Physics Colloquium

Clusters of galaxies are the largest gravitationally bound systems in the universe. They each contain of thousands of galaxies and are approximately 6 million light years across. A study of clusters near and far helps us
determine how they formed and evolved and how the cluster environment affects galaxy evolution. The spatial extent and distribution of clusters and galaxies can be directly related to cold dark matter (CDM) models of structure formation in the Universe. I will give a description of my contributions to the study of clusters and galaxies both in the optical and X-rays (which reveals the presence of a 100 million K intra-cluster gas with a maximum density of only 10 particles per cubic meter). Because of their large size and masses, clusters have only formed about 3.3 Gyr. or more after the Big Bang (z <= about 2; age of the Universe about 13.7 Gyrs). A measure of the formation and evolution of clusters tells us about world models of Cosmology, and surveys of clusters will (and have already) shed light on the nature of Dark Energy as well as CDM. An X-ray survey of the type I've designed to search the deep sky for distant X-ray emitting clusters of galaxies will do much more than shed light on the nature of Dark Energy (as if that is not enough!) as I will describe.