Particle physics in cosmology

Particle physics, which deals with high energies, is extremely important in the behavior of the early universe, since it was so hot that the average energy density was very high. Because of this, scattering processes and decay of unstable particles are important in cosmology.

As a thumb rule, a scattering or a decay process is cosmologically important in a certain cosmological epoch if its relevant time scale is smaller or comparable to the time scale of the universe expansion, which is 1 / H with H being the Hubble constant at that time. This is roughly equal to the age of the universe at that time.