Primordial black holes through preheating instabilities in Starobinsky-like inflationary scenarios

The study of formation and evolution of primordial black holes (PBHs) in the early universe has attracted a lot of attention lately. The typical mechanism for PBH formation is based on the collapse of primordial density fluctuations when they re-enter the Hubble horizon. If these perturbations have a value above some particular threshold at the re-entry time, they can collapse into a black hole and its mass is determined by the horizon mass at this particular time. In this work we propose a novel way to estimate the formation of PBHs for the modes that are already sub-horizon and for which the density perturbation is above the threshold. To do so, we study the preheating stage after a Starobinsky-like inflation scenario, where the inflaton field oscillates around the bottom of the potential, which causes the universe to behave effectively as being matter-dominated (pressure-less).