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III
Cosmology
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Cosmology, the study of the universe as a whole, provides the
canvas on which the detailed nature of the physical world is painted by
the other fields of physics. This canvas is the space-time framework
upon which all our physical theories are constructed. The question of
boundary conditions in both space and time (e.g., the issue of origin) is
ultimately a cosmological one.
A second feature of cosmology that endows it with fundamental
importance as a field of physics is the fact that the properties of matter
are studied under the most extreme conditions, from the unimaginable
densities and temperatures of the early universe to the near-perfect
vacuum of intergalactic space. By comparison, experimenters in
terrestrial laboratories can only test our physical theories over a
narrow region of their supposed range of validity.
But this potential for expanding our understanding of physics comes
at a price the uncertainties introduced by the remoteness of our
cosmological laboratories. Because only passive experiments (i.e.,
observations) are possible, theory must play a particularly critical role
in the planning of experiments as well as the interpretation of data and
the distillation of knowledge. An additional difficulty arises because of
the uniqueness of the universe, which prevents us from determining
whether our universe has a particular property by chance or by
necessity. Related to this problem is our inability to isolate the system
under study; indeed the observer is inseparable from, and a product of,
the system and processes being investigated.
During the past two decades, cosmology has undergone a revolution
because of our increasing ability to observe the universe as it is now
and as it was in the remote past. We have extended the horizon of our
knowledge back in time, through the era of the quasars to that at which
the microwave background photons were released a time when the
density of the universe was 109 times higher than it is now. And relic
nuclei allow us to see back even farther, to a time when the universe
was only a few seconds old. Currently, theorists are attempting to
study still earlier times, by applying new ideas from particle physics
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86 COSMOf OGY
to the universe at age 10-35 S. A major objective for cosmology is to
extend and broaden our physical understanding of the early universe.
Equally exciting is our rapidly growing knowledge of the local
universe, out to say 108 light years. Major advances in astronomical
instrumentation and data-processing techniques have led to more
detailed studies of the physics of galaxies and clusters of galaxies-
data vital to understanding the origin and evolution of these basic
elements of our universe. Important puzzles, such as the nature of a
probable dark-matter component and the physics of galactic nuclei, are
stimulating a burst of theoretical and observational activity. We can
expect this area, so rich in basic phenomena, to continue to grow and
flourish, aided greatly by new layers of knowledge from major new
astronomical instruments.
Representative terms from entire chapter:
iii cosmology
