Argument for the radiation-dominated behavior of matter fields in the preinflationary era

Abstract: We investigate the leading-order behavior of matter fields in the preinflationary era using the semiclassical approximation. Many inflationary models assume without supporting arguments that the Universe was radiation dominated prior to inflation, leading to modifications of cosmological observables, such as the Cosmic Microwave Background power spectrum. In previous work, we demonstrated that conformally coupled scalar fields do have a radiation-like contribution to the stress-energy tensor at sufficiently early times. In this work, we extend these arguments to apply to massless spin-1 fields and massive or massless spin-$\frac{1}{2}$ fields. We find massless spin-1 fields always have a radiation-like contribution. For spin-$\frac{1}{2}$ fields, we find the contribution at early times is radiation-like assuming this is the dominant contribution to the stress-energy tensor.