The model of cosmological inflation explains how the temperature perturbations observed at the CMB may originate from vacuum fluctuations during an early epoch of accelerated expansion of the universe. On the experimental side, new probes of inflation are expected by the detection of a gravitational wave background in the next decades. Contributions to the gravitational wave signal may come from the thermal processes that lead to a heating-up period after inflation. However, a theoretical description of the transition from a vacuum-dominated to a radiation-dominated universe is by today neither realistic nor robust. We derive a general framework to embed the inflaton field within a thermal plasma and present a concrete example for a viable inflation scenario, capable of heating the universe dynamically. The amplitude of the corresponding gravitational wave signal depends peculiarly on the plasma constituents and the interactions between them.
