An analytic formula to calculate the reheating temperature via gravitational particle production

We present for smooth non-oscillating backgrounds an analytic formula which calculates the energy density of massive and massless particles created via gravitational particle production, thus giving the corresponding reheating temperature. It can be applied to models of Quintessential Inflation such as $\alpha$-attractors, and shows that for masses larger than the Hubble rate at the end of inflation, namely $H_{END}$, the reheating temperature is exponentially suppressed. On the contrary, for masses of the order of $H_{END}$ one obtains a maximum reheating temperature of the order of $10^7$ GeV. Finally, to overcome the constraints coming from the overproduction of Gravitational Waves in Quintessential Inflation, we have shown that the viable masses which ensure the Big Bang Nucleosynthesis success are in the range between {$2\times 10^{10}$ GeV and $4\times 10^{13}$ GeV}, leading to a maximum reheating temperature of the order {$10^5-10^7$ GeV}.