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The Observational Approach to Cosmology

THIS series of lectures concerns the observational approach to cosmology, to the study of the physical universe. From our home on the earth we look out into the dim distance, back into the dim past, and we strive to imagine the sort of world into which we are born. Observations now range through an immense volume of space: perhaps the nature of the universe may be inferred from the appearance of the sample we explore. Theory presents us with an infinite array of possible universes, logically consistent systems: perhaps our information is now sufficient to identify among them the particular type, or family of types, which includes the actual universe we inhabit. At any rate, astronomy has developed to the point where, for the first time, such attempts are justifiable. Empirical investigations have definitely entered the field of cosmology. Already, certain conclusions can be drawn from the explorations. The long process of elimination and successive approximation has begun.

Cosmology lay for ages in the realm of sheer speculation, Rational arguments were introduced slowly until the critical period just two decades ago. Then theory invaded the field in force, and rapidly exploited the possibilities offered by general relativity. Later still, a dozen years ago, observations crossed the frontiers of the stellar, system, and swept out into the universe at large. The observable region of space, our sample of the universe, is now defined, and a preliminary reconnaissance has been completed.

Very recently, the reconnaissance has been followed by accurate surveys that extend out to the practical limits of the largest telescope in operation. Further significant extensions are not expected until more powerful methods are developed, or still greater telescopes are constructed. Therefore, the time is appropriate for a provisional interpretation of the data already available. The conclusions are tentative but they are none the less impressive, for once again, as in the days of Copernicus, we seem to face a choice between a finite, small-scale universe and a universe indefinitely large plus a new principle of nature.

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