Institute for Advanced Study Informal Astrophysics Seminar

After reionization, a largely uniform ~1 Rydberg background pervaded the Universe, keeping the intergalactic hydrogen extremely ionized. The characteristics of this background depend on the properties of the sources (quasars and galaxies) and the absorbers (Lyman-limit systems). Modeling the sources is difficult, but I will argue that the absorbers are likely captured in cosmological simulations, at least at high redshifts. Quick evolution (in a time < 0.1 H(z)^-1) in the ionizing background is observed at z=6, which has (controversially) been interpreted as indicating the end of reionization. I will explain why this evolution must owe to the absorbers and discuss how such quick evolution could arise. The only direct probe of the metagalactic ionizing background comes from comparing the hydrogen Lyman-alpha forest to the HeII Lyman-alpha forest in the same sightline. Previous attempts to measure background fluctuations from this comparison found order-of-magnitude fluctuations on 1-10 Mpc scales, in conflict with theoretical expectations. I will show that these previous analyses were flawed, and that the data is in fact consistent with the expectation of an almost uniform background. This comparison also allows us to infer the hardness of the EUV background (constraining the quasar versus stellar contribution) and allows us to constrain quasar lifetimes (via the transverse proximity effect). Unfortunately, all does not end well for the cosmic EUV.