Deuterium and Helium Absorption at High Redshift: Mapping the Abundance, Density and Ionization of Primordial Gas

Spectra of quasars at high redshift with high resolution and high signal-to-noise allow in favorable circumstances detection of absorption by deuterium and ultimately measurement of its primordial abundance. Ultraviolet spectra of high redshift quasars allow measurement of absorption by the most abundant cosmic absorber, singly ionized helium, thereby mapping gas even in the most rarefied cosmic voids. These new techniques already provide significant constraints on cosmological models but will soon become much more precise.