The thermal history of the cosmological gas

The aim of this lecture is to briefly review the thermal history of the baryonic content of the universe after the epoch Z ≈ 1400, at which the primeval plasma recombines in the standard model. The different processes that might determine its physical state are presented as well as the status of our knowledge of its properties at different epoch and the observational constraints that can be set. The uncertainty on this history is large because of the very poor constraints on possible reheating mechanisms. Nevertheless, such processes should have existed and they might be a fundamental key of galaxy formation.