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- ~ Opportunities OBSERVATIONS FROM SPACE Cosmologists are eagerly looking forward to the observations from Earth orbit planned for the coming decade. A broad range of the electromagnetic spectrum will be covered by the proposed missions- the Gamma Ray Observatory (GRO), the Advanced X-Ray Astrophys- ics Facility (AXAF), the Hubble Space Telescope (FIST), the Space Infrared Telescope Facility (SIRTF), the Cosmic Background Explorer (COBE), the Large Deployable Reflector (LDR), and an antenna in space to extend the Very-Long-Baseline Array. Previous astronomical satellites have brilliantly demonstrated that deeper exploration of space, in many spectral regions, holds great potential for making new discoveries, solving old problems, and raising important new ques- tions. The recent results from the Infrared Astronomy Satellite (IRAS) are an example of the scientific power of well-planned observations from space. A list of planned studies and possible discoveries is long and exciting; we can mention here only a few examples directly relevant to current cosmological problems. The many discoveries of IRAS highlight the untapped richness of the infrared sky, so long obscured by atmospheric absorption and emis- sion. For cosmology the infrared region holds special promise because, as illustrated in Figure 13.1, this is where one may at last see the birth of galaxies. The burst of starlight expected to accompany galaxy 101

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102 COSMOLOG Y d o3- ~n a, ~ ~o2- o 0.4 - 0.04 Photon Energy (eV} 4040 ~05 ~ ~o-!5 Microwave Background I Galaxy) t~ Formation / . do-15 do- lo ~ o-5 -40 -13 405 Wavelength Now (cm Gamma rays| Xraysil~ FIR) Radio Visible Microwovo' -0.0001 3 ID -0.001 _. -0.04 -0.' o ID - FIGURE 13.1 This figure shows the extent to which we can explore the universe throughout the spectrum of electromagnetic radiation (in terms of either the redshift of sources or, equivalently, how far back in time we see them). The darkly shaded areas show the extent of our present knowledge. The lightly shaded area shows the region that we can never view directly because the photons are either scattered by electrons or collide with other photons, producing electron-positron pairs. The dashed boundary surrounds the region where we may see galaxies in their early phase of development. formation may have been redshifted by cosmological expansion into the infrared region. The detection and study of primeval galaxies will give us a major foothold in the little understood epoch between z = 103 and z = 1, and perhaps will also be the evolution of large-scale struc- ture will be clarified. The National Aeronautics and Space Administra- tion (NASA) is planning to capitalize on the success of lRAS by orbiting a cryogenic telescope with an aperture of about 1 meter SIRTF. With pointing and imaging capability, SIRTF will be more sensitive than IRAS by factors of 109 to 103. Additionally, it will have more wavelength coverage and spectroscopic capability. A major part of SIRTF's scientific program will be a deep search for primeval galaxies.

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OPPORTUNITIES 103 Also of great cosmological significance is the planned role of the HST in the measurement of the extragalactic distance scale, the expansion rate of the present universe (Ho), and the deceleration parameter (q01. High spatial resolution (better than 0.1 arcsec) and broad spectral coverage will allow more detailed observations of nearby and distant galaxies, leading to better understanding of the physical properties of galaxies including evolution. Thus, the HST is expected to play an important part in improving the classical cosmo- logical observations over the next decade. The MST's unique angular resolution and ability to measure redshifts at z > 1 suggest other observations of cosmological interest. A simple, but important, observation will be to see whether the shape of galaxies is evolving. Do the thin disks so prominent in most nearby giant galaxies persist back to z1? A1SO7 the HST will be a great help in charting the way for deep redshift surveys. Because of the large observing time required, the bulk of this work will be done from the ground (see the following section), but calibrations and minisurveys from space will be important benchmarks for these surveys. The HST will be our best means for studying supernova events in deep space. Currently, supernovae are being studied as possible cosmic distance indicators and as a possible alternative to galaxies as probes for measuring q0. The advantage of using supernovae is that there is a good chance for theoretical understanding of the spectral and time depen- dence of their flux without needing to assume that they are standards of luminosity. One more example: MST's spectrometers operating at ultraviolet wavelengths (inaccessible from the ground) will be able to probe the thermal history of the intergalactic medium, which has been strongly influenced by the formation of structure in the universe. Thus, constraints can be set on the epoch of galaxy formation and on the nature of dark matter. The COBE was designed specifically as a cosmological satellite, to make detailed measurements of the 3-K radiation and to look for an infrared background flux. High spectral accuracy will permit a search for distortions in the sensitive region over and around the blackbody peak (A ~ 2 mm), and large-scale (>7) anisotropy will be accurately measured at A = 9, 6, and 3 mm. Because of limitations on the size of its antennas, COBE will not look for anisotropy at small angular scales. AXAF was highly recommended by the report of the Astronomy Survey Committee as an instrument sure to make important contribu- tions to broad areas of astronomy, including cosmology. Since the hot plasmas at the cores of some clusters of galaxies are strong x-ray emitters, AXAF will be able to make detailed measurements of these

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104 COSMOLOGY sources at redshifts of z = 1 to 2. Hundreds of sources per square degree are expected, with the number depending on the cosmological parameter q0 and possible evolution. AXAF's ability to carry out detailed studies of these distant sources promises important new data from a little-known cosmological epoch. In addition, AXAF will provide much better data on nearby clusters of galaxies than was possible with the Einstein satellite. It will measure accurate tempera- ture gradients as well as density gradients in the hot plasma in rich clusters. These will yield model-independent measurements of the gravitational potentials of the clusters and thereby trace the possible dark matter in the outer regions of the clusters. The LDR is currently envisioned as a 30-m telescope, with diffrac- tion limited at A ~ 30 ~m. Two important cosmological observations are being anticipated: a sensitive measurement of small-scale anisotropy in the 3-K radiation and a search for primeval galaxies at z ~ 3 using the reflector as a light bucket at A ~ 1-4 ~m. Currently, LDR offers our best hope of pushing small-scale anisotropy measurements to levels of ATIT ~ 10-6, in pursuit of the primordial density fluctuation spectrum. Above the atmosphere LDR offers the low-noise, broadband capability needed for sensitive measurements near the peak of the spectrum at A ~ 2 mm. CONTINUED GROUND-BASED OBSERVATIONS Most of what we know about the universe has been learned from interpreting observations made with ground-based instruments. Many of the data come from large telescopes at major observatories, but some important contributions have been made with small, special- purpose instruments. Always, the role of the theorist with a good understanding of the observations is an essential one, perhaps more so than in most areas of physics. The prospects for exciting ground-based work over the next decade are excellent; there is no shortage of important problems. Because of the crowded schedule of broad-based science for the HST, only critical cosmological observations of relatively short dura- tion can be made. Ground-based observatories will continue to be our main sources of data about the universe. The rapid pace of develop- ments in extragalactic astronomy indicates that we are only just entering the age of discovery. The Astronomy Survey Committee discusses a broad range of opportunities for ground-based telescopes; here we emphasize only a few of particular current interest to cosmic physics.

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OPPORTUN/T/ES 1 05 Two major themes of current work are to measure q`, and to understand the origin and evolution of large-scale structure in the universe. Recent redshift surveys of large numbers of nearby galaxies have greatly increased our understanding of kinematics and galaxy clustering in the local universe. It is important to extend this under- standing to redshifts of zI if possible. The joint distribution of galaxy redshifts and magnitudes will measure large-scale clustering of galaxies and afford a much clearer understanding of the evolution of structure in the universe, which depends on q`' and on the nature of dark matter. Such a survey is technically feasible with current and planned tele- scopes. Curiously, the interpretation of data from a deep survey program would be limited in part by our lack of systematic, baseline knowledge of nearby galaxies. Such fundamental studies are well within the reach of present technology, but they have not been done. There is a perception among observers that such long-term programs, however important, cannot be undertaken because of uncertainties in funding and the allocation of telescope time. Currently, our deepest look into the big bang is provided by measurements of the abundance of light nuclei. Astronomical obser- vations of these abundances need to be extended to more sources and to even better accuracy. More theoretical work must be done to find and understand all possible astrophysical production and destruction mechanisms. As a cornerstone of our current hot big-bang cosmolog- ical model the nucleosynthesis argument must be as sound as possible. Similarly, there is still much to be learned from further studies of the 3-K radiation. The spectrum near the blackbody peak needs to be measured still more accurately, large-scale anisotropy measurements can be improved (especially at millimeter wavelengths), and better polarization searches can be made. Fine-scale anisotropy measure- ments, of great importance to the understanding of primordial fluctu- ations, should be pursued from aircraft or balloons if necessary. Little is known about anisotropy on intermediate scales (~1~; all angular scales are potentially interesting and should be probed to the highest possible precision. The critical question of the nature of the dark matter that appears to dominate the present universe must be addressed by predicting and searching for signatures of the various candidates. Some possible signatures that have been suggested include x rays from accreting black holes, infrared radiation from very-low-mass stars' ultraviolet photons from the decay of massive neutrinos, direct detection of magnetic monopoles, and the photons from axion decay induced by magnetic fields. Theoretical studies will continue to impose constraints' such as

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106 COSMOLOG Y the limits on cosmological monopole flux imposed by the existence of a galactic magnetic field (the Parker limit). Particle accelerators are not generally regarded as astrophysical observatories, but the discovery of a stable weakly interacting, massive particle could have a profound effect on cosmology. PARTICLE PHYSICS AND COSMOLOGY Conventional cosmology, if correct, places some important con- straints on particle physics; examples are the allowed number of neutrino types and the allowed ranges of masses and half-lives of neutrinos. The new particle physics has generated some exceedingly stimulating ideas in cosmology and has great potential for influencing future thinking and directions; for example, the discovery of Higgs particles would be of major importance in lending credence to the inflation scenario. Within the decade the width of the neutral interme- diate-vector boson Z and the partial width due to neutrino pairs may be measured. Since the number of neutrino types affects nucleosyn- thesis, the measurement directly tests the big-bang model. Many particle-physics experiments of interest to cosmologists do not use accelerators. One class of such experiments tests the predictions of theories, such as Grand Unification, which have implications in cos- mology. Examples are the searches for proton decay and for an electric dipole moment of the neutron. Other experiments, such as those attempting direct detection of dark-matter candidates, offer the hope of a decisive resolution of important cosmological problems. THEORY Given the limited and indirect observational basis of cosmology, it is essential that theorists range broadly in their search for interpretations and for crucial observational and experimental tests. Fortunately, the field is sufficiently exciting to attract excellent theorists in graduate school and from other areas of physics and astronomy. It is impossible to anticipate where theory might go in the near future, but we briefly mention a few of the current promising ideas. On the particle-physics side, the successful quantization of gravity seems essential for penetrating the mysterious Planck era. Perhaps only then will physics be able to address the question of initial conditions. Currently quantum gravity enjoys great popularity among gravitation, particle, and cosmological theorists. There has recently been much study of universes with more than four dimensions,

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OPPOR TUNI TIES 107 motivated in part by supergravity theories, which attempt to unify gravity with the other three forces. (See the discussion under Quantum Gravity in Chapter 8.) An intriguing possibility is that these theories might lead to an understanding of the origin of space-time itself. Another difficult task is to develop the theory and consequences of symmetry-breaking transitions in the early universe. For example, the time-dependent transition that may cause inflation needs to be better understood. On the astrophysical side, one attempts to understand the structure of the universe as it is now and to infer from that what it must have been like in the past. Essential to such a program are detailed studies of the complicated processes occurring during the nonlinear develop- ment of a multicomponent system of radiation and one or more dark-matter candidates. The processes must be understood from the present back to a time before the radiation decoupled from the matter. Such studies may invoke a wide variety of possible scenarios, but they must mesh with a rich texture of observations. In many cases extensive numerical computation is essential, and here a barrier to progress is the somewhat irregular and informal coupling of theorists to the frontiers of progress in computing technology. We also expect analytic methods to continue to provide new ideas and important guidance for observers. Finally, we must bear in mind that the search for viable alternative cosmological models should continue. As an example, cold big-bang models in which the microwave background was produced by stars and thermalized by dust at an early epoch cannot be dismissed; they can give a present ratio of photons to baryons in agreement with the observed value. A major difficulty with such models is that no natural way to produce the observed deuterium has yet been found. Another class of nonstandard models are those that were initially chaotic rather than smooth. Is it possible that some process like particle production smoothed them out? What fraction of such models could evolve to resemble the present universe? What is the effect of an inflationary epoch on such models?