Gravitational wave background from non-Abelian reheating after axion-like inflation

Abstract

A pseudoscalar inflaton φ, coupled to the topological charge density FF̃ of a non-Abelian sector, can decay to gauge bosons (φ ⟶ gg), which may thermalize rapidly. The friction felt by φ is then increased by non-Abelian “strong sphalerons”, leading to a self-amplifying process that can efficiently heat up the medium. We determine a lower bound for the gravitational wave production rate from such a process, originating via hydrodynamic fluctuations and particle collisions, in terms of a minimal number of parameters. Only a moderate fraction of energy density is converted to gravitational waves, suggesting that non-Abelian models may avoid the overproduction observed in some Abelian cases.