2
The Science Behind the Recommendations



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Astronomy and Astrophysics in the New Millennium 2 The Science Behind the Recommendations

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Astronomy and Astrophysics in the New Millennium A VISION FOR ASTRONOMY AND ASTROPHYSICS IN THE NEW CENTURY In the year 1000 AD there were astronomers in only a few places on Earth: in Asia, particularly China, in the Middle East, and in Mesoamerica. These astronomers were aware of only six of the nine planets that orbit the Sun. Although they studied the stars, they did not know that the stars were like the Sun, nor did they have any concept of their distances from Earth. By the year 2000 AD, humanity’s horizons had expanded to include the entire universe. We now know that our Sun is but one of 100 billion stars in the Milky Way Galaxy, which is but one of about 100 billion galaxies in the visible universe. More remarkably, our telescopes have been able to peer billions of years into the past to see the universe when it was young—in one case, when it was only a few hundred thousand years old. All these observations can be interpreted in terms of the inflationary Big Bang theory, which describes how the universe has evolved since the first 10−36 seconds of cosmic time. It is impossible to predict where astronomy will be in the year 3000 AD. But it is clear that for the foreseeable future, the defining questions for astronomy and astrophysics will be these: How did the universe begin, how did it evolve from the soup of elementary particles into the structures seen today, and what is its destiny? How do galaxies form and evolve? How do stars form and evolve? How do planets form and evolve? Is there life elsewhere in the universe? Researchers now have at least the beginnings of observational data that are relevant to all of these questions. However, a relatively complete answer exists for only one of them—how stars evolve. The development and observational validation of the theory of stellar evolution was one of the great triumphs of 20th-century astrophysics. For the 21st century, the long-term goal is to develop a comprehensive understanding of the formation, evolution, and destiny of the universe and its constituent galaxies, stars, and planets—including the Milky Way, the Sun, and Earth.

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Astronomy and Astrophysics in the New Millennium In order to do this, the committee believes that astronomers must do the following: Map the galaxies, gas, and dark matter in the universe, and survey the stars and planets in the Galaxy. Such complete surveys will reveal, for example, the formation of galaxies in the early universe and their evolution to the present, the evolution of primordial gas from the Big Bang into matter enriched with all the elements by stars and supernovae, the formation of stars and planets from collapsing gas clouds, the variety and abundance of planetary systems in the Galaxy, and the distribution and nature of the dark matter that constitutes most of the matter in the universe. Search for life beyond Earth, and, if it is found, determine its nature and its distribution in the Galaxy. This goal is so challenging and of such importance that it could occupy astronomers for the foreseeable future. The search for evidence of life beyond Earth through remote observation is a major focus of the new interdisciplinary field of astrobiology. Use the universe as a unique laboratory to test the known laws of physics in regimes that are not accessible on Earth and to search for new physics. It is remarkable that the laws of physics developed on Earth appear to be consistent with phenomena occurring billions of light-years away and under conditions far more extreme than those for which the laws were derived and tested. However, researchers have only begun to probe the conditions near the event horizons of black holes or in the very early universe, where the tests of the laws of physics will be much more stringent and where new physical processes may be revealed that shed light on the unification of the forces and particles of nature. Develop a conceptual framework that accounts for all that astronomers have observed. As with all scientific theories, such a framework must be subject to continual checks by further observation. For the new decade, astronomers are poised to make progress in five particular areas: Determining the large-scale properties of the universe: its age, the nature (amount and distribution) of the matter and energy that make it up, and the history of its expansion; Studying the dawn of the modern universe, when the first stars and galaxies formed; Understanding the formation and evolution of black holes of all sizes;

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Astronomy and Astrophysics in the New Millennium TABLE 2.1 Science Goals for the New Initiatives   Initiativea Science Goal Primaryb Secondaryb Determining large-scale properties of the universe NGST, GSMT, LSST (MAP, Planck, SIM) Con-X Studying the dawn of the modern universe NGST, SKA, LOFAR (ALMA) Con-X, EVLA, SAFIR, GLAST, LISA, EXIST, SPST Understanding black holes Con-X, GLAST, LISA, EXIST, ARISE EVLA, LSST, VERITAS, SAFIR Studying star formation and planets NGST, GSMT, EVLA, LSST, TPF, SAFIR, TSIP, CARMA, SPST (ALMA, SIM, SIRTF, SOFIA) AST, SDO, Con-X, EXIST Understanding the effects of the astronomical environment on Earth LSST, AST, SDO, FASR GLAST NOTE: Acronyms are defined in the appendix. aMissions and facilities listed in parentheses are those that were recommended previously but have not yet begun operation. bProjects or missions listed in the “primary” category are expected to make major contributions toward addressing the stated goal, while “secondary” projects or missions would have capabilities that address the goal to a lesser degree. Studying the formation of stars and their planetary systems, and the birth and evolution of giant and terrestrial planets; and Understanding the effects of the astronomical environment on Earth. Table 2.1 lists these science goals and the new initiatives that will address them. In addition, the time is ripe for using astronomy as a gateway to enhance the public’s understanding of science and as a catalyst to improve teachers’ education in science and to advance interdisciplinary training of the technical work force.

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Astronomy and Astrophysics in the New Millennium THE FORMATION AND EVOLUTION OF PLANETS The discovery of extrasolar planets in the past decade was one of the most remarkable achievements of the 20th century and represented the culmination of centuries of speculation about planets orbiting stars other than our Sun. These observations confirmed for the first time that a significant fraction of the stars in the Milky Way Galaxy have planetary systems; at the same time, the observations brought the surprising news that a number of planetary systems are very different from our solar system. In fact, the first extrasolar planetary system discovered is quite exotic: Although it involves terrestrial-mass planets, the central star is not a normal star like the Sun, but a rapidly spinning neutron star. The first planet detected around a Sun-like star is much more massive than Earth. Its mass is at least half that of Jupiter, the largest planet in the solar system, but its orbit is only one-tenth as large as that of the innermost planet, Mercury (Figure 2.1). Further discoveries indicate that such “hot Jupiters”—gas giant planets orbiting 100 times closer to the host star than their analogs in our own solar system—are surprisingly common, being found around a few percent of all solar-type stars. It may even be that our own planetary system is the exception and hot Jupiters the rule. We are witnessing the birth of a new observational science of planetary systems. The new measurements of masses and orbital distances of planets demand explanation. The first step is to carry out a census of extrasolar planetary systems in order to answer the following questions: What fraction of stars have planetary systems? How many planets are there in a typical system, and what are their masses and distances from the central star? How do these characteristics depend on the mass of the star, its age, and whether it has a binary companion? Astronomers have a number of methods to detect extrasolar planets: astrometry, measurement of Doppler shifts, photometry, observations of gravitational microlensing, and direct imaging. SIM will utilize astrometry, a method that uses the back-and-forth motion of stars in the sky to infer the presence of an orbiting planet, to increase the census of Jovian-mass planets orbiting at relatively large distances from their central stars. GSMT and other ground-based telescopes will measure small shifts in the wavelengths of the observed radiation, or the Doppler shifts, caused by the motion of stars toward and away from us as the planets orbit the stars. The Doppler method has been used almost exclusively in the past decade and favors small orbital separation and

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Astronomy and Astrophysics in the New Millennium FIGURE 2.1 The discovery of the first planet orbiting a Sun-like star outside the solar system was made by observing small oscillations in the radial velocity Vr of the star 51 Pegasi. These oscillations are caused by the planet as it orbits the star every 4.2 days. The phase represents the time in units of the 4.2-day cycle. Courtesy of M. Mayor, D. Queloz, and S. Udry (Universite de Geneve). Reprinted by permission from Nature 378:355-359, copyright 1995 Macmillan Magazines Ltd. relatively large planets. Photometry measures the small decrease in the light from a star when a planet orbits between the observer and the star, partially eclipsing the star. Because photometry depends on a favorable inclination of the orbit, surveys of a large number of stars are required to find the frequency of planetary systems. Space-based photometry is sufficiently precise that it could extend the census to planets with masses as low as those of the terrestrial planets. Sensitive photometry of distant stars can also reveal planets through gravitational microlensing: The

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Astronomy and Astrophysics in the New Millennium gravitational field of an intervening faint star close to the line of sight to a distant star acts as a lens that amplifies the light of the distant star; planets orbiting the intervening star can change the amplification in a detectable manner. However, these methods all detect planets indirectly by their small perturbations of the light from the central star. The ultimate goal is to see and study the radiation from the planets themselves. Direct imaging of giant planets can be done from the ground with adaptive optics, but TPF or an enhanced NGST is needed for terrestrial planets. Once direct imaging is possible, radiation from extrasolar planets can be analyzed to characterize the atmospheres of the planets: How do the atmospheres depend on the mass of the planet, its separation from its host star, and the mass of the host star? Do any of the planets appear habitable? Are there any biological “marker materials” such as methane, molecular oxygen, or ozone? Observation of the atmospheres is extremely challenging, owing to confusion with the enormously brighter host star. TPF is designed to address this problem by using interferometry to null out the radiation from the host star; with the addition of an occulter NGST may contribute to this goal. The planetary census, together with new observations of protoplanetary disks, will provide the data needed to understand planet formation. Observations over the past two decades have established that protostars are accompanied by disks of gas and dust. These disks are believed to feed the growth of the stars and are regions where planets could form. Today’s instruments do not have the resolution or the sensitivity to find evidence for the existence of planets in protostellar disks, but ALMA, NGST, and TPF will. Theory shows that gas giants should create gaps in the disks that will be readily observable by these powerful instruments. Young giant planets (≤10 million years old) will emit enough radiation in the near infrared to be detectable by both NGST and GSMT in the nearby molecular clouds where star formation is occurring. These observations will reveal how protostellar disks evolve and the conditions under which planets can form. The existing census of extrasolar planets already indicates a surprising number of massive planets orbiting extremely close to the central star. Are these planets formed in the outer regions of the disk and then pushed into tighter orbits by the gravitational interaction with the disk material or with other planets? The Sun is in the minority in not having a stellar companion. Now do companion stars affect planet formation? Most stars form in large clusters containing massive stars, such as the cluster associated with the Trapezium in Orion. What is the effect of such an environment on

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Astronomy and Astrophysics in the New Millennium planet formation? Hubble pictures showing the destruction of protostellar disks in the Orion Nebula (Figure 2.2) suggest that such an environment is very hostile to planet formation. Some recent discoveries within our own solar system point the way toward another approach to filling in some details of the picture of planet formation and evolution. The Kuiper Belt consists of a ring or disk of subplanetary bodies circling the Sun beyond Neptune. Some 200 Kuiper Belt objects (KBOs) are now known, with diameters mostly in the 100- to 800-km range (Figure 2.3). Smaller KBOs are too faint to have been detected in existing surveys; larger ones almost certainly exist but await detection by deep, all-sky surveys such as will be conducted by LSST. It is thought that as many as 10 more objects of Pluto size (with a diameter of 2,000 km) await discovery. These KBOs are but the tip of an iceberg. Probably 100,000 objects larger than 100 km exist at distances 30 to 50 times Earth’s distance from the Sun. The number of objects larger than 1 km lies in the range of 1 billion to 10 billion. These objects are fossil remnants of the Sun’s planetary accretion disk, and their motions provide direct evidence of the protoplanetary disk’s physical characteristics. Collisions between these objects provide a long-term source for tiny dust particles in the solar system. Similar dust disks have been detected recently around some other main-sequence stars. The Kuiper Belt is probably the source of most short-period comets. Near-infrared spectra of the KBOs capitalizing on the huge light-collecting capability of GSMT will, for the first time, reveal the composition of comets in their pristine state, prior to entry into the inner solar system. The atmospheres of planets can be studied primarily in our own solar system. Except for Uranus, the gas giant planets emit more energy than they receive from the Sun. Their internal heat production drives complex and poorly understood systems of convection. The main external manifestations include differential rotation (as in the Sun) and energetic, weather-like, circulation patterns at the visible cloud tops. Planetary convection also powers dynamo action, causing the gas giants to support huge radio-bright magnetospheres. New adaptive optics systems on large-aperture telescopes will provide 10-milliarcsec resolution in the near infrared (Figure 2.4), enabling the study of long-term changes in planetary circulation (at Jupiter, 10 milliarcsec = 35 km; at Neptune, 200 km). Such studies will also provide the context for in situ investigations by NASA spacecraft. Near-Earth objects (NEOs) are asteroids with orbits that bring them close to Earth. The orbits of many NEOs actually cross that of Earth,

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Astronomy and Astrophysics in the New Millennium FIGURE 2.2 Protoplanetary disks in the Orion Nebula. These dark silhouetted disks, sometimes surrounded by bright ionized gas flows as seen in the cometary shape above, are being destroyed by intense ultraviolet radiation from nearby massive stars. The rapidity of their destruction may interrupt planet formation in these disks. Courtesy of C.R. O’Dell (Rice University) and NASA.

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Astronomy and Astrophysics in the New Millennium FIGURE 2.3 Plan view of the solar system, showing the orbits of the 200 Kuiper Belt objects (KBOs) known as of October 1999. Red orbits denote KBOs in orbits that are in resonance with Neptune, including Pluto; blue orbits show nonresonant or “classical” KBOs; and the large, eccentric orbits with labels denote KBOs that have been scattered by the gravity of the giant planets. The orbit of Jupiter at 5 AU (AU = astronomical unit, the distance from Earth to the Sun) is shown for scale. Observations with LSST should increase the number of known KBOs to 10,000, permitting intensive investigation of the dynamical structure imprinted on this fossil protoplanetary disk by the formation process. Courtesy of D. Jewitt (University of Hawaii).

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Astronomy and Astrophysics in the New Millennium FIGURE 2.4 An image of Neptune taken by the Keck Adaptive Optics Facility in the methane absorption band at 1.17 µm. The angular resolution of this image is approximately 0.04 arcsec, about an order of magnitude better than the resolution obtained without adaptive optics. Courtesy of the W.M. Keck Observatory Adaptive Optics Team. (This figure originally appeared in Publications of the Astronomical Society of the Pacific [Wizinowich, P., et al., 2000, vol. 112, pp. 315-319], copyright 2000, Astronomical Society of the Pacific; reproduced with permission of the Editors.) making NEOs an impact threat to our planet. Extrapolations from existing data suggest that about 1,000 NEOs are larger than 1 km in diameter, and that between 100,000 and 1 million are larger than 100 m. The effects of past NEO impacts on Earth range from the destruction of hundreds of square miles of Siberian forest at Tunguska in 1908 by a relatively small NEO to substantial disruption of the biosphere at the end of the Cretaceous period some 60 million years ago by a large (10-km) NEO. Interplanetary space is vast, so the probability of a substantial NEO hitting Earth is small: For example, it is estimated that the probability that an NEO larger than 300 m will strike Earth during this century is about 1 percent. Nonetheless, it behooves us to learn much more about these objects. Over a decade, LSST will discover 90 percent of the NEOs larger than 300 m, providing information about the origin of these objects in the process. However, comets also pose a substantial impact hazard, as was dramatically illustrated by the impact of Comet Shoemaker-Levy on Jupiter (Figure 2.5). Although LSST will discover much about comets, it will not provide long-term warning of potentially hazardous long-period comets.

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Astronomy and Astrophysics in the New Millennium the actual merger, enabling accurate prediction of the final event so that it could be observed by telescopes sensitive to the entire range of electromagnetic radiation. Observation of such a merger would provide a unique test of Einstein’s theory of general relativity in the case of strong gravitational fields. Further discussion of what scientists can learn about black holes can be found in the physics survey report Gravitational Physics: Exploring the Structure of Space and Time (NRC, 1999). Galactic nuclei can become extremely luminous as a result of intense bursts of star formation or the presence of a supermassive black hole. FIGURE 2.16 The jet produced by the central black hole in the galaxy M87. The Very Large Array (VLA) image at the upper left shows the radio emission powered by the jet. The Hubble Space Telescope (HST) image at the upper right shows the narrow jet at similar resolution. Finally, the Very Long Baseline Array (VLBA) image at the bottom, with more than 100 times the resolution of the HST image, is the closest view of the origin of such a jet yet obtained. Courtesy of NRAO, STScI, W. Junor (University of New Mexico), J.A. Biretta and M. Livio (STScI), and NASA. Reprinted by permission from Nature 401:891-892, copyright 1999 Macmillan Magazines Ltd.

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Astronomy and Astrophysics in the New Millennium These “starbursts” may be associated with the initial formation of the galaxy, or they may be triggered by an interaction with another galaxy. Starbursts are of great interest because they represent an extreme form of star formation that is not understood; for example, it is not known whether they produce the same distribution of stellar masses as that observed in our galaxy. Distinguishing starbursts from supermassive black holes is complicated by the fact that AGNs are often shrouded in dust, so that much of the direct emission is hidden from view. Long wavelengths penetrate the dust more readily, so the EVLA, SAFIR, and NGST with an extension into the thermal infrared are all suitable for separating the two phenomena. Very-high-energy photons can also penetrate the dust, so Constellation-X and EXIST will provide relevant data as well. Active galactic nuclei may be the source of ultrahigh-energy cosmic rays (gamma-ray bursts and intergalactic shocks have also been suggested as the source of these enigmatic particles). These cosmic rays are generally assumed to be protons that have been accelerated to very high energies. The energies are so large—equivalent to the energy of 1 billion to 100 billion protons at rest—that these cosmic rays can propagate only a limited distance before losing their energy through interactions with the cosmic microwave background radiation. Ongoing experiments with the Fly’s Eye in Utah and proposed experiments with the Southern Hemisphere Pierre Auger Observatory project will add greatly to our knowledge of these cosmic rays, particularly if the experiments are able to identify their sources. THE UNIVERSE Observations by NGST should witness the first light from distant galaxies. Long before the stars that emitted this light were formed, the matter making up the galaxies had to accumulate from the intergalactic medium. This process of galaxy formation occurred within the background of an expanding universe. How has the universe evolved through cosmic time? How did structures such as galaxies and clusters of galaxies develop in the expanding universe? Finally, observations show that not all the matter that makes up galaxies and clusters of galaxies is visible: What in fact is the composition of the universe?

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Astronomy and Astrophysics in the New Millennium THE EVOLUTION OF THE UNIVERSE Evidence indicates that somewhat more than 10 billion years ago the universe was created in a titanic explosion—the Big Bang. What may have preceded this event is unknown. The Big Bang theory allows us to trace the evolution of the universe back to a time when it was just a soup of elementary particles—a few microseconds after the beginning. Researchers have promising ideas that would enable extending understanding back to a time before particles existed, when even the largest objects in the universe were quantum fluctuations. How has the universe expanded since the Big Bang? Astronomers measure the expansion of the universe through the redshift of the radiation observed. The greater the redshift of light from an observed object, the more the universe has expanded since that radiation was emitted. The relationship between the redshift and time—the calibration of the cosmic clock—determines how long ago the radiation was emitted (see Figure 2.12). Using the speed of light to convert time to distance, this relationship can be also be used to determine the geometry of the universe (whether space is flat or curved). The current time scale for the expansion is set by a parameter known as the Hubble constant, which gives the relation between redshift and distance. Using HST and other telescopes, it has been possible to establish the value of the Hubble constant with an accuracy approaching 10 percent. In order to derive the age of the universe from the measured value of the Hubble constant, it is necessary to know how the expansion has accelerated or decelerated with time. The history of the expansion of the universe depends on the total density of matter in the universe (both ordinary matter and dark matter) and on the possibly non-zero “cosmological constant,” which might characterize a sort of “dark energy” in the universe. These parameters determine the geometry of the universe and its ultimate fate, whether it will expand forever or eventually recollapse. Theory suggests that the geometry of the universe is flat; in this case, the total density of matter and energy is said to have its “critical” value. Observations of distant clusters of galaxies indicate that the density of matter is about 30 percent of the critical value. One of the most exciting developments of the past decade has been the discovery that the cosmological constant may not be zero—our universe appears to be filled with dark energy. This discovery is based on

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Astronomy and Astrophysics in the New Millennium two independent sets of observations. First, astronomers have found a way to determine the luminosity of Type Ia supernovae from the rate at which their light declines. Knowledge of the luminosity enables the determination (or calculation) of the distance to such a supernova by measuring its brightness. The results show that distant supernovae appear fainter than expected, suggesting that the expansion of the universe is accelerating. When combined with other data, the observations of supernovae lead to the conclusion that dark energy makes up perhaps 70 percent of the total density of matter and energy. Second, observations of fluctuations in the cosmic microwave background (discussed below) strongly suggest that the universe is indeed flat, so that the total density of matter and energy is at the critical value. Since estimates of the masses of clusters of galaxies show that the matter density of the universe has only about 30 percent of the critical value, it follows that the dark energy must make up the remaining 70 percent. Together with the value of the Hubble constant determined above, the estimated values of the matter and energy densities yield an age for the universe of about 14 billion years. During this decade, observers and theorists will work to understand and extend these observations. Confirmation that dark energy exists, with a density that rivals that of matter, would be a physical discovery of the most fundamental significance. Planned observations of the cosmic microwave background will provide more accurate values of the cosmological parameters, including the density of ordinary matter. This value of the matter density, when compared to an equally precise determination derived from a measurement of the primeval deuterium abundance, will allow a fundamental consistency test of the standard cosmology. Recent measurements of the deuterium abundance in distant galaxies indicate that this test is feasible; however, a definitive measurement of deuterium is still needed. NGST will permit the observation of many supernovae at high redshifts, to confirm whether the universe is actually accelerating. Discovery of a much larger number of supernovae with LSST, followed up by more sensitive and precise measurements from ground- or space-based telescopes, will permit the cosmic clock to be calibrated with much greater precision. It should then be possible to determine whether the cosmological constant is really constant, as Einstein assumed, or evolving with time, as some current theories suggest.

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Astronomy and Astrophysics in the New Millennium THE EVOLUTION OF STRUCTURE IN THE UNIVERSE The seeds of the structure of the universe down to the scale of galaxies, and probably even smaller, were planted by tiny quantum fluctuations in the first instants of the Big Bang. In order to study how the large-scale structure in the universe grew from these seeds, it is necessary to study how galaxies are distributed in space today. Surveys of galaxies carried out more than a decade ago revealed large voids where few galaxies were visible, and other regions where the density of galaxies was enhanced on scales up to 300 million light-years in extent. Surveys of galaxies during the past decade have shown that this appears to be the limiting scale on which large fluctuations in density occur: On larger scales, the universe appears to be smooth. Surveys under way now, particularly the Sloan Digital Sky Survey, will provide a far more accurate map of the distribution of galaxies in the nearby universe. Direct evidence for the early fluctuations that led to this structure is imprinted on the oldest radiation in the universe, the cosmic microwave background (CMB). This radiation was emitted at a redshift of about 1,000, or a time only several hundred thousand years after the Big Bang, when the temperature of the radiation was somewhat less than that at the surface of the Sun. Today, the temperature of the background radiation is 1,000 times lower, just 3 degrees above absolute zero, having been cooled by the expansion of the universe. This radiation was observed with remarkable accuracy by the Cosmic Background Explorer (COBE), launched in 1989. Data from this satellite showed that the radiation had the theoretically predicted spectrum of a blackbody. COBE data also revealed tiny spatial ripples in the intensity of the radiation (Figure 2.17), indicative of density fluctuations that could lead to the observed large-scale structure of the universe. This set of satellite observations provided, for the first time, direct experimental evidence for a basic paradigm of scientists’ cosmological speculations and established the quantitative basis for all subsequent work in this field. By design, the COBE satellite had very low angular resolution, and therefore it was able to measure structure in the background radiation only on the largest scales. The characteristics of the background radiation on smaller scales depend on the matter and energy content of the universe; in concert with studies at lower redshifts, such as the Sloan Digital Sky Survey and searches for supernovae, these data can be used to determine all the fundamental properties of the universe, including its age and the amount of matter and energy it contains. Recent observa-

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Astronomy and Astrophysics in the New Millennium tions imply that the total density of matter and energy is very close to what is needed to make the geometry of the universe flat (see Figure 2.18). NASA’s MAP, the European Space Agency’s Planck Surveyor satellite, the ground-based Cosmic Background Imager, and future balloon observations will dramatically increase the sensitivity of studies of the background radiation. In addition to measuring the fundamental cosmological parameters with great precision, these missions will provide stringent tests of current cosmological theories. Ground-based studies will measure the distortion of the spectrum of the background radiation caused by the hot gas in intervening clusters of galaxies. Combined with observations by Constellation-X of the properties of this hot gas, these observations will enable researchers to determine the distances to these clusters, constrain the value of the Hubble constant, and probe the large-scale geometry of the universe. One aspect of the cosmic microwave background that these missions will only begin to investigate is its polarization. Gravitational waves excited during the first instants after the Big Bang should have produced effects that polarized the background radiation. More precise measure- FIGURE 2.17 The COBE satellite detected tiny variations in the intensity of the cosmic microwave background. The amplitude of the temperature fluctuations is only about 0.00001 K, which reflects the smoothness of the universe at the time this radiation was emitted, and dramatically confirms the theoretical expectation that the universe began from a dense, hot, highly uniform state. The COBE data sets were developed by NASA’s Goddard Space Flight Center under the guidance of the COBE Science Working Group and were provided by the National Space Science Data Center.

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Astronomy and Astrophysics in the New Millennium FIGURE 2.18 The spectrum of the primordial sound produced by the Big Bang. The sound waves can be observed through the fluctuations they produce in the temperature of the cosmic microwave background. Plotted is the mean-square temperature difference between two points in the sky as a function of their angular separation parameterized by the multipole number l (angular separation ~ 180 degrees/l). The observations were made with the BOOMERANG and MAXIMA balloon-borne telescopes; data from the COBE differential microwave radiometers (DMR) are also included. The peak in the spectrum at about 1 degree (l ~ 200) indicates that the universe is nearly spatially flat. The data can be fit well by models (such as that shown by the solid blue curve) in which only a small fraction of the matter is normal baryonic matter. Courtesy of the BOOMERANG and MAXIMA collaborations.

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Astronomy and Astrophysics in the New Millennium ments of the properties of this polarization—to be made by the generation of CMB missions beyond Planck—will enable a direct test of the current paradigm of inflationary cosmology, and at the same time they will shed light on the physics of processes that occurred in the early universe at energies far above those accessible to Earth-bound accelerators. COMPOSITION OF THE UNIVERSE Ordinary matter is made up of the same atoms as are known to us on Earth. The nucleus of an atom consists of protons and neutrons. The electrons encircling the nucleus are equal in number to the protons, although some of these electrons are stripped from the atom if the atom is ionized. Atoms can combine together into molecules, which in turn combine together to form all the matter we see on Earth. Atoms can produce light, and by observing light from stars astronomers have concluded that the stars, too, are made up of atoms. But when astronomers observe larger objects, such as the outer parts of galaxies or entire clusters of galaxies, they have found that the amount of matter they see in glowing gas and stars is not enough to hold these objects together by gravity. They therefore have postulated a form of matter too faint to see through its radiation: dark matter. The current state of knowledge of the composition of the universe is shown in Figure 2.19. As discussed above, recent observations have suggested that the total density of matter and energy is the critical value necessary for a flat universe. Of this total critical value, about two-thirds is dark energy, whose nature is unknown, and one-third is matter. Ordinary matter is about 5 percent of the total, and luminous stars make up only about 0.5 percent. Where is the ordinary matter that is not in luminous stars? A leading contender for at least some of this missing ordinary matter is hot intergalactic gas, and Constellation-X will test this hypothesis. An even greater mystery is the nature of the matter that is not made up of atoms—the dark matter. Some of this matter is composed of neutrinos left over from the Big Bang. Although the uncertainty in their mass makes it difficult to determine exactly how much, astrophysical observations suggest that neutrinos do not account for the bulk of the dark matter. The rest is believed to be in the form of dark matter particles or objects that move relatively slowly, and are therefore called “cold” dark matter. Determination of the nature of this cold dark matter is one of the great unsolved problems in modern astrophysics.

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Astronomy and Astrophysics in the New Millennium FIGURE 2.19 The makeup of our universe. Two-thirds of the matter and energy in the universe is in the form of a mysterious form of dark energy that is causing the expansion of the universe to speed up, rather than slow down. The other third is in the form of matter, the bulk of which is dark and which scientists believe is composed of slowly moving elementary particles (cold dark matter) remaining from the earliest moments after the birth of the universe. All forms of ordinary matter account for only about 5 percent of the total, of which only about one-tenth is in stars and a very tiny amount is in the periodic table’s heavier elements (carbon, nitrogen, oxygen, and so on). The idea of particle dark matter was reinforced by recent indications that neutrinos have mass and thereby account for almost as much mass as do stars. Adapted from a drawing courtesy of M. Turner (University of Chicago).

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Astronomy and Astrophysics in the New Millennium The large-scale distribution of the dark matter can be studied through observations of gravitational lensing. Studies of gravitational lensing have given astronomers their best look at the distribution of dark matter both in clusters of galaxies and around some individual galaxies. In this decade, surveys of galaxies over vast areas of the sky with LSST and other telescopes will provide lensing data that describe the dark matter distribution over supercluster scales—information crucial for understanding the growth of large-scale structure. Two leading possibilities for the makeup of dark matter are (1) elementary particles left over from the earliest moments of creation and (2) objects of stellar mass (massive compact halo objects, or MACHOs). It is a mark of the uncertainty in this field that these two candidates differ in mass by more than 57 orders of magnitude. Theorists predicted that MACHOs, though too faint to be detected by their own emission, could be detected by gravitational lensing as well: The light of the background star would be amplified as the MACHO passed in front of the star. During the past decade, several groups independently detected this phenomenon, which is called microlensing because the mass of the lens is so small compared with that of galaxies (Figure 2.20). The nature of the MACHOs is a significant mystery: Are they stars made up of ordinary matter, or are they objects made up of an exotic form of matter? Accurate determination of their masses would help resolve this question, but to date, definitive measurements have not been possible; the best estimate is that the typical mass of a MACHO is somewhat less than a solar mass. By resolving the apparent motion of the stars that are imaged by the MACHOs, SIM will measure the masses of the MACHOs. Studies of microlensing have had several important spinoffs, including resolution of the surface of the star being lensed, and demonstration that it should be possible to detect planets as small as Earth through microlensing observations, as discussed in “The Formation and Evolution of Planets” section of this chapter. As yet it is unclear how much MACHOs contribute to the dark matter in the Galaxy. If MACHOs are made of ordinary matter, then they cannot account for the bulk of the dark matter known to exist in the universe or even in our own galaxy. As a result, a number of efforts are under way in laboratories around the world to discover the particle dark matter that may be holding our own Milky Way together. There are two important

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Astronomy and Astrophysics in the New Millennium FIGURE 2.20 The first gravitational microlensing light curve, showing the amplification of the light of a background star by the gravitational field of an intervening object. These intervening objects, of unknown nature, may contribute to the dark matter in the Galaxy. The similarity of the curves in red light and blue light helps confirm that the brightening is caused by gravitational lensing. Courtesy of the MACHO collaboration. Reprinted by permission from Nature 365:621-623, copyright 1993 Macmillan Magazines Ltd. ongoing efforts in the United States: (1) the Cryogenic Dark Matter Search II, a search for a particle with roughly atomic mass called the neutralino, and (2) the U.S. Axion Experiment, a search for an extremely light dark matter particle called the axion. The existence of the neutralino is a prediction of superstring theory, a bold and promising attempt to unify gravity with the other forces of nature. The discovery that neutralinos or axions are the dark matter that binds our own galaxy would shed light not only on the astrophysical dark matter problem, but also on the unification of the fundamental forces and particles of nature.