The University of Chicago will lead a consortium of six institutions to design and utilize an eight-meter off-axis telescope at Amundsen-Scott South Pole Station in order to carry out a unique and powerful survey of galaxy clusters to unprecedented red shifts. This survey will allow the study of integrated cluster abundance and its red shift evolution, and will give us precise cosmological constraints, which are completely independent of those from supernovae distance and Cosmic Microwave Background anisotropy measurements. The sample size and redshift depth will be ideally suited for using cluster abundance evolution to determine the equation of state of the dark energy component of the universe. One of the most important discoveries in cosmology is that it appears that much, if not most, of the mass in the Universe is made not of stars and glowing gas that is familiar to us from astronomical images of the sky, but of what has been termed dark matter. This dark matter emits little or no light or other electromagnetic radiation, and so far makes its presence known only through the gravitational force it exerts upon the luminous matter. There is some indication that the dark matter may in fact not even be baryonic. Just what fraction of the mass is in the form of non-interacting non-baryonic particles is of great interest to cosmologists and physicists. Measuring the mass in baryons along with the total mass in a region of the Universe that could be considered a "fair sample" would provide a crucial direct determination of the dark matter content. In recent years, just such a test-bed has been found in the guise of massive clusters of galaxies. These clusters contain large amounts of gas (baryons) in the form of a highly ionized atmosphere of gas at temperatures of millions of degrees, which emit X-rays. Nearly all of the baryons in the clusters are believed to be in the hot phase, and so it is likely that we are truly measuring the baryonic mass in the cluster. In addition to emitting X-rays, the hot cluster gas also scatters the cosmic microwave background (CMB) radiation. This scattering, called the Sunyaev-Zeldovich effect (SZE), is measurable using radio telescopes. The CMB has a blackbody temperature of 2.7 K which and a radiation spectrum that peaks at a wavelength of around lmm. Observations at centimeter wavelengths see the SZE as a distortion of the spectrum, i.e. as a reduction in the brightness toward the cluster as the photons are scattered toward a higher energy level. The observed decrement is proportional to the integrated electron density along the line-of-sight through the cluster. The Sunyaev-Zeldovich effect is important to the study of cosmology and the CMB for two main reasons. 1) The observed "hotspots" created by the kinetic effect will distort the power spectrum of CMB anisotropies. These need to be separated from the primary anisotropies in order to probe properties of inflation. 2) The thermal SZ effect can be measured and combined with x-ray observations in order to determine values of cosmological parameters, in particular the Hubble constant.
