Clusters of Primordial Black Holes

Abstract

The Primordial Black Holes (PBHs) are a well-established probe for new physics in the very early Universe. We discuss here the possibility of PBH agglomeration into clusters that may have several prominent observable features. The clusters can form due to closed domain walls appearance in the natural and hybrid inflation models whose subsequent evolution leads to PBH formation. The dynamical evolution of such clusters discussed here is of crucial importance. Such a model inherits all the advantages of uniformly distributed PBHs, like possible explanation of supermassive black holes existence (origin of the early quasars), the binary black hole mergers registered by LIGO/Virgo through gravitational waves, which could provide ways to test the model in future, the contribution to reionization of the Universe. If PBHs form clusters, they could alleviate or completely avoid existing constraints on the abundance of uniformly distributed PBHs, thus allowing PBH to be a viable dark matter candidate. Most of the existing constraints on uniform PBH density should be re-considered to the case of PBH clustering. Furthermore, unidentified cosmic gamma-ray point-like sources could be (partially) accounted. We conclude that models leading to PBH clustering are favored compared to models predicting the uniform distribution of PBHs.

The fate of dense scalar stars

“…2. If RSSs are stable for a long time, they could undergo dynamical interactions including the i) formation of binaries [62] and collisions [65] with subsequent emission of primordial GWs that would result in a stochastic background; ii) clustering and formation of heavier bound objects [66], with possible collapse to heavy black holes that could survive even after the decay of the fields that compose the original star, and constitute a dark matter candidate. The richness of these dynamics would be enhanced if the stars collapse to black holes, since the longer lifetime would make it easier for the black holes to cluster.…”

Cosmoparticle physics of dark matter

“…In this aspect detection of signals from binary BH coalescence in the gravitational wave experiments [77][78][79][80][81] may be considered as a positive evidence for this scenario. Repeatedly detected signals localized in the same place would provide successive support in its favor or exclusion [75,82]. The existing statistics is evidently not sufficient to make any definite conclusion on this possibility.…”

Removing the conspiracy of BSM physics and BSM cosmology

“…PBHs with mass M ≥ 10 15 g should survive to the present time and represent a specific form of dark matter. It was noticed in [61] that taking into account PBH formation in clusters the constraints on PBH contribution into the total density [62] can be relaxed and even the possibility of PBH dominant dark matter is not excluded. It would make primordial nonhomogeneities in the form of PBHs the dominant matter content of the modern nonhomogeneities.…”

“…In this aspect detection of signals from binary BH coalescence in the gravitational wave experiments [63,64,65,66,67] may be considered as a positive evidence for this scenario [35]. Repeatedly detected signals localized in the same place would provide successive support in its favor or exclusion [35,61,68]. The existing statistics is evidently not sufficient to make any definite conclusion on this possibility.…”