  We introduce a method where particle physics processes in cosmology may be
calculated by the usual perturbative flat space quantum field theory through an
effective Minkowski space description at small time intervals provided that the
running of the effective particle masses are sufficiently slow. We discuss the
necessary conditions for the applicability of this method and illustrate the
method through a simple example. This method has the advantage of avoiding the
effects of gravitational particle creation in the calculation of rates and
cross sections i.e. giving directly the rates and the cross sections due to the
scatterings or the decay processes.
