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 with H being the Hubble parameter, which varies with time. The expansion timescale 1 / H is roughly equal to the age of the universe at each point in time.