The reheating epoch following inflation sets the initial conditions for the subsequent thermal history of the Universe, but remains largely unconstrained prior to Big Bang nucleosynthesis. In many extensions of the Standard Model, the inflaton may decay simultaneously into visible and hidden sectors, leading to non-standard cosmological histories. One possibility is an asymmetric reheating in which unequal amounts of energy are deposited in the visible and dark sectors, resulting in different temperatures and potential entropy exchange between them. In this talk, I will study asymmetric reheating in a simple and instructive hidden sector framework: dark QED, consisting of a Dirac fermion dark matter candidate charged under a dark U(1)′ gauge symmetry and a massive dark photon coupled to the visible sector via kinetic mixing. A central part of the discussion will be the thermalisation of dark QED after inflation, during reheating. I will focus on thermalization in the weak-coupling and initially under-occupied regime. I will then discuss the evolution of the temperature ratio during the reheating process. I will finally explore the implications for thermal dark matter production, highlighting viable regions of parameter space where dark matter is thermalized at a temperature different from that of the Standard Model.
