Stochastic gravitational waves associated with primordial black holes formed during an early matter era

Abstract

An early matter-dominated (eMD) era may have preceded the standard radiation-dominated era. During an eMD era, primordial black holes (PBHs) can be formed due to the gravitational collapse of a fraction of high-density peaks. If their formation probability is sufficiently high, PBHs formed during an eMD era could play interesting cosmological roles, such as accounting for part of or entire dark matter, and/or causing PBH mergers which might have been or would be detected by ongoing and future gravitational-wave detectors. In addition to PBHs, gravitationally collapsed, virialized objects can also be formed during an eMD era. We discuss gravitational waves associated with the formation of such collapsed structures when PBH formation probability is relatively high during an eMD era, and show that such GWs can be observable in future.