My research focuses on using quasar absorption lines to study the intergalactic medium (IGM) when the Universe was less than a few billion years old. I am particularly interested in determining when and how the IGM became re-ionized by the first stars and active galactic nuclei. The IGM contains most of the dark matter and baryons in the Universe, and provides the raw materials for galaxy formation. My group uses quasar absorption lines to measure the basic properties of the baryons, such as their temperature, chemical composition, and ionization state as a function of cosmic time. These properties reflect the impact of galaxies and quasars on the IGM, and thus provide insight into the evolution of luminous objects from very early times up to the present day. I am also interested in using the IGM to determine the nature of dark matter.