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Energy Density, Temperature and Entropy Dynamics in Perturbative Reheating

We discuss the perturbative decay of the energy density of a non standard inflaton field $ρ_ϕ$ and the corresponding creation of the energy density of the relativistic fields $ρ_r$ at the end of inflation, in the perfect fluid description, refining some concepts and providing some new computations. In particular, the process is characterized by two fundamental time scales. The first one, $t_\text{max}$, occurs when the energy density $ρ_r$ reaches its largest value, slightly after the beginning of the reheating phase. The second one, $t_\text{reh}$, is the time in which the reheating is completely realized and the thermalization is attained. By assuming a non-instantaneous reheating phase, we are able to derive the energy densities and the temperatures of the produced relativistic bath at $t_\text{max}$ and $t_\text{reh}$, as well as the value of the corresponding horizon entropy $S_\text{hor}$, for an Equation-of-State (EoS) parameter $w\ne 0$.