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Executive Summary We are at a special moment in our journey to understand the universe and the physical laws that govern it. More than ever before astronomical discoveries are driving the frontiers of elementary particle physics, and more than ever before our knowledge of the elementary particles is driving progress in understanding the universe and its contents. The Committee on the Physics of the Universe was convened in recognition of the deep connections that exist between quarks and the cosmos. THE QUESTIONS Both disciplines—physics and astronomy—have seen stunning progress within their own realms of study in the past two decades. The advances made by physicists in understanding the deepest inner workings of matter, space, and time and by astronomers in understanding the universe as a whole as well as the objects within it have brought these scientists together in new ways. The questions now being asked about the universe at its two extremes—the very large and the very small—are inextricably intertwined, both in the asking and in the answering, and astronomers and physicists have been brought together to address questions that capture everyone’s imagination. The answers to these questions strain the limits of human ingenuity, but the questions themselves are crystalline in their clarity and simplicity. In framing this report, the committee has seized on 11 particularly direct questions that encapsulate most of the physics and astrophysics discussed here. They do not cover all of these fields but focus instead on the interface between them. They are also questions that we have a good chance of answering in the next decade, or should be thinking about answering in
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following decades. Among them are the most profound questions that human beings have ever posed about the cosmos. The fact that they are ripe now, or soon will be, further highlights how exciting the possibilities of this moment are. The 11 questions are these: What Is Dark Matter? Astronomers have shown that the objects in the universe, from galaxies a million times smaller than ours to the largest clusters of galaxies, are held together by a form of matter different from what we are made of and that gives off no light. This matter probably consists of one or more as-yet-undiscovered elementary particles, and aggregations of it produce the gravitational pull leading to the formation of galaxies and large-scale structures in the universe. At the same time these particles may be streaming through our Earth-bound laboratories. What Is the Nature of Dark Energy? Recent measurements indicate that the expansion of the universe is speeding up rather than slowing down. This discovery contradicts the fundamental idea that gravity is always attractive. It calls for the presence of a form of energy, dubbed “dark energy,” whose gravity is repulsive and whose nature determines the destiny of our universe. How Did the Universe Begin? There is evidence that during its earliest moments the universe underwent a tremendous burst of expansion, known as inflation, so that the largest objects in the universe had their origins in subatomic quantum fuzz. The underlying physical cause of this inflation is a mystery. Did Einstein Have the Last Word on Gravity? Black holes are ubiquitous in the universe, and their intense gravity can be explored. The effects of strong gravity in the early universe have observable consequences. Einstein’s theory should work as well in these situations as it does in the solar system. A complete theory of gravity should incorporate quantum effects—Einstein’s theory of gravity does not—or explain why they are not relevant.
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What Are the Masses of the Neutrinos, and How Have They Shaped the Evolution of the Universe? Cosmology tells us that neutrinos must be abundantly present in the universe today. Physicists have found evidence that they have a small mass, which implies that cosmic neutrinos account for as much mass as do stars. The pattern of neutrino masses can reveal much about how nature’s forces are unified, how the elements in the periodic table were made, and possibly even the origin of ordinary matter. How Do Cosmic Accelerators Work and What Are They Accelerating? Physicists have detected an amazing variety of energetic phenomena in the universe, including beams of particles of unexpectedly high energy but of unknown origin. In laboratory accelerators, we can produce beams of energetic particles, but the energy of these cosmic beams far exceeds any energies produced on Earth. Are Protons Unstable? The matter of which we are made is the tiny residue of the annihilation of matter and antimatter that emerged from the earliest universe in not-quite-equal amounts. The existence of this tiny imbalance may be tied to a hypothesized instability of protons, the simplest form of matter, and to a slight preference for the formation of matter over antimatter built into the laws of physics. What Are the New States of Matter at Exceedingly High Density and Temperature? The theory of how protons and neutrons form the atomic nuclei of the chemical elements is well developed. At higher densities, neutrons and protons may dissolve into an undifferentiated soup of quarks and gluons, which can be probed in heavy-ion accelerators. Densities beyond nuclear densities occur and can be probed in neutron stars, and still higher densities and temperatures existed in the early universe. Are There Additional Space-Time Dimensions? In trying to extend Einstein’s theory and to understand the quantum nature of gravity, particle physicists have posited the existence of space-
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time dimensions beyond those that we know. Their existence could have implications for the birth and evolution of the universe, could affect the interactions of the fundamental particles, and could alter the force of gravity at short distances. How Were the Elements from Iron to Uranium Made? Scientists’ understanding of the production of elements up to iron in stars and supernovae is fairly complete. Important details concerning the production of the elements from iron to uranium remain puzzling. Is a New Theory of Matter and Light Needed at the Highest Energies? Matter and radiation in the laboratory appear to be extraordinarily well described by the laws of quantum mechanics, electromagnetism, and their unification as quantum electrodynamics. The universe presents us with places and objects, such as neutron stars and the sources of gamma ray bursts, where the conditions are far more extreme than anything we can reproduce on Earth that can be used to test these basic theories. Each question reveals the interdependence between discovering the physical laws that govern the universe and understanding its birth and evolution and the objects within it. The whole of each question is greater than the sum of the astronomy part and the physics part of which it is made. Viewed from a perspective that includes both astronomy and physics, these questions take on a greater urgency and importance. Taken as a whole, the questions address an emerging model of the universe that connects physics at the most microscopic scales to the properties of the universe and its contents on the largest physical scales. This bold construction relies on extrapolating physics tested today in the laboratory and within the solar system to the most exotic astronomical objects and to the first moments of the universe. Is this ambitious extrapolation correct? Do we have a coherent model? Is it consistent? By measuring the basic properties of the universe, of black holes, and of elementary particles in very different ways, we can either falsify this ambitious vision of the universe or establish it as a central part of our scientific view. The science, remarkable in its richness, cuts across the traditional boundaries of astronomy and physics. It brings together the frontier in the
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quest for an understanding of the very nature of space and time with the frontier in the quest for an understanding of the origin and earliest evolution of the universe and of the most exotic objects within it. Realizing the extraordinary opportunities at hand will require a new, crosscutting approach that goes beyond viewing this science as astronomy or physics and that brings to bear the techniques of both astronomy and physics, telescopes and accelerators, and ground- and space-based instruments. The goal then is to create a new strategy. The obstacles are sometimes disciplinary and sometimes institutional, because the science lies at the interface of two mature disciplines and crosses the boundaries of three U.S. funding agencies: the Department of Energy (DOE), the National Aeronautics and Space Administration (NASA), and the National Science Foundation (NSF). If a cross-disciplinary, cross-agency approach can be mounted, the committee believes that a great leap can be made in understanding the universe and the laws that govern it. The second part of the charge to the committee was to recommend a plan of action for NASA, NSF, and DOE. In Chapter 7, it does so. First, the committee reviewed the projects in both astronomy and physics that have been started (or are slated to start) and are especially relevant to realizing the science opportunities that have been identified. Next, it turned its attention to new initiatives that will help to answer the 11 questions. The committee summarizes its strategy in the seven recommendations described below. Within these recommendations the committee discusses six future projects that are critical to realizing the great opportunities before us. Three of them—the Large Synoptic Survey Telescope, the Laser Interferometer Space Antenna, and the Constellation-X Observatory—were previously identified and recommended for priority by the 2001 National Research Council decadal survey of astronomy, Astronomy and Astrophysics in the New Millennium, on the basis of their ability to address important problems in astronomy. The committee adds its support, on the basis of the ability of the projects to also address science at the intersection of astronomy and physics. The other three projects—a wide-field telescope in space; a deep underground laboratory; and a cosmic microwave background polarization experiment—are truly new initiatives that have not been previously recommended by other NRC reports. The committee hopes that these new projects will be carried out or at least started on the same time scale as the projects discussed in the astronomy decadal survey, i.e., over the next 10 years or so. The initiative outlined by the committee’s recommendations can realize many of the special scientific opportunities for advancing our understand
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ing of the universe and the laws that govern it, but not within the budgets of the three agencies as they stand. The answer is not simply to trim the existing programs in physics and astronomy to make room for these new projects, because many of these existing programs—created to address exciting and timely questions squarely within physics or astronomy—are also critical to answering the 11 questions at the interface of the two disciplines. New funds will be needed to realize the grand opportunities before us. These opportunities are so compelling that some projects have already attracted international partners and others are likely to do so. THE RECOMMENDATIONS Listed below are the committee’s seven recommendations for research and research coordination needed to address the 11 science questions. 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 research and development to bring the needed experiments to fruition. Cosmic inflation holds that all the structures we see in the universe today—galaxies, clusters of galaxies, voids, and the great walls of galaxies—originated from subatomic quantum fluctuations that were stretched to astrophysical size during a tremendous spurt of expansion (inflation). Quantum fluctuations in the fabric of space-time itself lead to a cosmic sea of gravitational waves that can be detected by their polarization signature in the cosmic microwave background radiation. Determine the properties of dark energy. The committee supports the Large Synoptic Survey Telescope project, which has significant promise for shedding light on the 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. The discovery that the expansion of the universe is speeding up and not slowing down through the study of distant supernovae has revealed the presence of a mysterious new energy form that accounts for two-thirds of all the matter and energy in the universe. Because of its diffuse nature, this energy can only be probed through its effect on the expansion of the universe. The NRC’s most recent astronomy decadal survey recommended
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building the Large Synoptic Survey Telescope to study transient phenomena in the universe; the telescope will also have significant ability to probe dark energy. To fully characterize the expansion history and probe the dark energy will require a wide-field telescope in space (such as the Supernova/ Acceleration Probe) to discover and precisely measure the light from very distant supernovae. Determine the neutrino masses, the constituents of the 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. Neutrino mass, new stable forms of matter, and the instability of the proton are all predictions of theories that unify the forces of nature. Fully addressing all three questions requires a laboratory that is well shielded from the cosmic-ray particles that constantly bombard the surface of Earth. Use space to probe the basic laws of physics. The committee supports the Constellation-X and Laser Interferometer Space Antenna missions, which hold great 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. The universe provides a laboratory for exploring the laws of physics in regimes that are beyond the reach of terrestrial laboratories. The NRC’s most recent astronomy decadal survey recommended the Constellation-X Observatory and the Laser Interferometer Space Antenna on the basis of their great potential for astronomical discovery. These missions will be able to uniquely test Einstein’s theory in regimes where gravity is very strong: near the event horizons of black holes and near the surfaces of neutron stars. For this reason, the committee adds its support for the recommendations of the astronomy decadal survey. 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.
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The highest-energy particles accessible to us are produced by natural accelerators throughout the universe and arrive on Earth as high-energy gamma rays, neutrinos, and cosmic rays. A full understanding of how these particles are produced and accelerated could shed light on the unification of nature’s forces. The Southern Auger array in Argentina is crucial to solving the mystery of the highest-energy cosmic rays. Discern the physical principles that govern extreme astrophysical environments through the laboratory study of high-energy-density physics. The committee recommends that the agencies cooperate in bringing together the different scientific communities that can foster this rapidly developing field. Unique laboratory facilities such as high-power lasers, high-energy accelerators, and plasma confinement devices can be used to explore physics in extreme environments as well as to simulate the conditions needed to understand some of the most interesting objects in the universe, including gamma-ray bursts. The field of high-energy-density physics is in its infancy, and to fulfill its potential, it must draw on expertise from astrophysics, laser physics, magnetic confinement and particle beam research, numerical simulation, and atomic physics. 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. The scientific opportunities the committee identified cut across the disciplines of physics and astronomy as well as the boundaries of DOE, NASA, and NSF. No agency has complete ownership of the science. The unique capabilities of all three, as well as cooperation and coordination between them, will be required to realize these special opportunities. The Committee on the Physics of the Universe believes that recent discoveries and technological developments make the time ripe to greatly advance our understanding of the origin and fate of the universe and of the laws that govern it. Its 11 questions convey the magnitude of the opportunity before us. The committee believes that implementing these seven recommendations will greatly advance our understanding of the universe and perhaps even our place within it.
Representative terms from entire chapter: