Cosmology and Extragalactic Astronomy (Studies of Objects Beyond the Mi 1ky Way) Distances to extragalactic objects are very great. For this reason, most galaxies, although as luminous as several billion stars, are as difficult to discern and study as the faintest stars visible in our own galaxy. 1) The Size of the Universe. Measurements of the size of the universe depend on our ability not only to observe the most distant (and faint) objects but also to develop technigues for distance measurement. The identification of distant objects is extremely marginal because of their faintness and is complicated by the presence of a bright sky background "against" which they must be viewed from the ground. Even small increases in sky brightness can render these objects undetectable. 2) Curvature of the Universe. Ever since its beginning 15 billion years ago, our expanding universe has probably been decelerating as the attractive gravitational forces between all massive objects operate to counteract the initial expansion. This deceleration, however, is not large at the present time. At an earlier epoch, objects were closer together and the effects of gravity were stronger. The finite speed of light affords us an opportunity to study the ancient universe—but only at distances of M5 billion light-years. A guestion of nearly universal interest is the ultimate fate of our universe: will it ever fall back on itself (closed), or will it expand forever (open)? The answer will come directly from measuring the deceleration of the universe. 3) Formation Era of Galaxies. All galaxies are believed to have formed as soon as the early universe cooled to a few thousand degrees (some 100,000 years after the big bang) but before the expansion of the universe spread the available matter over large distances. The formation of galaxies is extremely complex, and our comprehension of existing galaxies is incomplete without a good idea of the initial conditions of formation. 4) Open or Closed Universe? In addition to the difficult observational strategy of measuring the deceleration of the universe, the guestion of an open or closed universe can be settled by measuring all of the mass in the universe. At present, the visible forms of mass accessible to
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