### CHAPTER I

THE OBSERVABLE REGION AS A SAMPLE OF THE UNIVERSE

*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.