A Thermal Universe \([T_{RH} \rightarrow T_{\text{CMB}}]\)

Once the Universe has terminated its inflationary phase, and produced a thermal bath through the inflaton decay, its evolution follows the evolution of a classical plasma in an expanding Universe. The law undergoes (relativistic) statistical laws, and one can apply our knowledge of this field to the production and decoupling of elements, from neutrino to dark matter. We propose in this chapter to review in details the thermal evolution of the primordial plasma, and the possibility to produce weakly interacting massive particles (WIMPs) from it.