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Frontiers of Astrophysics . LARGE-SCALE STRUCTURE IN THE UNIVERSE Probes of Large-Scale Structure As seen on photographs taken with the largest telescopes, galaxies appear to drift in the depths of space like motes in a sunbeam. Everywhere they are clumped in groups containing a few galaxies, and occasionally in clusters of a thousand or more. Some clusters clump in superclusters 50 megaparsecs or more across. On even larger size scales, however, groups and clusters of galaxies seem to be distributed nearly at random, the number in a given volume of space being about the same throughout the Universe. This uniformity of the distribution of matter on very large scales invites comparison between the observations and a simple model of the Universe, or cosmology, derived for a uniform distribution of matter from Einstein's General Theory of Relativity. According to this model, the geometry of space-time is curved by matter, and the curvature forces the matter to move: at any epoch the Universe must be either expanding or contracting. Hubble's discovery in 1929 that the Universe is actually expanding forces us to confront a bizarre implication of the theory: that an expanding Universe must have originated in a powerful explosion-referred to as the big bang before which neither time nor space had any meaning. In the half-century since Hubble's momentous discovery, astron 37

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38 ASTRONOMY AND ASTROPHYSICS FOR TEIE 1980's omers have been probing the Universe in space and in time. Using radio and optical telescopes, they have found objects so distant that they are receding from us at 90 percent of the speed of light. With microwave antennas, they have discovered a faint radio noise that they interpret as the remnant of the big bang itself. From the theory of the nuclear reactions that must have taken place during the first 3 minutes, they have calculated the abundances of key elements and isotopes such as hydrogen, deuterium, and helium, which were produced in the big bang; with ground-based telescopes and ultra- violet spectrographs in Earth orbit they have verified that the actual relative numbers of these atoms in space agree surprisingly well with theoretical predictions. The big bang has become the standard model with which to com- pare observations. This is not to say that it is completely correct: the data are imprecise; their interpretation may be in error; and the theory could be wrong. A central problem for the future is the further development of the big-bang model and its testing against all avail- able observations. The big-bang model requires that matter is distributed uniformly on large scales. By using a variety of approaches, it is now possible to test whether this is true. Observers have used apparent magni- tudes as a rough measure of the distances of galaxies; plotting the directions of galaxies in various distance ranges, they have found that on scales exceeding 100 megaparsecs, galaxies are distributed rather uniformly. One can obtain the precise location of each galaxy in three dimensions by determining its red shift spectroscopically. Recording the spectrum photographically is time consuming, but the recent development of electronic array detectors has speeded up the recording of spectra so greatly that red-shift surveys of thousands of galaxies are now possible. The resulting three-dimensional distri- bution appears to be uniform on the largest scales. It is anticipated that red-shift surveys of much more distant galaxies will be com- pleted during the 1980's. X-ray and gamma-ray astronomy also tell us about the large-scale distribution of matter. A diffuse background emission not attributable to known sources appears in both spectral regions; its near isotropy proves that it cannot originate within the Galaxy but must instead originate at distances comparable with the size of the Universe itself. The High-Energy Astronomical Observatory-1 (HEAo-1) x-ray ob- servatory established that the x-ray background is highly isotropic and that its spectrum between a few and about 60 kiloelectron volts (keV) agrees closely with the radiation expected from a gas having

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Frontiers of Astrophysics 39 a temperature of about 500 million degrees, leading to the suggestion that such gas is distributed uniformly between the galaxies. The Einstein (HEAo-2) x-ray observatory, on the other hand, discovered that individual quasars at large distances are powerful x-ray sources in the few-keV range powerful enough, in fact, that quasars at even larger distances than can be detected individually by the Einstein x- ray observatory must account for a substantial fraction of the ob- served x-ray background in the few-keV range. As some quasars have also been found to be powerful gamma-ray sources, the gamma- ray background may also be due to quasars. It is still not clear, however, how quasar spectra would sum up so as to mimic the spectrum of hot gas. The Advanced X-Ray Astrophysics Facility (AX~) recommended in this report can observe sources 100 times fainter than could the Einstein x-ray observatory and can thus determine whether faint quasars account for the observed background at ener- gies of a few keV. Measurements of faint quasars by the Gamma Ray Observatory (GRO) will give similar information for the back- ground at gamma-ray energies. If it proves that the x-ray and/or gamma-ray backgrounds are actually due to quasars, the fact that the background is highly isotropic requires that matter at great dis- tances is distributed very uniformly. If, on the other hand, inter- galactic gas is responsible for at least part of the x-ray background, one can infer that it is distributed uniformly; moreover, the amount of gas required is an important datum for the theory of evolution of galaxies. The cosmic microwave background radiation also gives information about the large-scale structure of the Universe. Precise measurements have revealed a smooth variation in its intensity over the sky that is attributable to the Earth's motion through the cosmos. The ob- served variation is unexpectedly large, corresponding to a velocity of 500 km/see for the Local Group of galaxies with respect to distant matter. The same measurements reveal no other certain variations larger than 0.03 percent, indicating that the Universe was highly uniform at the time the background radiation last interacted with matter. Ground-based experiments indicate that the spectrum of the microwave background radiation does not deviate significantly from thermal, as predicted by the big-bang model, but a balloonborne submillimeter experiment points to discrepancies that are difficult to explain. Both variations in intensity with direction and deviations from a thermal spectrum will be measured over the entire spectral range with improved precision (about 0.01 percent) by the Cosmic Background Explorer (COBE) mission planned by NASA.

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40 Expansion Time Scale ASTRONOMY AND ASTROPHYSICS FOR THE 1980's The big-bang model predicts that galaxies should move away from each other with velocities that are proportional to their separations. Slipher and Hubble found evidence for such a relationship in exten- sive measurements of the brightnesses and red shifts of galaxies during the 1920's; the constant of proportionality between a galaxy's velocity and its distance is called the Hubble constant. According to relativistic models, the reciprocal of the Hubble constant (the "Hub- ble time") is roughly equal to the present age of the Universe that is, the time that has elapsed since the big bang. Determining the value of the Hubble time requires the measure- ment of the distances of remote galaxies, using a "ladder" of inter- connected distance scales determined by different methods; each step of the ladder reaches further into space. Hubble's own estimate for it was 2 billion years. It has since been revised several times to 5, then to 10, and then to 20 billion years; the latest estimates are between 10 billion and 20 billion years. Each revision has been the result of a major advance in understanding the properties of stars or galaxies that are used to construct the ladder of distance scales. The value of the Hubble time enters all cosmological calculations in a fundamental way. To find its true value, each step of the ladder of distance scales must be secure, and any contributions to the ve- locities of galaxies that are not due to the expansion of the Universe must be taken into account. An example of the latter effect is the motion of the Local Group of galaxies revealed by study of the cosmic background radiation; when this is taken into account, a more con- sistent set of data for the Hubble time emerges. Refinement of the distance ladder will take much more work. Development of more precise astrometric methods, as recommended in this report, will make possible a more accurate measurement of the distance to the Hyades star cluster, the first step in the ladder of cosmic distance scales. Because of its extremely faint limiting mag- nitude, Space Telescope (ST) will for the first time resolve Cepheid variable stars in the Virgo cluster, thereby eliminating an uncertain intermediate step of the distance ladder. The continued deployment of advanced optical detectors at ground-based telescopes will make possible the rapid measurement of red shifts of galaxies at moderately large distances, where the velocity field should be one of nearly pure expansion; ST can determine the distances of the same galaxies by comparing the brightness of their globular clusters with the bright

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Frontiers of Astrophysics 41 ness of those in the galaxies of the Virgo cluster, whose distances are known accurately. The Early Universe The cosmic microwave background radiation carries information about the Universe before it was about 1/100,000 of its present age, so the COBE experiment is fundamental to studies of the early Universe. Other clues depend on the nucleosynthesis of various elements and isotopes in the first 3 minutes. Theoretical predictions of their abun- dances depend critically on the amount of ordinary matter present during that period. If the amount is low, the resulting deuterium abundance would be high and the helium abundance low; if the amount is high, the opposite would be the case. Present information on the abundance of deuterium and helium in interstellar space in our Galaxy, taken at face value, indicates that the amount of matter is too low by a factor of 10 for its gravitation to be able to halt the expansion of the Universe. However, helium has been produced and deuterium has been destroyed in stars, so present abundances in the Galaxy may not be the same as in the primordial gas that emerged from the big bang. Abundances in intergalactic gas, if it exists, should be primordial. Astronomers have discovered absorption-line systems in distant quasars that probably originate either in clouds formed by the out- ward ejection of thick shells of gas from the quasar itself or in in- tergalactic clouds lying along the line of sight. In the first case, the phenomenon would resemble the late stages occurring in the stellar outbursts known as novae. In the second case, the clouds should contain very little carbon or other medium-weight elements, which are telltale signs of stellar nucleosynthesis, because such gas would never have been inside a galaxy. The gas in such clouds would be a good candidate for the study of primordial helium and deuterium. Observations of helium and deuterium in such gas, however, must be made at much shorter wavelengths than are accessible to ground- based observatories; they require ST. With ST we can study helium lines in clouds of red shift greater than unity and deuterium lines in clouds of all but very low red shifts. Groups, Clusters, and Superclusters The grouping of galaxies on various size scales can be studied by calculating the statistical correlations between the observed positions

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42 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's of galaxies. In earlier studies, the apparent magnitudes of galaxies were taken as measures of their distances, and their positions then follow from their observed directions in the sky. The calculated correlations between the positions derived in this way decrease as an inverse power of the distances between pairs of galaxies. A simple model to explain this is based on gravitational clustering of point masses, which are initially distributed at random but which then move under their mutual gravitation as the Universe expands. This model reproduces many of the features of the ob- served clustering of galaxies, so that galaxies may have formed early in the expansion of the Universe and clumped together later by gravitation. Recent observational work, however, has brought out an un- expected new feature in the distribution of galaxies. Aided by red- shift measurements, which furnish the distances of galaxies much more accurately than estimates based on their apparent magni- tudes, astronomers have found that groups of galaxies outside of clusters are not sprinkled at random through space but instead lie in great sheets between the clusters, leaving vast empty re- gions between. To explain this may require a new theoretical model. in which Galaxies formed rather late. At first, giant tur- bulent cells of gas collided, compressing the gas into sheets; only after the sheets formed did the galaxies condense from them and then begin to clump together as in the earlier model. Two kinds of data are required if we are to understand the formation and clumping of galaxies. First, red-shift surveys em- bracing a large number of galaxies are needed. For the nearer galaxies, it is feasible to obtain red sniffs w^tn currently ava'^A- able telescopes of moderate size, equipped with array detectors and fast spectrographs. To penetrate more deeply into space, however, large telescopes will be needed. Telescopes of the 5-m class will make important contributions, but only a new tele- scope of the 15-m class, such as the New Technology Telescope (NTT), can measure the red shifts of galaxies at large distances rapidly enough to accumulate the required number of galaxies. The raw speed of NIT, made possible by its order-of-magnitude increase in collecting area over the previous largest telescopes, is critical for this project. A A ~ ~ ~ ^ ^ ~ ~ ~ ~ ~ A _ A ~ ~-~ O ~ ~ -id. .1 .1 1 Hidden Mass and the Fate of the universe For the past 20 years, astronomers have been increasingly puzzled by the ,ihidden mass,, problem: the matter that constitutes most of

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Frontiers of Astrophysics 43 the mass of the Universe is invisible. The spectra of galaxies indicate that, like our own Milky Way Galaxy, they contain normal stars; however, the internal motions in galaxies are so large that they would fly apart if the only gravitational attraction holding them together were that of the stars we see. There must be additional mass present in some form that is hidden from our immediate view- enough to supply the gravitational attaction required for stability. The rotational velocities observed in spiral galaxies demonstrate that the amount of hidden mass inside a given radius increases approximately linearly with radius out to distances of nearly 100 kiloparsecs. Similar results emerge from studies of groups of two or more galaxies: their masses must be at least 10 times greater than the masses of all the visible stars in them. Solution of the hidden-mass puzzle is a major goal of astronomy in the decade ahead. The first task is to find how it is distributed. The velocities of globular clusters in the outer reaches of galaxies reflect the strength of the local gravitational field and hence the distribution of mass in the parent galaxy. Since globular clusters in even relatively nearby galaxies are extremely faint, spectroscopic measurements of their velocities can be made only with a telescope as large as NIT. Galaxies themselves can serve as probes of the distribution of mass in clusters and superclusters of galaxies. Since galaxies are much brighter than globular clusters, work on clusters of galaxies is already proceeding with intermediate-sized telescopes. However, measurements of velocities of galaxies in distant clusters are essential to determine how the distribution of mass has changed with time; this will require observations with NIT. Various possibilities have been suggested to account for hidden mass: diffuse gas, massive neutrinos, collapsed stars (white dwarfs, neutron stars, black holes), and faint red dwarfs. Diffuse gas can be ruled out as a dominant component of either galaxies or clusters of galaxies through radio, optical, and x-ray ob- servations; although 100-million-degree gas exists in clusters of gal- axies, the amounts are not sufficient to hold the clusters together. Massive neutrinos, if they exist, might fall into clusters of galaxies, and possibly even into galaxies themselves, thus contributing to the hidden mass. Collapsed stars of various types could in principle constitute much of the hidden mass; however, such stars are the descendants of massive main-sequence stars and so would dominate the total mass only if, at early epochs of star formation, massive stars dominated the total mass of main-sequence stars. Just the contrary is observed to be the case for star formation in our Galaxy near the Sun: faint

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44 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's red dwarfs, which are of low mass, are so numerous that they ac- count for most of the mass bound up in stars. One could speculate that there were many more massive stars in the outer parts of galaxies during the early stages of galaxy evolution, so that large numbers of collapsed stars would exist there today. However, if that were so, one would expect a higher concentration of heavy elements in the outer parts of the galaxies, since massive stars synthesize heavy elements and eject them into the interstellar medium; this is contrary to observation. Faint red dwarfs could also account for the hidden mass, as large numbers of them in the outer parts of galaxies would be consistent with both the lower concentrations of heavy elements and the lower light levels observed there. It may just prove possible to test this hypothesis by using the recent discovery that red dwarfs are rela- tively luminous sources of coronal x rays. AXAF will be able to detect such red dwarfs by observing their integrated coronal x-ray emission if they are numerous enough. The hidden-mass problem is intimately connected with the ques- tion of the ultimate fate of the Universe. According to the big-bang model, the Universe will continue to expand forever if the amount of matter in it is less than a critical value calculated to be between 0.5 x 10-29 and 2 x 10-29 g in each cubic centimeter. If the amount of matter exceeds the critical value, the present expansion will reverse at some time in the distant future, and the Universe will collapse back into a singular state similar to the big bang. The observations of deuterium and helium discussed earlier suggest that the amount of ordinary matter is only 10 percent of the critical value, so that only massive neutrinos could raise it above the critical value. A lower limit on the total amount of mass in all forms is obtained from the masses of clumps in the distribution of galaxies; current estimates suggest that the aggregate amount of matter in such clumps may be as much as 40 percent of the critical value. Since this is larger than the upper limit on the amount of ordinary matter obtained from observations of helium and deuterium, massive neutrinos may con- ceivably account for most of the matter in the Universe. Massive neutrinos are discussed further in the last section of this chapter. EVOLUTION OF GALAXIES The Study of Galaxies Like the Galaxy in which we live, the 100 billion or more galaxies in the visible Universe are fascinating systems in their own right. As

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Frontiers of Astrophysics 45 the nuclear and gravitational energy stored in them is released, it is likely that galaxies evolve toward objects evermore structured and compact. Among the variety of forms that galaxies take, Hubble discerned several recurrent patterns spirals, ellipticals, lenticulars, and irreg- ulars; these patterns have still not been completely explained theo- retically. Ellipticals and lenticulars are nearly devoid of interstellar gas and dust, while spirals and irregulars contain gas and dust, as well as young stars formed recently from them. Until recently, the gas and dust in spiral galaxies other than our own could be studied with high angular resolution only at optical wavelengths, by imaging the dark interstellar dust clouds and the luminous gas clouds heated by bright young stars. Now the Very Large Array (VLA) radio tele- scope can image galaxies both in the 21-cm line produced by inter- stellar atomic hydrogen and in the synchrotron radiation produced by relativistic electrons gyrating in interstellar magnetic fields; it can thus trace the distribution and state of the interstellar medium with angular resolution comparable with that of optical telescopes. As in all fields of astronomy, spectroscopy is the key to deeper understanding. Ground-based optical spectroscopy of galaxies dem- onstrates that a major component of most galaxies is stars of various masses and ages, like those in our Galaxy. However, present ground- based telescopes are hard pressed to obtain the spectra of extremely faint subsystems of galaxies, such as individual giant stars, regions of ionized gas, and globular clusters; they are too small to permit collection of photons at a sufficiently high rate. NTT, with its order- of-magnitude increase in collecting area, can obtain the spectra of such objects, thus making possible a whole new range of studies related to chemical composition, distribution of stellar masses, and rotational and random velocities within galaxies. For a galaxy of a given red shift, NTT will make possible studies with much higher spectral resolution; for the same spectral resolution, it can carry out studies on galaxies of much higher red shift. The latter capability is crucial for analysis of objects of large red shift that will be discovered by ST. One of the most striking capabilities of the new instruments rec- ommended for the 1980's is the systematic exploration of the de- pendence of various galactic properties on red shift at greater and greater cosmological distances. Big-bang models of the Universe pre- dict such a dependence because the evolution of galaxies with time translates into changes with lookback time, and hence with red shift. ST and NTT will be able for the first time to observe galaxies with red shifts substantially exceeding unity, corresponding to lookback times

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46 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's that are more than half of the Hubble time. ST can image such distant objects because the sharpness of its images makes them stand out against the background, and NIT can obtain their spectra because it has a much larger collecting area than present large telescopes. If the matter comprising the inner parts of galaxies has already settled into an equilibrium state within considerably less than a billion years after the big bang, the forms of galaxies would not depend sensitively on red shift out to red shifts of 10 or so. However, the evolution of stars and the conversion of interstellar gas into stars proceeds much more slowly and should be observable at much lower red shifts. The spectra of isolated elliptical galaxies should manifest subtle changes that reflect the evolution of the stars that they contain, while isolated spiral galaxies should in addition manifest the progressive depletion of interstellar matter, as well as its enrichment in heavy elements produced by supernova explosions. A major indirect effect will be the reduction in the number of short-lived massive stars as the gas required to form them is depleted. Failure to observe such basic predictions of big-bang theory would force major revisions in current thinking. Formation of Galaxies The first relativistic models of the big-bang Universe were derived by Friedmann in 1922. For simplicity, he assumed that matter is distributed absolutely uniformly. Although this assumption conflicts with the existence of stars and galaxies, the model is useful because matter is in fact distributed quite uniformly when averaged over large distances. Still, the origin of galaxies in a big-bang model is an unresolved problem. Many properties of galaxies can be explained at least qualitatively if it is assumed that they originated in small fluctuations in the amount of local matter in the early Universe. At that time, the be- havior of matter was governed by the pressure exerted by the cosmic background radiation. Two types of density fluctuations could have existed. One type, so-called isothermal fluctuations, would have led to gravitationally unstable clumps of matter if they had involved more than 105 to 106 solar masses; another type, adiabatic fluctua- tions, would have led to gravitationally unstable clumps if they had involved more than 10~3 to 10~4 solar masses. In both cases, instability would have set in about 100,000 years after the big bang, and as a result, the matter in the fluctuations would soon cease to participate in the cosmic expansion, would then become more dense as self

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Frontiers of Astrophysics 47 gravitation drew the gas together, and would ultimately form discrete gas clouds of various masses. The Cosmic Background Explorer (COBE) satellite will yield impor- tant information on the proposed instability process by observing the disturbances in the background radiation that would accompany any density fluctuations in the early Universe. Adiabatic fluctuations, which involve variations in temperature and hence in the intensity of the cosmic background radiation, would result in intensity vari- ations on angular scales of a few degrees if the masses involved in the fluctuations are about those of clusters of galaxies. Complemen- tary information about fluctuations on the smaller angular scales corresponding to individual galaxies (less than a degree) will be obtained by the Large Deployable Reflector (LDR) in space. The theory of adiabatic fluctuations has been worked out in detail for the case in which there is a random collection of initial fluctuations of various sizes and masses. Fluctuations involving 10~3 to 10~4 solar masses, usually identified with groups and clusters of galaxies, should form clouds first; the formation of galaxies would have taken place later within these clusters and groups. Clusters and superclusters con- taining more than 10~3 to 10~4 solar masses must have formed through later gravitational clustering of the original mass aggregations of this size. Alternatively, isothermal fluctuations may have dominated the in- itial stages of galaxy formation. In this case, the first objects to form must have had masses from 105 to 106 solar masses, and galaxies must have been built up later by gravitational clustering of these smaller objects. The fact that globular clusters containing 105 to 106 solar masses are so common would be a natural result of isothermal fluctuations. If galaxies formed out of objects having 105 to 106 solar masses, then groups and clusters of galaxies must have formed sub- sequently through gravitational clustering of the galaxies themselves. This process can be modeled with computers by treating each galaxy as a point mass and calculating its gravitational interactions with its neighbors. Extensive simulations of gravitational clustering have been carried out in this way during the past decade; the results agree with observations in some respects, but they do not predict the large holes devoid of galaxies that have been observed between clusters. It is still uncertain whether galaxies or clusters of galaxies originated first. None of the existing computer simulations of either galaxy collapse or clustering addresses the origin of the fluctuations themselves. Current attempts to answer this important question, based on Grand Unified Theories of elementary particles, are encouraging.

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9o ASTRONOMY AND ASTROPHYSICS FOR THE 1980's for obtaining radial-velocity measurements of the required precision; sustained programs of observations of many candidate stars are now required. Astronomers have expended great effort to make milli- arcsecond-position measurements on nearby stars, but until recently the required precision has not been available. With the development of the optical astrometric techniques recommended in this report, however, it should be possible to observe many stars with a precision exceeding 1 milliarcsecond and by this means to detect Jupiter-sized planets around nearby stars, if they exist. Space astrometry should ultimately yield much higher positional accuracy, leading to the still more interesting prospect of detecting Earth-sized planets. Far-in- frared interferometric observations from space could also reveal planets around nearby stars. Search for Extraterrestrial Intelligence Even if other planets are detected, it will still be difficult to infer whether life is present; to do so directly would require imaging the planet itself with as yet undreamed-of resolution. Only if the planet harbors intelligent life capable of producing electromagnetic signals detectable at Earth is there at present any hope of finding life outside our solar system. It is a remarkable fact that radio and television signals generated copiously on Earth could be detected at distances of many parsecs by civilizations that, like ours, would otherwise have no way of knowing of the existence of details of the planet that is our home. Should the human race search seriously for signals from other possible civilizations? Much has been written about this question, both on a technical and a philosophical level. Reception of intelligent signals from space could have a dramatic effect on human affairs, as did contact between the native peoples of the New World and the technologically more advanced peoples of Europe. The effects would be beneficial, if the information could be deciphered and should prove generally useful; on the other hand, they could be harmful if humanity is not ready to use the information wisely. The technology is now available to make significant searches of this kind. The 300-m radio telescope of the National Astronomy and Ionosphere Center at Arecibo, Puerto Rico, is capable of receiving a message beamed at us from any of the hundreds of billions of stars in our Galaxy, provided the civilization sending the message were transmitting with a facility similar to that at Arecibo. Several searches for such extraterrestrial signals have already been undertaken, so far

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Frontiers of Astrophysics 91 with negative results, but the rate of improvement of communica- tions technology is so rapid that each search has been far more sensitive than its predecessors. We are entering an era when it is technically possible both to detect planets around nearby stars and to detect signals from intel- ligent life on planets immensely farther away, even if we cannot detect the more distant planets themselves. Both investigations would bear directly on important scientific questions. Our interest in the tiny fraction of the matter in the solar system that condensed into planets is heightened by the fact that life has developed on at least one of them. Have condensations to planets and the origin of life occurred elsewhere as well? And has that life evolved into com- municative intelligence, with which we human beings might be able to enter a conversation about life in the Universe? These questions reach far beyond astronomy, and even beyond science as we currently think of it. Yet astronomers, who are in a sense commissioned by the public to keep an eye on the Universe, feel bound to ask them and to point out how we might begin to try to answer them. It is for these reasons that the Committee recom- mends that in the 1980's an astronomical Search for Extraterrestrial Intelligence be initiated as a long-term effort. ASTRONOMY AND THE FORCES OF NATURE Energy Sources in the Universe In the 1970's, physicists have made substantial progress toward re- alizing an age-old dream-the understanding of all the forces in nature as different aspects of a single fundamental force. A theory that unifies electromagnetic and weak nuclear forces has been suc- cessfully developed along with a comprehensive theory of the strong nuclear force; new theories aimed at unifying both of these theories are now being proposed. Astronomical data have played a role in these developments and may play an even greater role in the future. Newton's law of gravitation, formulated in precise mathematical terms, set the stage for the investigation of the forces of nature that continues today. We now realize that chemical energy, such as that released in the burning of fossil fuels, is due to the action of electrical forces within atoms. Holding electrons in orbits around nuclei just as gravitation holds planets in their orbits around the Sun, these forces release energy whenever an electron drops into a lower orbit. Magnetic forces result from the motion of electrically charged par

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92 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's ticles. In the 1860's, Maxwell unified electrical and magnetic forces in a single theory, called electromagnetic theory, which also explains electromagnetic radiation as a wave that sustains itself through a constant interplay between electrical and magnetic energy. By the enr1 of the nineteenth century, both gravitational and electromagnetic forces were well understood at a certain level. O Early in the twentieth century a series of important experiments revealed that the orbits of electrons are qualitatively different from those of planets. The position of a planet can be predicted precisely from a knowledge of the gravitational force acting on it, but the best one can do with an electron is to predict its probability of being at various possible positions. The impossibility of doing any better, embodied in Heisenberg's Uncertainty Principle, is an essential fea- ture of what is now known as quantum theory. Today, the melding of electromagnetic theory and quantum theory, called quantum elec- trodynamics or QED, is unchallenged in its ability to describe elec- tromagnetic phenomena. A shining goal of contemporary physics is to bring the understanding of all the forces of nature up to the standard of QED. Sunlight is electromagnetic radiation, and the form in which the energy of sunlight is stored by plants is chemical energy; both forms of energy are embraced by QED. What about the energy stored in the Sun, which it emits as sunlight? Early suggestions included elec- tromagnetic radiation trapped within the Sun, chemical energy stored in its atoms and molecules, and the energy due to the gravitational attraction between all of its atoms. However, none of these forms of energy is adequate to keep the Sun shining for its known age of 4.5 billion years. The solution to this problem was reached in the early 1920's, when it was recognized that a new form of energy discovered in the laboratory, nuclear energy-which is released, for example, when the nuclear force between hydrogen nuclei (protons) draws them together to form helium nuclei-could keep the Sun shining for many billions of years. Nuclear interactions come into play only at very high temperatures; only then do nuclei have sufficient speeds to overcome their mutual electrical repulsion. Thus, nuclear forces play a role in astronomy only where matter is extremely hot, as in the interiors of stars or in the searing heat of the big-bang explosion. Laboratory studies of nuclear reactions show that there are actually two types of nuclear force, strong and weak; the latter is associated with an unusual particle called the neutrino.

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Frontiers of Astrophysics Two Puzzles: Solar Neutrinos and Hidden Mass 93 Neutrinos can penetrate the entire Sun, so weak is the force with which they interact with matter. Detectors placed beside nuclear reactors, which are copious sources of neutrinos, can record only a minute fraction of those emitted. Despite the great difficulty of de- tecting them, the role of neutrinos in astronomical research has be- come increasingly important. The current theory of stellar energy generation predicts that large numbers of neutrinos are produced in the fusion of hydrogen to helium in the deep interior of the Sun. Because this theory is critical to our understanding of stellar structure and evolution generally, it is important to test this prediction by measuring the flux of solar neutrinos at the Earth. The observed flux of neutrinos is less than one third of that predicted from the most carefully constructed models of the solar interior. Among various proposed explanations of this discrepancy is the possibility that neutrinos behave differently from what has been assumed until recently. In a completely different area of research, it has been proposed that the problem of hidden mass in galaxies might be resolved if the rest mass of neutrinos were not zero, as usually assumed. From calculations of the number of neutrinos produced in the big bang, one finds that neutrinos could supply the hidden mass in galaxy clusters if they possess a rest mass about 1/10,000 that of the electron. There are thus two astronomical problems that might be resolved if neutrinos prove to have properties not previously known. Theo- retical physicists have recently suggested a resolution of both of these problems. The recently developed unified theory of weak and elec- tromagnetic forces is based on a principle called gauge invariance and is therefore referred to as "the gauge theory of weak and elec- tromagnetic interactions." So far it has succeeded in explaining all the various phenomena involved with both electromagnetic and weak nuclear forces. The gauge theory of weak and electromagnetic interactions in its original form says nothing about the problems of solar neutrinos or hidden mass. However, pursuing the principle of gauge invariance behind it, physicists have constructed a theory of the strong nuclear force, called quantum chromodynamics, or QCD. This theory pos- tulates the existence of elementary particles that combine to form protons and neutrons, called quarks. The success of QCD in explain- ing the results of experiments in elementary-particle physics gives

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94 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's increasing confidence that it is the correct theory of the strong force that binds neutrons and protons into atomic nuclei. Spurred by the success of the gauge theory of weak and electro- magnetic interactions and of QCD, physicists are now trying to find an even more general gauge theory, called "Grand Unified Theory," that incorporates both. Some theories of this type predict that there should be the three types of electrons that are actually observed, as well as three corresponding types of neutrinos, called e, mu, and taut In some versions of the theory, e, mu, and tan neutrinos are regarded as three aspects of the same basic neutrino, which has a finite rest mass and which oscillates back and forth among its three aspects. Although the nuclear reactions in the Sun emit only e neu- trinos, according to some Grand Unified Theories neutrino oscilla- tions would be expected to occur long before the neutrinos reached the Earth, so that at the Earth one would observe a random mixture of e, mu, and tan neutrinos. Since the Homestake Mine apparatus is sensitive only to e neutrinos, a factor-of-3 discrepancy would thereby be explained. Oscillations can occur only if neutrinos have a finite rest mass. If the value of the rest mass were in the right range, it would have a dramatic bearing on our understanding of the hidden-mass problem and of the ultimate fate of the Universe. Theories involving several different types of neutrinos are con- strained by calculations of the properties of the early Universe. If there were more than about four types of neutrinos, their contri- bution to the gravitational acceleration in the early Universe would have been so great that there would not have been sufficient time for primordial neutrons to decay; there would then be more helium in the Universe than is actually observed. Thus, current astronomical observations eliminate some versions of Grand Unified Theories. A critical experiment endorsed earlier in this report will help to shed light on the true nature of neutrinos. The gallium solar neutrino experiment will be sensitive to neutrinos of much lower energy than those measured by the 37C1 detector in the Hamestake Mine. The flux of such lower-energy neutrinos can confidently be calculated from the observed luminosity of the Sun, independently of the details of solar models. If there is a discrepancy between the predicted and observed values of the solar neutrino flux in the gallium experiment, it could be an indication that neutrinos oscillate and have a finite neutrino rest mass. There may also be powerful sources of high-energy neutrinos among the many sites of violent activity observed to occur on both stellar

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Frontiers of Astrophysics 95 Chlorine solar-neutrino detector deep ire the Homestake Mine, Lead, South Dakota. (Photo courtesy of R. Davis, Jr., Brookhave~z National Laboratory) and galactic scales. Despite the difficulty of detecting such neutrinos and the weak fluxes to be expected because of the distances to the sources, the study of energetic-neutrino detectors with possible as- tronomical applications is appropriate for the coming decade. An interesting possibility for such study is the proposed observation of neutrino-induced reactions in seawater employing arrays of photo- multipliers to detect the associated Cerenkov radiation. Before the First Three Minutes Although astronomical data now available appear to be in agreement with the predictions of big-bang cosmology, the big-bang model cannot yet be considered conclusively proven, so that it is of the greatest importance to test its predictions however possible. In par- ticular, the model predicts that the cosmic microwave background originated as high-temperature radiation in the first few minutes of time. As the Universe expanded, according to this view, the radiation cooled to its present observed temperature, about 3 degrees above absolute zero. When the Universe was about 1/10,000 of a second old, its temperature was a trillion degrees, so hot that the radiation

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96 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's present created about 100 million proton-antiproton pairs for every proton now observed in the Universe. As time passed, these pairs annihilated, leaving behind only the very small fraction by which the number of protons exceeds the number of antiprotons. Had this excess not existed, the number of protons in the present Universe would have been 10 billion times smaller, and there would not have been sufficient matter in the Universe for the formation of galaxies, stars, and planets. What caused the excess of matter over antimatter implied by this big-bang scenario? Until recently, physicists had regarded the excess as a fact as inexplicable as the existence of the Universe itself. Re- cently, it has been suggested that Grand Unified Theories provide an explanation: very heavy particles present in the first 10-38 sec of the history of the Universe decayed, creating in the process slightly greater numbers of protons than antiprotons. This prediction can be tested in a straightforward way, for if protons can be created they must also decay. As the lifetime of the proton estimated from Grand Unified Theory is 100 billion billion times the age of the Universe, physicists are not concerned that the Universe will soon evaporate. On the other hand, the predicted proton lifetime is sufficiently short that one such decay will occur in a ton of material each year. Ex- periments are now in progress to detect such events. The Limits Of Gravitation Gravity keeps us on the Earth, binds the Earth to the Sun, and slows the expansion of the Universe. Newton described it as a force, while Einstein, in his General Theory of Relativity, interpreted gravitational forces in terms of the curvature of space-time. Einstein's theory, unlike Newton's, is believed to be valid for very strong gravitational fields and for bodies moving close to the speed of light; it is therefore crucial for an understanding of systems such as neutron stars, black holes, and the expanding Universe. The General Theory of Relativity predicts that when any non- spherical body collapses to form a compact object or a black hole, it emits a new form of energy called gravitational radiation. Although this radiation is predicted to be extremely difficult to detect, several research groups are now building detectors thousands of times more sensitive than those available during the 1970's. Parallel efforts to calculate the amount of gravitational radiation emitted by collapse indicate that, if the planned development of new instrument concepts succeeds, we might hope to detect an event within two decades-

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Frontiers of Astrophysics 97 even earlier if there should be a new supernova within the Galaxy. The recently confirmed, slow decrease in the orbital period of the binary pulsar has already been interpreted as the result of gravita- tional radiation from a close pair of neutron stars. While efforts to develop a quantum theory of gravitation have not yet succeeded, there is reason to believe that quantum effects should occur near black holes, where space-time curvature is high. The quantum theory of elementary particles predicts that even in vacuum, particle-antiparticle pairs are constantly being produced and anni- hilated in an interval of time too short to observe. If this effect should occur near a black hole, one member of the pair may fall into the black hole before the pair annihilates. Zero-mass particles, including photons, are created similarly; the black hole thus appears to the outside world as a source of radiation, ultimately evaporating as a result of the energy lost. Black holes of all sizes could have been created in the big bang; in particular, those having masses about of 10~5 g (the mass of a small mountain on Earth) would just be evap- orating now, giving rise at the ends of their lives to bursts of gamma radiation. Such radiation from evaporating black holes has been searched for, and, although the Gamma Ray Observatory will con- tinue the quest, so far none has been found. It thus appears that primordial black holes with masses less than that of a mountain cannot make up a significant fraction of the mass of the Universe. The theory of black-hole evaporation depends on the quantum nature of strong nuclear forces but not on the quantum nature of gravitation. Although no convincing theory of gravitation that in- corporates the quantum principle has yet been produced, it is con- jectured that the quantum effects must become important whenever the radius of curvature of space-time becomes less than the so-called Planck length, 10-33 cm. Such conditions are thought to have occurred in the Universe at times before 10~3 sec and at temperatures above 1032 deg. Because the energies and temperatures characteristic of Grand Unified Theories are remarkably close to these values, some physicists believe that a theory should be possible that incorporates all four forces in nature into one "Super-Grand" force at energies only slightly higher than those relevant to Grand Unification. A prime hope for such a theory is that it will yield, almost as a by- product, the correct theory of quantum gravitation. Attempts in this direction have so far met with little if any success, but the devel- opment of such a theory could be considered to be the ultimate challenge to physics at present. The notion of force, as a law governing matter once created, fails

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98 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's to take account of the process of creation itself. Is it possible, as astrophysics pushes the frontiers of time back to the moment of cosmic creation, that the existence of the Universe will be recognized as a consequence of the nature of the fundamental force? Is it possible that the potential existence of the world somehow calls it into exis- tence? Such questions, once believed outside the range of science, are now arising in scientific thought.

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The primary mirror for the Space Telescope being inspected after figuring. Photo courtesy of the National Aeronautics and Space Administration)