Baryons: What, When and Where?

Jason X. Prochaska
UC Santa Cruz


We review the current state of empirical knowledge of the total budget of baryonic matter in the Universe as observed since the epoch of reionization. Our summary examines on three milestone redshifts since the reionization of H in the IGM, z = 3, 1 and 0 with emphasis on the endpoints. We review the observational techniques used to discover and characterize the phases of baryons. In this talk, I will emphasize the open questions regarding this research, of which there are embarrassingly many. See astro-ph/0805.4635 for a write-up of this material.