The primordial black holes that disappeared: connections to dark matter and MHz-GHz gravitational Waves

Abstract

In the post-LIGO era, there has been a lot of focus on primordial black holes (PBHs) heavier than ∼ 1015g as potential dark matter (DM) candidates. We point out that the branch of the PBH family that disappeared — PBHs lighter than ∼ 109g that ostensibly Hawking evaporated away in the early Universe — also constitute an interesting frontier for DM physics. Hawking evaporation itself serves as a portal through which such PBHs can illuminate new physics, for example by emitting dark sector particles. Taking a simple DM scalar singlet model as a template, we compute the abundance and mass of PBHs that could have provided, by Hawking evaporation, the correct DM relic density. We consider two classes of such PBHs: those originating from curvature perturbations generated by inflation, and those originating from false vacuum collapse during a first-order phase transition. For PBHs of both origins we compute the gravitational wave (GW) signals emanating from their formation stage: from second-order effects in the case of curvature perturbations, and from sound waves in the case of phase transitions. The GW signals have peak frequencies in the MHz-GHz range typical of such light PBHs. We compute the strength of such GWs compatible with the observed DM relic density, and find that the GW signal morphology can in principle allow one to distinguish between the two PBH formation histories.

Recycled dark matter

“…In this section, we discuss the mass distribution of PBHs formed during a FOPT following [36,46,47]. The conditions for PBH formation in our model will be very similar, with one major difference: since the leakage of ϕ particles into the true vacuum is suppressed in our case, the size range of false vacuum pockets that can potentially collapse to PBHs is determined by the annihilation into SM particles, as shown in eq.…”

“…In the case that most (or all) of the pockets collapse into PBHs, we expect an early PBH dominated Universe, since almost all the energy content of the dark sector which is of the order of percentage of the radiation energy density ends up in PBHs. It is worth mentioning that the high temperature of FOPT connected to recycling DM by PBHs gives rise to the gravitational wave signatures in the ultra high frequency MHz-GHz range [47,53].…”

Gravitational wave probe of primordial black hole origin via superradiance

“…The literature on probing the PBHs utilizing the features in the gravitational waves is quite large [17,[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62]. 1 However, our previous study was one of the first ones to conduct a study that uses the cumulative GW spectra [68].…”

Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves

“…and light primordial black holes (PBH) [22][23][24][25][26][27][28][29] -it is important to explore GWs in the MHz, GHz, or even higher frequency ranges [20].…”