Primordial reheating in $f(R)$ cosmology by spontaneous decay of scalarons

We employ a viable $f(R)$ gravity model capable of giving an inflationary phase in order to study the subsequent reheating phase due to particle creation at the expense of energy in the scalaron field. Since quantum mechanics is expected to play a dominant role in particle creation, we formulate a plausible scenario of reheating obeying Heisenberg’s uncertainty principle that imposes constraints on the particles created in the configuration space. We show that, so long as the energy available in the scalaron field is sufficient to populate the entire configuration space, the energy density of the particles grows, attaining a maximum value giving an efficient reheating. Beyond this maximum, the available energy becomes insufficient to populate the entire configuration space leading to a declining energy density. We further find that there is a negligible growth of energy density in the inflationary phase that lasts for $\sim {10}^7{t}_{\mathrm{P}}$, although particles are constantly created in this phase. The subsequent reheating phase spans for $\sim {10}^{11}{t}_{\mathrm{P}}$ and it begins with a well-defined preheating stage lasting for $\sim {10}^5{t}_{\mathrm{P}}$, making a cross-over to a thermilization regime. The temperature at the beginning of the thermilization is found to be $T_{\mathrm{t}\mathrm{h}}\sim {10}^{12}$ GeV, whereas the reheating temperature is estimated as $T_r\sim {10}^{13}$ GeV. Importantly, these estimates follow from a single parameter, the scalaron mass, $M\sim {10}^{-5}{M}_{\mathrm{P}}$.