ABSTRACT
This chapter presents hotter and hotter phases in the history of the universe. Elementary particle physics plays a crucial role in the early universe during the radiation-dominated regime. The chapter explains the number of ultra-relativistic degrees of freedom, determining the expansion rate at a given temperature. This quantity provides an important connection between cosmological observables and particle physics. However, the early universe behaves as a sort of particle collider but with an important difference. In a particle collider the beam energy is fixed by the experimental setup. For example, in the Large Hadron Collider two collinear anti-circulating proton beams with equal energy collide. Derive also an expression, always in integral form, for the number densities of electrons and positrons as a function of temperature. Inelastic processes contribute not only to establishing kinetic equilibrium, but also chemical equilibrium for the number of neutrinos. Thermal equilibrium is realised when both kinetic and chemical equilibrium are achieved.
