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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 85
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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.
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