspiral of supermassive black hole binaries (108 to 1010 solar masses) in the frequency range 10–9 to 10–6 Hz, and perhaps to resolve a few of the brighter systems. With modest enhancements to the existing arrays, a positive detection is likely by the latter part of the decade. Such measurements will fix the merger rate of supermassive black holes and provide unique constraints on models of black hole growth. In the low-frequency (10−5 to 10−1 Hz) portion of the spectrum, accessible only from space, there are likely to be so many sources that a background of waves from weaker systems will be a dominant source of noise. Above this noise it will still be possible to detect black hole mergers in the mass range 3 × 103 to 107 solar masses out to redshift 20 or greater. These observations will reveal the masses and spins of the black holes and will indicate the merger rate as a function of distance. For example, with typical sensitivity parameters for a space interferometer, including noise from foreground sources, for two 106 M◉ nonspinning black holes merging at z = 10, the total mass of the system could be measured to 0.1 percent, and at z = 1, to 0.001 percent. This information can be used to test theories of how galaxies and black holes coevolve, and to determine the relative importance of gas accretion and mergers in massive black hole growth. If the basic ideas of massive black hole growth are quali- tatively correct, tens to hundreds of events per year for inspirals at the high-mass end may be detectable. For inspirals at the low-mass end, the rates are highly uncer- tain. Observations of low-redshift systems can be used to confirm the existence of intermediate-mass (500 to 104 solar mass) black holes and to probe their properties. Also visible out to z ≈ 2 will be the capture of stellar remnants (mostly black holes and neutron stars) by supermassive black holes in galactic nuclei. Observations 6 Only nonsymmetric distributions of mass emit gravitational waves. Spherically symmetric dis- tributions of charge or mass emit neither electromagnetic nor gravitational waves. Distributions of charge with a dipolar symmetry emit electromagnetic waves; however, there are no dipolar gravita- tional waves.

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Panel rePorts—new worlds, new HorIzons 46 of these “extreme mass ratio inspirals” (EMRIs) will allow for precision measure- ments of the mass and spin of the central black holes, and the rate of these mergers as a function of eccentricity can be used to constrain models of galactic cores. The most common signals detected from space, however, will be from bi- nary systems of white dwarfs in our own galaxy, and tens of thousands of these systems are expected to be individually resolved. The masses and positions of all short-period binaries (periods less than about 11 minutes) in our galaxy will be measured, providing unique constraints on population-synthesis models and pro- viding a new window for the study of white-dwarf interiors through tidally induced oscillations. Joint electromagnetic/gravitational wave observations of white-dwarf binaries can be used to constrain the mass of the graviton and the polarization pattern of gravitational waves. Once again, opening up several decades of a new observational spectrum is fertile ground for surprises, from the exotic (for example, cosmic superstrings) to others that are wholly unanticipated. Looking for New Physics with Gravitational Waves The gravitational waves in the universe today preserve a record of all macro- scopic mass-energy flows over the entire history of the universe. They can be used to probe aspects of new physics never before explored. The possibilities include first- order phase transitions leading to bubble nucleation and collision, the dynamics of extra spatial dimensions, inflationary reheating, and a writhing network of cosmic (super)strings. The radiation from these and other exotic processes occurring in the early universe when the temperature was 0.1 to 1,000 TeV will have been redshifted to the frequency range explored by a space-based instrument. This nice coincidence means that gravitational waves have the potential to explore weak-scale physics. Binary black hole mergers will provide stringent tests of general relativity (Figure 1.10). These systems are “simple,” consisting of pure space-time curvature, while their strong signals, even when emitted from cosmological distances, can dominate over noise in space-based measurements. Because the final inspiral and merger of two compact bodies are dominated by their mutual gravity, the orbit and gravitational wave signal will reflect strong-field, dynamical, curved space-time general relativity in its full glory. Detailed comparisons between the measured waveforms and theoretical waveforms calculated from combinations of analytical and numerical solutions of Einstein’s equations will give a rich variety of tests of the theory in a regime that has hitherto been inaccessible to experiment or observation. Also detectable will be “ringdown” waves, emitted by the distorted black hole produced by the merger as it settles down to a stationary state. These waves have discrete frequencies and damping rates that depend on the mass and spin of the hole. Thus, measurements of the ringdown will test whether geometry obeys the no-hair theorems predicted by general relativity.

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rePort Panel cosMoloGy fundaMental PHysIcs 47 of tHe on and FIGURE 1.10 Simulations of black hole mergers suggest that spin may play a critical role in how black holes merge; depending on the magnitude and alignment of the spins, the mergers could be very rapid or could experi- ence a momentary “hang-up,” with significant consequences for the observed waveform. Shown in this figure are the final few orbits of two black holes (one in red, the other in blue), with arrows denoting their spin mag- nitude and direction. These complex spin-induced orbital effects are the consequences of “frame dragging,” a fundamental prediction of Einstein’s theory that has been probed in the solar system using Gravity Probe B, Laser Geodynamics (LAGEOS) satellites, and lunar laser ranging, and has been hinted at in observations of accretion onto neutron stars and black holes. Observing the effects of frame dragging in such an extreme environment would be a stunning test of general relativity. Furthermore, with spinning progenitors, the final black hole could experience substantial recoil resulting from the emission of linear momentum in the gravitational waves, large enough to eject it from the host galaxy. SOURCE: Reprinted with permission from M. Campanelli, C.O. Lousto, and Y. Zlochower, Spinning-black-hole binaries: The orbital hang-up, Physical Review (Section) D: Particles and Fluids 74:041501, 2006, copyright 2006 by the American Physical Society. EMRIs may be observed by a space-based detector and can provide incredibly precise quantitative tests of the space-time geometry of black holes. Over the 104 to 105 eccentric, precessing orbits traced out by the smaller mass, the emitted waves encode details about the space-time structure of the larger hole with a variety of distinct signatures. In addition to providing determinations of the black hole’s mass and angular momentum to fractions of a percent, the observations can also be used to test whether the space-time that encodes the waves is the unique Kerr geometry that general relativity predicts for rotating black holes. Gravitational waves can also be used to test specific theories alternative to gen- eral relativity. If gravity propagates with a speed that depends on wavelength (for example if the putative “graviton” were massive), a strong constraint on its mass could be placed by searching for deviations in the phases of the arriving waves from the general-relativistic predictions. It may be possible to test whether the prediction of only two transverse quadrupolar modes is correct. If astronomers can detect the

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Panel rePorts—new worlds, new HorIzons 48 electromagnetic signature associated with the inspiral, then differences in arrival times would place powerful constraints on the graviton mass. Binary black hole inspirals provide standard candles free of calibrations for measuring the distance to the source. For these “clean” systems, there is an expected relationship between the waveform shape and the luminosity of the source. A space-based detector could measure luminosity distances to a few percent at red- shift 2, and to tens of percent at z = 10. At the same time, because of the changing orientation of the space array with respect to the source, its sky location could be determined to 10 arcminutes for massive inspirals at z = 1. This positional infor- mation can be used to search for electromagnetic counterparts, which can in turn be used to measure the redshift. The combination of several such measurements could give a dark energy bound that begins to be competitive with conventional dark energy approaches. Absent an electromagnetic counterpart, statistical associa- tions of galaxies with EMRIs can be used to measure the Hubble constant to a few percent, and to place constraints on the acceleration rate. Realizing the Discovery Potential A number of steps are needed to realize the discovery potential of gravitational wave astronomy. The first is to complete the upgrade of the network of ground- based, high-frequency interferometers in the United States and Europe. If these ar- rays achieve their proposed sensitivities of roughly 4 × 10–24/(Hz)1/2 at 100 Hz, there will be a very good chance of discovery of gravitational waves from stellar remnant inspirals and mergers. Development of pulsar timing arrays with the capability of detecting and timing approximately 40 millisecond pulsars with 100-nanosecond accuracy should be completed. Initially, the primary need is increased time alloca- tions at existing facilities, with advanced kilometer-scale detector arrays envisioned for the future. For the low-frequency band between 10−5 and 10−1 Hz, a space-based detec- tor is essential, with sensitivity capable of detecting massive black hole mergers to redshift 20 with a signal-to-noise ratio of at least 10. This requirement translates roughly to a strain sensitivity of 3 × 10–21/(Hz)1/2 in the millihertz range. The science to be learned from gravitational wave detections will be greatly en- hanced if observations of the same phenomena can be done in the electromagnetic (and possibly the neutrino) window. Because many gravitational wave sources are transient in nature (mergers, collapse), this will require wide-angle, high-cadence electromagnetic surveys, together with potentially rapid follow-up observations of sources with large telescopes. Observing electromagnetic counterparts will aid in source identification, sky localization, and redshift determination, and will make possible novel measurements of the distance-redshift relation. For massive black hole inspirals, a space detector could give weeks of warning of the final merger

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rePort Panel cosMoloGy fundaMental PHysIcs 49 of tHe on and BOX 1.4 Conclusions Relating to Gravitational Waves by the Science Frontiers Panel on Cosmology and Fundamental Physics Goals • Detect gravitational waves from mergers of neutron stars and stellar mass black holes. • Detect gravitational waves from inspirals and mergers of supermassive black holes at cosmologi- cal distances. • Achieve high signal-to-noise ratio measurements of black hole mergers to test general relativity in the strong-field, highly dynamical regime. • Identify electromagnetic counterparts to gravitational-wave sources. • Open a radically new window on the universe, with the potential to reveal new phenomena in stellar-scale astrophysics, early-universe physics, or other unanticipated areas. Needed Capabilities • A space-based gravitational wave interferometer probing the 10–5 to 10–1 Hz frequency range to the sensitivity limits imposed by astrophysical “foreground” noise from galactic binaries. • Ground-based interferometers probing the 10- to 1,000-Hz range with the sensitivity to detect neutron star mergers at 300-Mpc distances. • Pulsar-timing arrays probing the nanohertz frequency range with the sensitivity to detect the stochastic background from supermassive black hole binaries. • Time-domain electromagnetic facilities with the sensitivity, speed, and flexibility needed to find the counterparts of gravitational wave sources. event together with degree-accurate source positions, permitting narrower fields to be viewed. Panel Conclusions Regarding Gravitational Waves The conclusions of the panel with respect to gravitational waves are presented in Box 1.4. Included are goals and needed capabilities in this area. THEORY AND SYNTHESIS Theory is and has been essential to what we choose to observe and how we arrange to do so. Many of the ideas that are central to the next decade’s empirical investigations—inflation, supersymmetric dark matter, neutrino oscillations, black holes, and gravitational waves—began life decades ago as theoretical speculations. Many of the tools that are being used for these investigations—CMB polarization, weak lensing, BAO observations, laser interferometers—grew out of theoretical studies that started long before the methods were technically feasible.

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Panel rePorts—new worlds, new HorIzons 50 As these methods are moving toward implementation, theory plays an im- portant role in designing experiments, optimizing methods of signal extraction, and understanding and mitigating systematic errors. Theory and observation are so closely intertwined in investigations of cosmology and fundamental physics that it is often difficult to define the border between them. Examples include predicting the signals and backgrounds for dark-matter-detection observations, calculating the impact of intrinsic galaxy alignments on weak-lensing measure- ments, and computing waveforms for template matching in gravitational wave searches. Theoretical advances often amplify the scientific return of a data set or experiment well beyond its initial design. Potentially powerful techniques that are subjects of active theoretical scrutiny include redshift-space distortions as a preci- sion measure of structure growth, and scale-dependent galaxy bias as a sensitive probe of primordial non-Gaussianity. As data come in, theory assumes the pivotal role in tying them back to the underlying physics, whether it be computer models of core-collapse supernovae, phase transitions in the early universe, or extensions of the standard model of particle physics. Finally, more speculative, exploratory theory may produce the breakthrough that leads to a natural explanation of cosmic acceleration, a compelling physical mechanism for inflation, or the prediction of an extraordinary gravitational wave phenomenon yet to be observed. The above examples illustrate four distinct modes of theoretical work that are essential to progress in the next decade: 1. Before observation. Development of new methods, identification of new observables, and statistical forecasting. 2. During observation. Design of experiments, calculation of systematic effects, and statistical analysis to optimize the use of the data. 3. After observation. Interpretation of empirical results in terms of underlying physical models. 4. Exploratory theory at the frontiers of current knowledge. Although often spec- ulative and high risk, this mode of theoretical research can lead to breakthrough ideas that transform the field. Advances in high-performance computing are driving rapid progress in many areas of cosmology and fundamental physics. Examples that are central to the themes of this report include numerical simulations of structure formation needed to interpret maps of the galaxy distribution or to predict signals of dark matter annihilation; computational studies of core collapse and thermonuclear superno- vae; calculations of gravitational wave emission from mergers of spinning black holes; statistical analyses of large and complex data sets from CMB observations, LSS surveys, neutrino observations, and gravitational wave searches; and massive searches through high-dimensional parameter spaces to evaluate the statistical

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rePort Panel cosMoloGy fundaMental PHysIcs 51 of tHe on and uncertainties from varied combinations of data sets. To exploit these advances requires both cutting-edge computer hardware and the software, personnel sup- port, and the training of researchers needed to maximize its scientific reach. This support is needed at many levels, from the handful of ultrapowerful machines that enable the most ambitious calculations, through the larger and more varied tier of supercomputers available at national and state-supported centers, and on to high-performance clusters in individual research groups and the networks of workstations and laptops by which scientists access these facilities and examine the results of their computations. Although the advances in computational theory are dramatic, it is often pencil- and-paper theory that leads to novel ideas or identifies the connections between seemingly disparate phenomena. The frontiers of cosmology today present grand theoretical challenges: rooting models of inflation in more fundamental descrip- tions of underlying physics; explaining the asymmetry between matter and an- timatter and thus the origin of the particles that form Earth and the life on its surface; describing the interior structure of black holes and explaining their entropy in terms of quantum gravity; determining whether there are spatial dimensions beyond the three of everyday experience; explaining the surprising magnitude of cosmic acceleration and the seeming coincidence of the densities of baryons, dark matter, and dark energy; and determining whether our observable cosmos is a fully representative sample of the universe or one of many disparate bubbles in a much larger inflationary sea. Robust support for the full span of theoretical activities is essential in order to reap the return from large investments in observational facili- ties over the next decade, and also to ensure that the scientific opportunities in the 2020-2030 decade will be as exciting as those of today. BIBLIOGRAPHY Bahcall, J. 2004. Solar neutrinos: A popular account. Encyclopedia of Physics. Edition 3, arXiv:physics/0411190. Bertone, G., D. Hooper, and J. Silk. 2005. Particle dark matter: Evidence, candidates and constraints. Physics Reports 405:279. Blumer, J., R. Engel, and J.R. Horanfel. 2009. Cosmic rays from the knee to the highest energies. Progress in Particle and Nuclear Physics 63:292. Camilleri, L., E. Lisi, and J. Wilkerson. 2008. Neutrino masses and mixings: Status and prospects. Annual Review of Nuclear and Particle Science 58:343-369. Fields, B., and K. Olive. 2006. Big bang nucleosynthesis. Nuclear Physics A 777:208. Frieman, J.A., M.S. Turner, and D. Huterer. 2008. Dark energy and the accelerating universe. Annual Review of Astronomy and Astrophysics 46:385-432. Hannestad, S. 2006. Primordial neutrinos. Annual Review of Nuclear and Particle Science 56:137-161. Hoffman, K.D. 2009. High energy neutrino telescopes. New Journal of Physics 11:055006. Hughes, S.A. 2003. Listening to the universe with gravitational-wave astronomy. Annals of Physics 303:142. Sathyaprakash, B.S., and B.F. Schutz. 2009. Physics, astrophysics and cosmology with gravitational waves. Living Reviews in Relativity 12:2. Available at http://www.livingreviews.org/lrr-2009-2. Sommers, P., and S. Westerhoff. 2009. Cosmic ray astronomy. New Journal of Physics 11:055004.

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Panel rePorts—new worlds, new HorIzons 52 Stecker, F.W., and S.T. Scully. 2009. Searching for new physics with ultra-high energy cosmic rays. New Journal of Physics 11:085003. Steigman, G. 2009. Primordial nucleosynthesis after WMAP. In Chemical Abundances in the Universe: Connecting First Stars to Planets, Proceedings of the IAU, Volume 265, arXiv 0909.3270. Uzan, J.-P. 2003. The fundamental constants and their variation: Observational status and theoretical motivations. Reviews of Modern Physics 75:403-455. Uzan, J.-P. 2009. Fundamental constants and tests of general relativity—Theoretical and cosmological consider- ations. Space Science Reviews 148:249-265.