Kinetics and Thermodynamics in Cosmology

We discuss the thermodynamics of elementary particles in the early Universe. We formulate the conditions for the establishment of thermal equilibrium and we discuss the properties of the canonical equilibrium distribution functions, including the conditions for the formation of Bose-Einstein condensates in cosmology. We compute frozen densities of massless and massive particles. It is proven that the equilibrium distribution of massless particles is not distorted by the cosmological expansion. On the other hand, it is shown that massless neutrinos in the early Universe deviate from equilibrium.