Abstract: After cosmic inflation and before the transition to radiation domination, the cosmic energy density may have been dominated during an extended period by an oscillating massive scalar condensate. During this period, sub-Hubble scale perturbations are subject to a metric preheating instability that leads to the growth of density perturbations which in many cases become non-linear already before the beginning of a radiation dominated Universe. The appearance of such early meta stable structures may lead to the formation of primordial black holes.
https://arxiv.org/abs/1002.3039
https://arxiv.org/abs/1907.04236
https://arxiv.org/abs/1710.06851
