| Clusters of galaxies have long been a valuable tool for cosmology, beginning in the 1930's with the discovery of dark matter in the nearby Coma cluster; then in the early 1990's providing evidence for a low matter density universe; and earlier this decade confirming the accelerating expansion of space through direct constraints on the distance-redshift relation. While these breakthroughs rested on observations of the properties of individual systems or small cluster samples, this thesis is focused on what can be learned from the cluster population as a whole. As the largest gravitationally bound systems in the Universe, the abundance, astrophysical properties and evolution of clusters provide a unique window into the formation and growth of cosmic structure.;Although the results described here demonstrate the power of cluster observations as probes of the growth of cosmic structure, they certainly do not represent the end of the story. The current data set can be applied to a variety of cosmological questions of interest, including neutrino masses, non-gaussianity of the initial density perturbation field, and modifications of General Relativity. Upcoming surveys using X-ray, optical and Sunyaev-Zel'dovich observations to search for clusters promise to greatly expand the available data, ensuring that cluster studies will remain a powerful approach to cosmology.;This thesis documents observations and analysis that have been applied to this problem, using the X-ray emission of hot, intracluster gas to identify and study massive clusters. Results include the first constraints on dark energy from this technique. The analysis produces tight cosmological constraints, and provides independent confirmation of the cosmological constant model. In addition, the data are used to probe the astrophysics of clusters, such as the presence and nature of excess heating of the intracluster medium, and the prevalence of dense, cooling cores in clusters. |
