Primordial magnetic fields from the post-inflationary universe

We explore cosmological magnetogenesis in the post-inflationary universe, when the inflaton oscillates around its potential minimum and the universe is effectively dominated by cold matter. During this epoch prior to reheating, large-scale magnetic fields can be significantly produced by the cosmological background. By considering magnetogenesis both during and after inflation, we demonstrate that magnetic fields stronger than 10 −15 G can be generated on Mpc scales without having strong couplings in the theory, or producing too large electric fields that would dominate the universe.