Editing Physical cosmology (section)

===Particle physics in cosmology===
{{main|Particle physics in cosmology}}

During the earliest moments of the universe, the average energy density was very high, making knowledge of [[particle physics]] critical to understanding this environment. Hence, [[scattering]] processes and [[particle decay|decay]] of unstable [[elementary particles]] are important for cosmological models of this period.

As a rule of thumb, a scattering or a decay process is cosmologically important in a certain epoch if the time scale describing that process is smaller than, or comparable to, the time scale of the expansion of the universe.{{clarify |date=April 2018 |reason=Isn't the opposite true – departures from equilibrium occur only when the process takes longer than the time scale? }} The time scale that describes the expansion of the universe is <math>1/H</math> with <math>H</math> being the [[Hubble parameter]], which varies with time. The expansion timescale <math>1/H</math> is roughly equal to the age of the universe at each point in time.