section_path: History of the universe/Particle physics in cosmology
section_name: Particle physics in cosmology
paragraph_text: 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 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. The time scale that describes the expansion of the universe is 1 / H {\displaystyle 1/H} with H {\displaystyle H} being the Hubble parameter, which varies with time. The expansion timescale 1 / H {\displaystyle 1/H} is roughly equal to the age of the universe at each point in time.