My research interests lie in theoretical astrophysics and UV/X-ray space astronomy. I work on theories of galaxy formation, reionization, first stars, primordial metal production, and numerical hydrodynamical simulations of high-redshift galaxies and the multi-phase intergalactic medium (IGM). With the Cosmic Origins Spectrograph on Hubble, we are conducting UV spectroscopic studies of missing baryons in the intergalactic medium, quasar ionizing spectra, mass estimates of super-massive black holes in quasars, and low-metallicity gas in the Galactic halo. We have found over 50% of the "missing baryons" in intergalactic Lyman-alpha and O VI absorbers. Recent research projects also include theoretical studies of escaping ionizing photons from galaxies, Galactic supernova shock- destruction of dust, and quasar outflows. I will be active in the Sloan-IV Digital Sky Survey and in planning for future NASA missions, including a large UV-Optical space telescope.