General SIGW source for reheating dynamics

Abstract

Working in an arbitrary gauge, we derive the source term for scalar-induced gravitational waves (SIGW) valid during a general reheating epoch. The dominant energy component is allowed to transition smoothly from an inflaton field to a set of fluids, possibly via a period of matter domination. Gauge invariance is verified up to second order.

Comments: 17 pages. v2: clarifications added, literature comparison extended, journal version