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5 Cosmology and Fundamental Physics The hot big bang cosmological model derived from Einstein's general theory of relativity provides a frame- work for understanding the evolution of the universe from a fraction of a second after its creation until the present, some 13 billion years later. The success of this model is an impressive confirmation of our present understanding of physics, including general relativity itself. In recent years, dramatic and accelerating progress has been evident in researchers' pursuit of the central goal of cosmological research an understanding of the evolution and content of the universe. Such understanding rests on knowledge of fundamental physics, which often can be tested and extended only by observation of phenomena in extreme conditions in the cosmos. The current rapid progress in this field is owing in large part to access to space-based observations throughout the electromagnetic spectrum. KEY THEMES Cosmology and fundamental physics cover a diversity of phenomena whose spatial and temporal scales and characteristic energies range from the very large to the very small. In summarizing the main areas relevant to space astronomy and astrophysics, this chapter focuses on the following themes: Origin and evolution of the universe; Contents of the universe; and New astrophysical windows and cosmic mysteries. ORIGIN AND EVOLUTION OF THE UNIVERSE According to the big bang model, the universe began as a hot, formless sea of quarks and other fundamental particles of nature. As the universe expanded, it cooled. Quarks combined to create neutrons and protons. Sometime later, neutrons and protons combined to form the nuclei of the simplest elements. Eventually, atoms formed and accumulated via gravity into the objects seen by astronomers today stars, galaxies, quasars, clusters of galaxies, superclusters, and filaments of galaxies such as the Great Wall (Figure 5.11. Four important areas for study that will test and enrich our understanding of this model can be identified. 47

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48 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS 1. Cosmic microwave background. The cosmic microwave background radiation (CMBR), which fills the universe and whose spectrum is precisely that of an object at a temperature of 2.73 K, is hard evidence that the universe was very hot and very dense in the beginning. It provides a snapshot of the universe in its infancy, a few hundred thousand years after the big bang. The intensity of the CMBR is nearly uniform across the sky, demon- strating that in the beginning the distribution of matter and radiation in the universe was astonishingly smooth. Small variations, amounting to about 0.001%, are now observed and indicate that the initial distribution of matter was slightly lumpy. This lumpiness, or inhomogeneity, was amplified by the action of gravity over billions of years to create ultimately all the structure seen in the universe today. 2. Light elements. Further evidence for an explosive beginning to the universe comes from the abundances of the light elements D, 3He, 4He, and 7Li. According to the hot big bang model these elements were produced by nuclear reactions when the universe was only a few seconds old. Their abundances in the atmospheres of old stars, interstellar clouds, and extremely metal-poor galaxies, thought to be the best contemporary samples of the early universe, provided the earliest confirmation of the hot big bang model. 3. Hubble constant. Theory predicts that the light from distant galaxies should be red-shifted in accord with Hubble's law; that is, redshift is proportional to distance. The measured redshifts of thousands of galaxies confirm this prediction. The light that astronomers now see was emitted by the most distant of these galaxies when the universe was one-sixth its present size and only a few billion years old. The expansion rate of the universe (quantified by the Hubble constant) indicates a beginning between 10 billion and 15 billion years ago. Current best estimates of the ages of the oldest stars are at the top of this range or even slightly beyond, but, obviously, the stars must be younger than the age of the universe itself. It is hoped that work during the coming decade will resolve this apparent contradiction. 4. Inflation. The hot big bang model also provides a foundation for discussing earlier times and addressing some of the most fundamental questions in the natural sciences. The high temperatures and densities that existed during the earliest moments of creation constitute a bridge between cosmology and the study of the fundamental particles and forces of nature. The theoretical unification of these particles and forces is one of the central goals of physics and is a key to unlocking the secrets of the earliest moments of creation. Conversely, the study of the universe may offer important clues about this hoped-for unification. A remarkable idea arising from the connection between the microscopic and the macroscopic is inflation. According to this theory the universe did not initially expand in a uniform manner, but, rather, underwent an early burst of enormous expansion. This expansion, or inflation, was driven by a very unusual form of energy the so- called false-vacuum energy. Inflation can explain why the CMBR has almost exactly the same intensity in every direction and why the density of matter in the universe is near the critical density the value required for gravitational attraction to ultimately halt the universal expansion. Inflation theory also makes two striking predictions: first, most of the matter in the universe exists in a form different from the atoms that compose everything we see around us; second, all structures observed today in the universe grew from subatomic, quantum fluctuations. Key Questions About the Origin and Evolution of the Universe The success of the hot big bang model has added urgency to some old questions and has also led to the formulation of newer, deeper ones. These important questions include the following: Geometry of the Universe How big and how old is the universe? Is the time back to the big bang really consistent with the ages of the oldest objects in the universe? Is space curved? . What is the ultimate fate of the universe? Is there a large-scale repulsive force, represented by the "cosmological constant," as originally suggested by Einstein?

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COSMOLOGY AND FUNDAMENTAL PHYSICS 49 Answering these simple, but profound, questions requires accurate measurements of the Hubble constant, Ha; the deceleration parameter, q0; and the average density of matter in the universe. Origin and Evolution of Structure u, While researchers are now fairly confident that all structure from the smallest galaxies to the great walls of galaxies seen in large-scale redshift surveys grew from small, primeval inhomogeneities in the distribution of matter, fundamental questions such as the following remain: What are the nature and the ordain of the primeval inhomogeneities? . How and when did the structure evolve? When were the first galaxies formed? Origin of the Universe The study of the unification of the forces of nature allows a scientific approach to addressing questions concerning the beginning of the universe itself. Key questions in this area include the following: . What launched the expansion of the universe? What is the origin of the cosmic microwave background radiation? Were there other big bangs? Recent Progress in Understanding the Origin and Evolution of the Universe Observations by NASA's Cosmic Background Explorer (COBE) satellite have led to major advances in current understanding of the most important cosmological fossil, the CMBR. COBE measured its temperature to four significant digits it is 2.728 + 0.002 K-and showed that it is the most perfect black body ever studied. These findings established beyond any doubt that the CMBR is the "echo" of the big bang. By mapping the dipolar variation in the CMBR's temperature across the sky, COBE determined Earth's velocity with respect to the cosmic rest frame to a precision of 1%. COBE also detected small variations (about 0.001%) in the intensity of the CMBR coming from different directions separated by angles of about 10 degrees and larger (Figure 5.24. This discovery provided the first evidence for the primeval lumpiness that under gravitational attraction grew into all the structure seen today, and it represented the first step toward establishing that all structure arose from subatomic quantum fluctuations. Since the COBE discovery, more than 10 other experiments, some ground-based, others on balloons, have also detected variations in the intensity of the CMBR. These measurements have confirmed the COBE result and are beginning to map out the inhomogeneities in the distribution of matter that gave rise to present structure. Knowledge of the distribution of galaxies today is important to understanding the~development of structure in the universe. The distribution of galaxies across the sky is readily found by classical astronomical techniques. To obtain the third dimension distance from Earth galaxy redshifts must be measured. Several large redshift surveys have been carried out, including one based on galaxies cataloged by the Infrared Astronomical Satellite (IRAS). The IRAS catalog is especially useful because it covers the entire sky (other catalogs miss much of the sky due to absorption of visible light by interstellar dust in our own galaxy, the Milky WayJ. All-sky coverage has allowed the mean mass density of a very large sample of the universe to be measured: based on a comparison of the distribution of IRAS-cataloged galaxies with Earth's velocity as determined by COBE, the average density was determined to be at least 30% and possibly as high as 100% of the critical density. Significant progress has been made toward the goal of determining Ho and q0. The discovery and study of Cepheid variable stars in galaxies in the Virgo cluster and in the Fornax and Leo groups by several teams using the Hubble Space Telescope (MST) represented a major step in the process of calibrating bright, so-called secondary

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so A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS indicators that can be seen at great distances (e.g., supernovae and bright spiral and elliptical galaxies). In addition, new determinations of Ho were made by physically based methods. Techniques of this kind include measurements of the temperature dip in the CMBR associated with hot, x-ray-emitting gas in clusters (the so- called Sunyaev-Zeldovich effect) and of the time delay associated with distant gravitational lenses. A reliable determination of Ho is now within sight. Very distant supernovae have recently been discovered, providing important means toward a determination of the deceleration parameter. HST and the Keck 10-meter telescope on Mauna Kea have opened new windows to the distant universe. HST has provided the deepest images of the universe ever, revealing galaxies and clusters apparently in the process of formation. The Keck telescope has provided a measurement of the temperature of the universe at a much earlier time, confirming a fundamental aspect of the big bang model that the temperature of the universe decreases as it expands and is providing the means to measure the abundance of deuterium in very old hydrogen clouds to compare with predictions of the big bang model. Future Directions for Understanding the Origin and Evolution of the Universe Cosmologists anticipate major advances in their understanding of the origin and evolution of the universe. Achievement of the 80-year-old quest to determine the geometry of the universe-that is, its structure on the largest possible scale is now within reach. Moreover, bold ideas that can extend current knowledge to within the tiniest fraction of a second of the beginning are ready for testing. Thus, not only will researchers' understanding of the universe be greatly advanced, but light will also be shed on the unification of the fundamental forces and particles of nature. The key to achieving these goals is precise cosmological measurements. The most important future directions for study include the following, in generally decreasing order of priority: 1. Determining the geometry of the universe. The values of the Hubble constant, the deceleration param- eter, and Einstein's cosmological constant, which determine the geometry of the universe, are notoriously difficult to measure. Thus a variety of approaches are needed. Observers are fortunate in having a powerful new tool for such measurements high-angular-resolution mapping, on scales of a few arc minutes and larger, of the anisot- ropy of the CMBR. The CMBR is crucially important because it offers a snapshot of the universe at a simpler time, long before stars, galaxies, and other structures existed, and there are very precise predictions for the anisotropy expected for different values of the cosmological parameters. Such mapping will reveal the inhomoge- neity in the distribution of matter that seeded all the structure seen today. Realizing the full implications of the variations in the CMBR will require a variety of ground- and space-based observations. Nonetheless, the scientific return clearly justifies the effort: rarely have theory and technology been better matched to address a major scientific question. Comparison between cosmological parameters determined from the CMBR and those measured by other techniques will provide vital tests of astronomers' current understanding about the geometry of the universe, and could reveal additional information. Astronomers will soon be able to determine the value of the Hubble constant to within 10% by using HST observations of bright secondary distance indicators. As mentioned, promising additional approaches include measurement of the Sunyaev-Zeldovich effect and measurement of the time delay associated with distant gravitational lenses; study of the motion of distant radio jets may also contribute. Ground- and space-based studies of distant supernovae may allow determination of the deceleration parameter and the cosmological constant. 2. Testing models of the origin and evolution of structure in the universe. Maps of the anisotropy of the CMBR can also test specific predictions of inflation theory (e.g., regarding the flatness of the universe, the specific pattern of the anisotropy, and the existence of exotic dark matter) and will test the most detailed model of how large-scale structure evolved, that is, the cold dark matter hypothesis. More generally, these maps will provide the basic data needed to test any alternative models of the early universe. They will allow determination of the total density of the universe and the density of ordinary matter to a precision of 5% or better. A comparison of the inhomogeneity that existed a few hundred thousand years after the beginning, as shown in maps of the CMBR's anisotropy, with that which exists today, as revealed in the large-scale redshift surveys (see Figure 5.1) now under

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COSMOLOGY AND FUNDAMENTAL PHYSICS 51 way (e.g., the Sloan Digital Sky Survey at Apache Point Observatory and the Two-Degree Field Survey at the Anglo-Australian Observatory), will be crucial to understanding how structure formed in the universe. 3. Measuring the polarization of the cosmic microwave background radiation. Some polarization of the CMBR is expected; if found it will offer an important test of current understanding of the origin of structure. Observations of polarization may also enable researchers to determine the epoch of primordial star formation. 4. Searching for deviations from a blackbody cosmic microwave background radiation spectrum at radio wavelengths. Such studies offer a unique window on exotic processes (e.g., decays of relic elementary particles) that might have occurred at early times. 5. Measuring the anisotropy of the x-ray background radiation. Studies of this type would provide a fundamental cosmological test, because the x-ray background comes largely from very distant galaxies. Its dipolar anisotropy should, therefore, coincide with the anisotropy of the microwave dipole, because matter averaged on a large scale should be at rest with respect to the CMBR. CONTENTS OF THE UNIVERSE Some of the most fundamental questions researchers ask concern the contents of the universe. Four signifi- cant facts seem secure: 1. Outnumbering of atoms by photons. Most of the known particles in the universe are the photons of the CMBR. The ultimate origin of these photons, which outnumber atoms by more than a billion to one, is unknown. Other photons of more recent origin those of the infrared, x-ray, and gamma-ray backgrounds pervade the universe; these photons were produced by distant stars, galaxies, and clusters long ago, and, although they are far fewer in number than the photons of the CMBR, they provide important clues about the formation of the objects from which they come. 2. Absence of antimatter. In an enormously large region around Earth, the universe is made of matter and not of an equal mixture of matter and antimatter. This imbalance is extremely puzzling since both matter and antimatter are predicted to have been almost equally abundant at the beginning. Because photons so greatly outnumber atoms, the general absence of antimatter today suggests that the amount of matter and antimatter present at early times was unequal by one part in a billion. It seems likely that this difference in abundance is linked to a slight asymmetry in the laws of particle physics favoring the creation of matter rather than antimatter. 3. Abundances of light elements. The cosmic abundances of the light elements, D, 3He, 4He, and Li, are generally in accord with those expected based on calculations of their production during the first few minutes after the big bang. Further, the relative abundances give astronomers their most precise estimate of the amount of ordinary (baryonic) matter about 5% of the critical density. Astronomers now know that only a fraction of ordinary matter has found its way into stars: most of the barvons mav exist in the. hot A: thnt norm Airy of galaxies and intergalactic Inns. , _ By, ~= ~~ r~ ~'~'~ < ~ -- -- - -r ~~ ~ 4. Existence of dark matter. As already emphasized, most of the mass in the universe is dark, neither emitting nor absorbing any form of electromagnetic radiation. Known to exist only by its gravitational influence, it is present on all scales: in galaxies, in clusters of galaxies, and, perhaps, in even larger concentrations of mass such as the so-called Great Attractor. Although the total amount of dark matter is still unknown, good evidence indicates that there is more dark matter than can be accounted for as ordinary matter. If it is dominant, dark matter must have determined how structure formed in the universe, and it will determine whether or not the universe expands forever. Key Questions About the Contents of the Universe Dark Matter Constructing a complete cosmology is impossible without a better understanding of dark matter, the dominant form of matter in the universe. Questions researchers need to answer include the following:

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52 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS What is the composition and what is the quantity of dark matter? How much of the universe is composed of ordinary matter, and how much exists in more exotic forms? What are the constituents of the exotic dark matter? . How is the dark matter distributed? Ordinary Matter The history of ordinary matter is crucial to constructing a complete cosmology. Questions researchers need to answer include the following: In what form does ordinary matter exist today? How has the distribution of baryons changed over time from primeval plasma to stars, hot gas in clusters of galaxies, warm gas between clusters, dead stars, and other forms that exist today? Matter and Antimatter Although particle physics provides a general framework for understanding the origin of the asymmetry between matter and antimatter, a detailed explanation remains to be found and tested. Astronomers still do not have a good answer to the following basic question: . Why is the universe made of matter and not of both matter and antimatter? Other Constituents The photons of the CMBR remained unknown until 1964. Possible other undiscovered constituents include superheavy magnetic rnonopoles, cosmic strings, and the tiny, exploding primordial black holes predicted by Stephen Hawking. Thus, a basic question still to be answered is this: Are there radically new constituents of the universe that have yet to be discovered? Recent Developments in Understanding the Contents of the Universe In addition to greatly improving current knowledge of the CMBR, data from COBE set important limits on the level of the cosmic infrared background radiation that is expected from the oldest stars and galaxies. Significant progress has also been made in understanding the extragalactic x-ray and gamma-ray background radiation. Measurements made by the Roentgensatellit (ROSAT), the Advanced Satellite for Cosmology and Astrophysics (ASCA), and the Compton Gamma-Ray Observatory (CGRO) indicate that active galaxies account for a large part-perhaps almost all of these backgrounds. Important advances in the understanding of dark matter include new estimates of the average density of matter based on comparing the spatial distribution of galaxies with galaxy velocities relative to the rest frame of the CMBR. As discussed above, these estimates strongly suggest that the density of matter is at least 30% of the critical density. One of the fundamental predictions of general relativity, the deflection of light by gravity, has been used to directly probe dark matter. Gravitational tensing of distant galaxies by intervening clusters has allowed the dark matter in the clusters to be mapped and weighed by both ground- and space-based observatories. Gravitational microlensing of stars in the nearest external galaxy, the Large Magellanic Cloud, has been used to discover dark baryons in the form of so-called massive compact halo objects (MACHOs) in the halo of the Milky Way. MACHOs may represent a new population of objects and may constitute between 10% and nearly 100% of the mass of the Milky Way's halo. X-ray telescopes such as ROSAT and ASCA have provided important information about the gas in galaxy clusters, showing that it represents between 10% and 20% of the total mass. The implications of this work are

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COSMOLOGY AND FUNDAMENTAL PHYSICS 53 twofold. First, most and perhaps all of the "dark" baryons in clusters exist in the form of hot gas. Second, the bulk of the matter in clusters is still of unknown composition. The hot gas appears to be the main repository of baryons in the universe today. The x-ray observations of clusters, which probe the amplitude of mass fluctuations, have been an important complement to galaxy redshift surveys, which probe the amplitude of the fluctuations in galaxy numbers. The most accurate determination of the abundance of ordinary matter, between 1% and 10% of the critical density, comes from measuring the cosmic abundance of D, 3He, 4He, and Li, the light elements synthesized a few seconds after the big bang. Systematic progress has been made toward determining the abundances of all four in samples of primordial matter that have not been significantly altered by stellar nuclear processes. Obsemationa1 evidence strongly suggests that most of the dark matter is not ordinary matter. In particular, the average density of the universe appears to be significantly higher than the baryonic density inferred from the cosmic abundances of the light elements. Moreover, it is difficult to construct a consistent cosmology with only baryons. Dark matter in the form of relic elementary particles provides the most natural explanation for the development of the structures seen today as the primeval matter inhomogeneities inferred from measurements of the CMBR's anisotropy. In the past 5 years, ultrasensitive ground-based experiments have begun to search for yet undetected elemen- tary particles that might constitute the dark matter in the halo of the Milky Way. Experiments are under way or under development to search for the hypothetical axion and neutralino. The properties of known particles that could contribute to the dark matter also continue to be investigated. If one of the three neutrino species has a mass between 2 eV and 30 eV, then some or all of the dark matter may be in the form of neutrinos produced in the big bang. A variety of experiments are searching for evidence of neutrino mass. There are hints from solar-neutrino experiments and an accelerator experiment that at least one neutrino species has mass. Future Directions for Understanding the Contents of the Universe Future directions for studies of the contents of the universe include the following, in no particular order: 1. Undertaking a multifaceted study of the amount, nature, and distribution of dark matter. Character- ization of dark matter is one of the most important problems in all of astrophysics, and many opportunities exist for significant advances. A number of new approaches for measuring the mean mass density of the universe, e.g., high-resolution mapping of the CMBR, could be used to determine the total amount of dark matter. Microlensing is an important new probe of baryonic dark matter; a small, space-based telescope operating in conjunction with existing ground-based telescopes could reveal much about the nature of the MACHOs discovered in the Milky Way's halo. More remains to be learned about the distribution of baryons and dark matter in clusters of galaxies from higher-resolution x-ray observations. Likewise, gravitational tensing could be used to probe directly the distribution of dark matter in clusters and in the halos of large spiral galaxies, and it might be used to search for concentrations of dark matter that are not associated with bright galaxies. Ground-based experiments will con- tinue to search directly for the elementary particles that contribute to the Milky Way's halo. The search for positrons, gamma rays, and antiprotons from dark-matter annihilations in the halo of the Milky Way is an important complementary approach that could provide the first direct evidence for nonbaryonic dark matter. 2. Determining the primeval abundances of the elements created in the big bang. Definitive measure- ment of the abundance of deuterium is a key test of primordial nucleosynthesis that might also pin down the density of ordinary matter to a precision of 10% or better. The Far-Ultraviolet Spectroscopic Explorer can be used to determine precisely the cosmic helium abundance. Other opportunities for more accurately determining the abundances of the light elements merit pursuit. 3. Investigating the infrared, ultraviolet, x-ray, and gamma-ray background radiation. Space-based observations of these backgrounds, which were produced in part by stars and galaxies in their infancy, offer a window on the origin and evolution of stars and galaxies. 4. Remaining alert to unexpected discoveries. It is important to be on the lookout for entirely new phenomena. The discovery of the most important cosmological relic, the cosmic microwave background radia

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54 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS lion, was serendipitous. The discovery of a single antihelium nucleus, a magnetic monopole, or an exploding, primordial black hole would profoundly change cosmology and might have implications for the unification of the particles and forces of nature. NEW ASTROPHYSICAL WINDOWS AND COSMIC MYSTERIES Astronomy is replete with examples of significant advances or astounding discoveries arising from the opening of new observational windows. Much of the bedrock of modern astronomy infrared, radio, x-ray, and gamma-ray astronomy grew out of exploitation of such windows. Several promising and exciting new windows are either now accessible or on the verge of being so. In addition, some well-explored windows present seemingly intractable mysteries. Current understanding singles out four important areas ripe for progress: 1. Gravitational radiation. The emission of gravitational radiation by accelerating masses is a crucial predic- tion of general relativity. Gravitational waves offer a promising new tool for observing the behavior of astronomi- cal systems under conditions of strongly nonlinear gravity and superhigh velocities. 2. Low-frequency radio waves. This window, spanning the spectral band from about 30 kHz to 30 MHz, is the last important region of the electromagnetic spectrum that is still largely unexplored. Space-based observa- tions are essential because such radio waves are generally unable to penetrate Earth's ionosphere. Many important astrophysical questions concerning the solar system, the galaxy, and the distant universe can be addressed by low- frequency radio observations with angular resolution near an arc minute. 3. Gamma-ray bursts. The nature and origin of gamma-ray bursts present one of the deepest mysteries in contemporary astrophysics. Celestial gamma-ray bursts were discovered serendipitously several decades ago by U.S. satellites designed to detect clandestine nuclear tests, and well over 1,000 have been detected subsequently by scientific satellites, chiefly CGRO. Yet until very recently none have been located with enough precision to enable identification with known astronomical objects. Possible explanations of the mysterious objects range from coalescing neutron stars and black holes at cosmological distances, to magnetic flares on the surfaces of neutron stars in large galactic halos. 4. Ultrahigh-energy cosmic rays. These particles, with energies in excess of some 10~9 eV, open a promising window onto astrophysical processes, cosmology, and fundamental physics. Because their deflection and contain- ment by galactic magnetic fields are negligible, particles with such energies must be extragalactic, and yet they are so energetic that they are blocked by interactions with the photons composing the CMBR within a few hundred megaparsecs, a distance that is small compared to the size of the universe. Their existence could signal new physics associated with grand unified theories of particle physics. Key Questions About New Astrophysical Windows and Cosmic Mysteries Often in astronomy, the opening of new observational windows has led to such novel discoveries that the relevant questions could not have been conceived beforehand. For unexplored territory such as the low-frequency radio band, it is difficult to anticipate what will be found, or even to formulate the right questions to ask. For other new windows and for unexplained observations in already-explored windows, the task of formulating key ques- tions is somewhat easier. These include the following: Gravitational Waves What are the characteristics of gravitational waves as predicted by general relativity? What could gravitational waves reveal about dynamical processes in the universe involving high speeds and strong gravity? Are there supermassive black holes in the centers of galaxies and quasars? How do supermassive black holes form, merge, and swallow up stars?

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COSMOLOGY AND FUNDAMENTAL PHYSICS Gamma-Ray Bursts 55 How can the recent discovery of a soft x-ray afterglow and a fading optical emission coincident with a gamma-ray burst constrain burst models? Are gamma-ray bursts located within the Milky Way's halo, or at cosmological distances? UTh~t ;~ the ~_~ ~ ~ ~ 41_ A_ 1 1 ~ . . - '7ll"L lo t11~ 1l"tUl~ U1 BAAS pA~Vv~v i~V3U~ArAvA~A~ 10r ine sudden, enormous energy releases of gamma-ray bursts and how do they compare with the relatively nearby soft gamma-ray repeaters in the galactic disk? This question is of particular significance if gamma-ray bursts occur at cosmological distances. What, if anything, can gamma-ray bursts reveal about the early universe? Ultrahigh-Energy Cosmic Rays What is the origin of cosmic rays with energies above 10~9 eV? Do these particles reveal more about cosmology or about new physics? Recent Developments in Understanding New Windows and Cosmic Mysteries The construction of a network of ground-based, kilometer-scale, laser-interferometric gravitational wave observatories in the United States and Europe (the LIGO and VIRGO projects, respectively began in the past decade with the first observations planned for 2000-2001. These instruments will be most sensitive to gravita tional waves in the 10- to 500-Hz band and will be capable of detecting the final orbital decay and coalescence of binary systems containing neutron stars or black holes at cosmological distances. Also detectable will be superno- vae within a few hundred megaparsecs and possibly a cosmological background of gravity waves. Cosmic gamma-ray bursts are distributed very uniformly over the sky, and their distribution of peak intensi- ties shows that most of the sources cannot reside within the galactic disk. Rather, the sources must occupy either an extended, spherically symmetric region such as an extremely large galactic halo or a cosmolo~icallv distant ~ . . . ~ . region of the universe. Possible observation of temporal stretching in the duration of faint gamma-ray bursts is suggestive of cosmological time dilation. If so, the sources must be located at redshifts of z ~ 1 to 2. If the sources are truly so remote and if they emit isotropically, then their energy output in gamma rays alone in less than a minute is comparable to the total energy radiated by a supernova. ~; ~11 ~.. _: A ~1 ~_ _ ~. . . . If the sources are this distant, they could pony De a unique tool for Anvestlgat'Ang a new fundamental phenomenon widespread throughout the universe. If not at cosmological distances, gamma-ray bursts may be the only observable manifestation of some unseen matter in a highly extended halo about our own galaxy. The recent discovery of the apparent repetition of a "classical" gamma-ray burst over a period of several days would require either gravitational tensing of an object at a cosmological distance (although differences in burst profile make this unlikely) or a source in the galactic halo. Indeed, the latter explanation may suggest that gamma-ray bursts are somehow related to the distinctly different population of so-called soft gamma-ray repeaters in the galactic disk. Recent precise positions for two soft gamma-ray repeaters and the identification of one with a persistent x-ray source strongly suggest that they are associated with supernova remnants and thus, presumably, neutron stars. However, the remarkable recent discov- ery by the Satellite per Astronomia in Raggi X (SAX) of the soft x-ray afterglow of a "classical" gamma-ray burst, subsequently identified with a fading optical source, points the way to possible identification of gamma-ray bursts: rapid follow-up observations at soft x-raY and optical wavelengths of bursts lockers ~o within an Ret of ~ felt, arc minutes or better. ~=,~ An. ~ ~^ v~v I_ ~ ~AA~Al BAA ~1~} ~1 ~ 1~ W A cosmic-ray air shower generated by a primary particle with energy as high as 3 x 102 eV (48 joules), the highest-energy particle event yet found in nature, has recently been detected by the Fly's Eye detector in Utah. If the primary particle is cosmological in origin, then the inferred energy exceeds a bound imposed by interaction with the CMBR. Although a number of objects have been considered as possible sources, no plausible object has been found within 50 Mpc in the direction of the detected event. Possible resolutions of this mystery include the existence of a hitherto unknown particle with very low interaction with the CMBR, but with sufficient interactions with matter to produce the observed air showers.

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56 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS Future Directions for Understanding New Astrophysical Windows and Cosmic Mysteries Priority directions for the study of new windows and cosmic mysteries include, in generally decreasing priority order, the following: 1. Searching for low-frequency gravitational waves. In the frequency band between 10 - and 10-i Hz, known or predicted sources of gravitational waves include binary systems of normal stars, white dwarfs or neutron stars in the Milky Way, formation or coalescence of massive black holes in galactic nuclei, and a cosmological background of waves. Observation of massive black hole mergers offers the opportunity to map the space-time structure in the vicinity of a black hole and thereby to test general relativity in a strong-field regime. Discovery and study of such massive black hole processes could also have a strong impact on theories of galactic structure and evolution. Because of seismic and gravity-gradient noise on Barth, searches for gravitational waves at frequencies lower than 10 Hz must be done in space. ----I ----rat ~ ~. ~. ~ . . ~. . 2. Testing the foundations of general relativity. Testing of the principles of general relativity, which underlies current theories of cosmology and gravitational radiation, is critically important. The principle of equivalence, one of the foundations of modern physics, asserts the equivalence of uniform acceleration and a uniform gravitational field. A critical test of this principle involves demonstrating whether the ratio of inertial mass to gravitational mass is the same for two different types of material. The best ground-based measurements, performed several decades ago, verified this equality to an accuracy of a few parts per trillion. A proposed space- based experiment could increase the sensitivity of this test by a factor of about 100,000. 3. Increasing the number of known gamma-ray burst sources and improving knowledge of their pre- cise locations. Over the next decade it is very likely that key questions of the distance scale of gamma-ray bursts can be answered by rapid follow-up observations of moderately well known (accurate to + a few arc minutes) burst locations at soft x-ray and optical wavelengths, as has recently occurred. Another approach is to obtain accurate (+ arc seconds) burst locations using gamma-ray burst detectors widely dispersed in the solar system or by using improved wide-field hard x-ray imaging detectors. A small sample of gamma-ray burst sources identified with host galaxies would point strongly to the cosmological model. A sensitivity test of halo models could be made by imaging and monitoring the halo of M31 in hard x rays. Understanding of the nature of the sources can be advanced by multiwavelength detections and by sensitive high-resolution spectra of gamma-ray bursts in the entire band from hard x rays to gamma rays. 4. Determining the origin of the ultrahigh-energy cosmic rays. Assessment requires improved data on the incident direction, mass, and energy of these events. Current indications of a correlation of events with the supergalactic plane will require much more air-shower data for confirmation. Substantial progress will be enabled by the planned international 5,000-sq.-km Pierre Auger Array. This facility, with elements in the United States and Argentina will allow an order-of-magnitude increase in exposure. A complementary experiment would involve an orbiting platform observing scintillations caused by air showers induced by cosmic rays from above the atmosphere. ~. . . . . . . . ~ - . - . 5. Exploring the low-frequency radio window. Observations from space are required, because from Earth's surface, observations at low radio frequencies are blocked by the ionosphere. In this window observers may find new sources of coherent emission, new classes of very steep spectrum sources, and "fossil" radio galaxies. Studies of the low-frequency spectra of galactic supernova remnants Pulsars and the extended Galactic nonthermal ~ . . ... . . .. . 1 ~ radiation will address questions concerning particle acceleration mechanisms, magnetic field strengths, and radia- tive lifetimes. The distribution of low-energy cosmic rays and diffuse ionized hydrogen in the Milky Way can be determined. Measurements of the angular broadening of extragalactic sources caused by interstellar scattering will provide data on the distribution and turbulent structure of the interstellar plasma. But the real excitement will be in exploring the new territory below 30 MHz, which is likely to uncover new phenomena.

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COSMOLOGY AND FUNDAMENTAL PHYSICS 57 CONCLUSIONS Taking into consideration the subjects discussed above, TGSAA identified the following scientific opportuni- ties as the highest priorities for space research in cosmology and fundamental physics during the next 5 to 10 years. They are listed from highest to lowest priority. Determine the geometry of the universe by measuring the anisotropy of the cosmic microwave background radiation. High-precision mapping over the full sky provides a powerful new technique for accurate determination of long-sought cosmological parameters. Comparison with improved results from classical astro- nomical techniques and other methods will-provide a firm basis for refining cosmological models. 2. Test theories for the origin and evolution of structure in the universe. Measurements of the primordial matter inhomogeneities revealed by maps of the cosmic microwave background radiation, when combined with measurements of the large-scale distribution of matter today, will test the currently favored theories of structure formation, inflation, and cold dark matter. Such data will also provide the observational facts needed to test alternative theones. 3. Determine the amount, distribution, and nature of dark matter. Many types of measurements can contribute to resolution of the questions concerning dark matter. Among these are mapping of the anisotropy of the cosmic microwave background radiation, study of gravitational tensing by objects ranging from MACHOs to distant galaxy clusters, measurement of the hot, x-ray-emitting gas in galaxy clusters, precise measurement of the primordial abundances of the light elements, and a direct search for new species of particles. The following scientific opportunities are important, but of lower priority than those listed above. They are shown in priority order. 1. Test predictions of general relativity in the strong-gravity regime. By initiating observations of low- frequency gravitational waves, astronomers can probe the strong gravitational fields near massive black holes and so extend the conditions under which the predictions of general relativity can be tested. Such studies will also open a unique new window for the study of gravitational interactions in a variety of astrophysical systems. 2. Investigate the nature of cosmic gamma-ray bursts and the origin of ultrahigh-energy cosmic rays. While definitive approaches for resolving these mysteries are hard to specify, understanding of both is likely to be improved by detection of more events with improved directional information. 3. Extend radio astronomy to ultralow frequencies. Measurement of radio waves blocked by Earth's ionosphere will likely enhance understanding of familiar phenomena and reveal new ones.