Gravitational reheating in Starobinsky inflation

We investigate the possibility of achieving post-inflationary reheating exclusively through the gravitational interaction in Starobinsky inflation, which itself assumes nothing but gravity. We consider the possibility that the reheating sector couples to gravity via a non-minimal coupling. Our analysis is performed both in a perturbative and in a non-perturbative approach, where particle production is computed from Bogoliubov coefficients. Our findings indicate that, for sufficiently large non-minimal coupling (ξ~few× 10), non-perturbative particle production allows for temperatures of order 10¹² GeV to be reached while also ensuring that the Universe becomes radiation-dominated. This shows that the gravitational interaction could be the sole responsible for reheating the Universe after inflation, without the need to assume other ad hoc inflaton interactions.