Abstract
In this first session, the theory of re-ionization is developed, and illustrated by cosmological simulations. The last scattering surface, or recombination, corresponds to the zredshift = 1 000 and the temperature 3 000 K(T(CMB) = 2.73 (1 + z)). By this date, fluctuations with an amplitude of the order of 10-3 would already be present in dark matter, if we were to explain the existence of very strongly non-linear structures today. Yet the fluctuations measured in the cosmic microwave background are only 10-6. What's more, thanks to the Hubble Space Telescope (HST) and its ultra-deep field, we can observe the first galaxies at around z = 10, when the Universe was only 500 million years old. Further constraints on the epoch of reionization come from absorptions in front of quasars. When the space between galaxies is still filled with atoms, the latter absorb all photons of sufficient energy to ionize hydrogen (13.6 eVor 912 AA), and in the quasar spectrum we see the Gunn-Peterson effect, i.e. total absorption, for quasars of z = 5 - 6. For closer quasars, only a Lyman-alpha forest is produced by the absorption of atomic gas in the vicinity of galaxies. The ionization potential of helium is higher than that of hydrogen, so helium does not reionize until much later. Reionization is slow at first, there are few stars and, above all, the density n of the Universe is high, at (1 + z)3, and the rate of recombination is n2. Then, it speeds up, the density decreases and the number of stars and galaxies increases. The number of electrons on the line of sight is a diagnostic of the duration of the reionization epoch, as electrons scatter photons from the cosmological background. The WMAP (Wilkinson Microwave Anisotropy Probe) space observatory and PLANCK indicate that reionization should be complete at z ~ 6. Numerical simulations need to take into account not only the radiative processing for hydrogen ionization, in addition to dynamics and star formation, but also how hydrogen is excited, in order to predict the low-frequency signal (VHF and UHF) to explore the epoch of reionization.
