Pinning down the primordial black hole formation mechanism with gamma-rays and gravitational waves

Abstract

Primordial black holes (PBHs) are predicted in many models via different formation mechanisms. Identifying the origin of PBHs is of the same importance as probing their existence. We propose to probe the asteroid-mass PBHs [𝒪(1017) g ≲ M ≲ 𝒪 (1022) g] with gamma-rays from Hawking radiation and the stochastic gravitational waves (GWs) from the early Universe. We consider four concrete formation mechanisms, including collapse from primordial curvature perturbations, first-order phase transitions, or cosmic strings, and derive the extended PBH mass functions of each mechanism for phenomenological study. The results demonstrate that by combining gamma-rays and GW signals we can probe PBHs up to 𝒪(1019) g and identify their physical origins.

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

“…Spherical over-dense regions with radius R at the onset of the phase transition typically shrink by a factor of ∼ 3 before they become smaller than the Schwarzschild radius, leading to PBH formation (right panel, figure 4 of [106]). We discuss this process at length below, highlighting the main results of [106] and [29].…”

“…The evolution of the energy density therefore depends on the ratio of the initial pocket size R and the pocket size R(t) at time t, with the scaling being quartic because ψ number density is enhanced by the suppressed volume while ψ particles are simultaneously being heated up by the wall. 6 The mass of the PBH, M PBH and the corresponding abundance β PBH can be calculated using the methods developed in [29]. 6 The total energy density of the pocket also receives contributions from Standard Model particles and is given by…”

“…While correlations between Hawking evaporation and GWs generated during the formation of PBHs have been used to study stable PBHs[27][28][29], the method also holds great potential for lighter PBHs that evaporated away. For instance, baryogenesis arising from the Hawking radiation of light PBHs has been studied by the current authors[68].…”

Probing the origin of primordial black holes through novel gravitational wave spectrum

“…The literature on probing the PBHs utilising the features in the gravitational waves is quite large [22,[43][44][45][46][47][48][49][50][51][52][53][54][55]. However, an approach to study the cumulative effect of the GW background from the creation mechanism of PBH as well as the gravitational interactions of the PBHs is yet to be rigorously performed.…”

“…Recently in refs. [43,53] it was proposed that SGWB spectra can be used to distinguish between creation mechanism of PBH; though the SGWB due to gravitational interactions of PBHs were not considered since for those mass ranges of PBH it would have been insignificant. Since we are considering gravitational interactions of the PBHs as one of the sources of SGWB, the mass of the PBHs are to be large enough for the SGWB to be significant.…”