Interaction rates in cosmology: heavy particle production and scattering

We study transition rates and cross sections from first principles in a spatially flat radiation dominated cosmology. consider model of scalar particles to scattering heavy particle production pair annihilation, drawing more general conclusions. The S-matrix formulation is ill suited these ubiquitous processes rapidly expanding introduce physically motivated adiabatic expansion that relies on wavelengths much smaller than the horizon at given time. leading order this dominates sections. Several important results are direct consequences cosmological redshift finite horizon: i) violation local Lorentz invariance, ii) freeze-out section time, iii) sub-threshold heavier as consequence uncertainty energy horizon, manifestation \emph{antizeno} effect. If dark matter produced via annihilation lighter species, yields an enhanced abundance. discuss several possible effects.