better ways of finding galaxies. Three basic avenues are being explored.Deeper surveys of the whole sky in visible light are being conductedusing highly sensitive detectors (called charge-coupled devices;CCDs) that can detect intrinsically faint galaxies, galaxies of lowsurface brightness, and distant galaxies. Near-infrared surveys ofthe sky at 2-micron wavelength will make it possible for the firsttime to observe the dip down near the plane of our galaxy, whosedust clouds obscure 35 percent of the sky in visible light. Finally,x-ray satellite surveys provide yet another means of mapping clustersof galaxies.

A fundamental question is whether one large section of the universelooks like another. In other words, how large do sections have tobe before they begin to appear statistically uniform? Unfortunately,a single ground-based observatory sees only a portion of the sky.To achieve the high degree of uniformity needed over the whole skyrequires careful surveys. There are three requirements: First, individualsurveys must cover as much of the celestial sphere as possible. Second,surveys must be closely coordinated and well standardized so thatthey can be knitted together. Finally, homogeneous all-sky surveysneed to be conducted by satellites above Earth, such as the InfraredAstronomy Satellite (IRAS), a joint mission of the United States,the United Kingdom, and The Netherlands that was flown in 1983.

A major goal for the next generation of surveys is to increase theirrange out to 3 billion light-years, roughly 20 percent of the radiusof the visible universe. On such scales cosmologists would be probingstructures that are the same size as the smallest structures in theCOBE microwave map. The clustering behavior of galaxies over an extremelywide range of scales could be measured and compared directly to theCMBR anisotropy with no extrapolation. This would tell us how thedensity fluctuations have evolved from the epoch of CMBR emission(the epoch of photon decoupling) to the present. This informationwould yield essential clues to the amount and nature of dark matterin the universe.

Theory of large-scale structure

Statistical description and theoretical modeling of the observedgalaxy distribution have been extremely productive over the pastdecade. Much of this modeling has been done with large computer simulationson the largest available supercomputers. This is a problem in the“grand challenge” class, with the goal of understanding in detail the formation ofstructure on both small and large scales. The models typically followthe evolution of a large patch of the universe. Calculations startwith random initial fluctuations as statistically predicted for differentcosmological parameters and different types of dark matter. The equationsgoverning the gravitational coupling, as well as other physical processes,are then solved numerically by the computer. Starting from smallamplitudes, the fluctuations become increasingly larger, as expectedfrom the gravitational instability picture. The computational resultscan then be compared to the observed properties of large-scale structurein the universe. With careful analysis, such comparisons can setconstraints on the amount and nature of dark matter. Some proposeddark-matter candidates have already been ruled out in this way. Figure 3 is a recent example of a numerical simulation, processed with similarselection criteria as for observational redshift surveys. The similarityin the voids and filaments shown in Figure 2 and Figure 3 is striking.

Clusters of galaxies, with size on the order of 3 million light-yearsand mass of 1015 Suns, are central to our understanding of structure. Astronomershave recently discovered that galactic cores are dense enough toact as gravitational lenses (discussed in section IV in “Gravitational Lenses”), that most of the baryonic (ordinary) matter within them is inthe form of hot gas, not galaxies, that dark matter constitutes approximately80 percent of their total mass, and that they show a considerableamount of substructure when examined at high spatial resolution.The abundance of clusters

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