ABSTRACT
Based on the cosmological principle the Hot Big Bang provides a successful account of the history of the Universe as far back as a few seconds after the Big Bang explosion. The Hot Big Bang is the history of the Universe, more or less until the present, when the Universe content is dominated first by radiation and then by matter. The Hot Big Bang does not account for the origin of these primordial perturbations, but inflation does. The history of the very early Universe is thus merely an extrapolation of the Hot Big Bang towards early times. The cosmic microwave background radiation (CMB) is one of the major pillars of support for the Big Bang model, since it demonstrates that the early Universe was hot and dense, disproving thereby the rival theory of a steady-state universe. After the decoupling of neutrinos, their distribution retains its black body form exactly as is the case for photon decoupling and the CMB.
