Mitchell Institute for Fundamental Physics & Astronomy
College Station, Texas 77843
The Bullet Cluster has been the subject of intense research in the last few years. This system is remarkably well-suited to addressing outstanding issues in both cosmology and fundamental physics. I will present measurements of the composition of this system, show the evidence for existence of dark matter, and describe limits that we placed on the intrinsic properties of dark matter particles. I will also present results from a much larger sample of Bullet-like clusters, MACSJ0025-1222, A520, and DLSCL J0916.2+2951 among others. In the second part of my talk I will describe our recent endeavors with Hubble Space Telescope that enabled us to detect galaxies at the time when the Universe was only a few precent of its current age. They are likely beacons of the epoch of reionization, which marked the end of the so-called ``Dark Ages’’ and signified the transformation of the universe from opaque to transparent. Clusters of galaxies, when used as cosmic telescopes, can greatly simplify the task of studying and finding these galaxies. With a massive cluster one can gain several magnitudes of magnification over a typical observing field, enabling imaging and spectroscopic studies of intrinsically lower-luminosity galaxies than would otherwise be observable, even with the largest telescopes. I will present the newest results that seem to suggest that the galaxies have started forming stars much earlier than previously thought.