Cosmologists work to better understand the contents, structure, and evolution of the universe over vast stretches of space and time and over enormous ranges of density, temperature, and energy. Theorists, observers, and experimenters use a diverse assortment of techniques and instruments to answer questions of the most fundamental kind. Progress over the past three decades, but especially since the maturing of space science, has been astonishing. Recent observational results from the COBE satellite have revolutionized how cosmologists think about structure in the universe, and current efforts to map the fluctuations in the CMBR on smaller angular scales promise to show us details of the thermal history of the universe and to measure its fundamental parameters. Observations of large-scale structure in the universe as measured by the galaxy distribution have made dramatic progress in recent years, challenging all available theoretical models. Our knowledge of large-scale flows is still incomplete, but, when the work is finished, cosmologists will be much closer to understanding the role of dark matter in the universe. The new generation of giant telescopes such as the 10-m Keck are making remarkable observations, and the Keck, along with its newly constructed twin and the two 8-m Gemini telescopes, will enable study of the distant universe in ways never before conceivable. The HST has already made seminal contributions, including a measurement of the Hubble constant that gives the universe a surprisingly young age. We expect dramatic advances in our understanding of galaxy and large-scale structure evolution, as well as new tests of global curvature, to emerge from these studies in the coming decade. We still have no clear idea of the nature of the ubiquitous dark matter, but it is almost certainly a remnant of the early universe. Several experiments are under way to detect, or eliminate, candidates for this dark matter, and these will make substantial progress in the coming decade.
The problems of cosmology are particularly well posed, but many
of the solutions have remained elusive for decades. At last, with
the accelerating influx of new data from numerous advanced experimental
and theoretical techniques, fundamental questions about the nature
of our physical world are beginning to be answered. Cosmologists
eagerly await the exciting new insights that will surely come
in the next decade. If another briefing on cosmology is written
10 years from now, it will undoubtedly bear little resemblance
to this one.