The scalar perturbations, the origin of the CMB anisotropies and the large scale structure, can induce gravitational waves (GWs) through the interaction at second order in perturbations. This kind of GW has a lot of attention recently because it can be used to investigate small-scale scalar perturbations, which could produce primordial black holes (PBHs) as dark matter and BHs detected by LIGO-Virgo collaborations. In this seminar, I will talk about the enhancement of the induced GWs, which occurs right after a sudden reheating at the end of the early matter-dominated era. This enhancement mechanism is named the “Poltergeist mechanism”. The induced GWs are much enhanced due to the fast oscillation of the subhorizon scalar perturbations after the reheating. This kind of GW enhancement could happen right after a PBH-dominated era.

References: arXiv:1904.12879, 1904.12878, 2003.10455.