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The Big Bang theory was bolstered in 1964 with the discovery of the cosmic background radiation (CBR), the remnants of the radiation released in the first second of the life of the universe. But the theory contained no solution to the "horizon" and "flatness" problems until 1979, when Alan Guth came to the rescue by adding inflation to the Big Bang. At only 10^-35 seconds following the Big Bang, cosmologists theorize that he universe underwent a critical phase transition that caused space to expand exponentially (i.e. inflate), flattening out any pre-existing curvature of space and thrusting once-intimately connected matter billions of light years apart.
Minute fluctuations in the CBR--footprints of the small ripples in the density of matter that led to the formation of today's galaxies and clusters--were only discovered in 1992, and new ground and space-based telescopes and X-ray satellites continue to reveal new information about the age, composition and structure of the universe.
With such a large volume of the universe mapped, cosmologists will be able to make firm quantitative measurements about the nature of large scale structure which any successful model must reproduce. The Sloan survey will also increase the sample of known quasars a hundred-fold to over one hundred thousand. Since quasars are the oldest and nost distant astronomical objects known, and thought to represent an early phase of galaxy evolution, the increased sample will allow astronomers to pin down the epoch of galaxy formation with more precision.
The Hubble Space Telescope, with its superb imaging capabilities, will continue to collect images of distant galaxies and clusters of galaxies. Because of the finite speed of light, the more distant the galaxy, the further into the past we are peering. Hubble can see what galaxies and clusters looked like many billions of years ago, providing astronomers with the essential data to say how galaxies evolve over time.
A particularly important key program of the HST is the measurement of the Hubble constant, which is a key parameter in cosmological theories and presently unknown to within a factor of two. By making careful distance measurements to a "ladder" of increasingly distant galaxies, the HST is expected to measure the Hubble constant to a precision of about 10 percent.
AXAF will be able to survey a large volume of the universe comparable to the Sloan Digital Sky Survey, thus providing cosmologists with an independent and complimentary way to map the large scale structure of the universe. Models of structure formation will therefore be double constrained to predict where the galaxies form, but also the thermal properties of the gas out of which they form--a much harder challenge!
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