# Particle physics 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 even to the time scale of the universe expansion, which is ${\displaystyle {\frac {1}{H}}}$ with ${\displaystyle H}$ being the time-dependent Hubble parameter. This is roughly equal to the age of the universe at that time.