Early Reionization in Cosmology

Since this is the first contribution to this meeting which is mainly involved in cosmology, I would like to put it into its cosmological perspective: In the standard model of cosmology we assume that the universe is homogeneous and isotropic on large enough scales. This assumption, originally probably made just by reasons of simplicity, leads to a so called Friedmann Lemâıtre universe which explains, e.g., the well established uniform Hubble expansion [1]. Within this model, the universe started out from an extremely dense, hot initial state and subsequently cooled by adiabatic expansion undergoing a series of phase transitions. At a temperature of about 0.1MeV≈ 109K, deuterons become stable and virtually all the neutrons present are bound into 4He. This leads to the well established abundances of the light elements. As the universe cools further, below about 3000oK there are no longer enough ionizing photons around to keep the Hydrogen Helium plasma ionized. The matter in the universe recombines to neutral H/He and the universe becomes transparent to the cosmic radiation field. Radiation can then propagate freely, influenced only by the cosmic expansion and redshifted to the 2.7oK microwave background which we observe today. At recombination, the age of the universe was t ≈ 2 × 105years, which corresponds to a redshift of zR ≈ 1100. We now want to discuss the possibility, that the cosmic plasma might have reionized again at some lower redshift zi < zR. The reason one might want to investigate this idea is twofold: • Observationally: The Gunn Peterson test, i.e. the absence of a Lyman-α trough in all the observed quasar spectra, shows that the intergalactic medium is ionized for z < 4.5 [2, 3]