Heating after inflation
At the end of inflation, most of the energy density of the Universe is stored in the zero mode of the inflaton field. In order for the thermal history of the Universe to start, this energy density must be transferred to the Standard Model particles in a direct or indirect way. The relocation of energy can proceed via perturbative decays or involve highly non-perturbative effects such as parametric resonance or tachyonic preheating. Depending on the model under consideration, these mechanisms can appear in isolation or coexist. A paradigmatic example of this latest possibility is the Combined Preheating scenario appearing in Higgs inflation.
Selected paper(s) on this topic: