LIGO/Virgo black holes and dark matter: the effect of spatial clustering

Abstract

We discuss the effect of clustering for the determination of the merger rate of binary black holes in the LIGO/Virgo mass range. While for a Poissonian initial distribution, and assuming isolated binaries, the allowed fraction of Primordial Black Holes (PBHs) to dark matter (DM) is a few percent, we show that this bound can be relaxed if PBHs are clustered. More precisely we show that for large clustering the merger rate can drop with increasing fraction of PBHs, introducing a degeneracy in the parameters of the theory consistent with a given present merger rate, and allowing all the DM to be in the form of stellar mass PBHs. This degeneracy can however be broken by looking at the evolution of the merger rate with redshift. For a simple clustering model that we consider, we show that the LIGO/Virgo projected sensitivity can disentangle, through the observation of a stochastic background of gravitational waves, different clustered distributions having the same present merger rate.

Primordial black holes as a dark matter candidate

“…While distant three-body interactions are expected to have little impact on isolated PBH binaries [211,223], three-body interactions in PBH clusters could significantly affect the properties of binaries and hence the predicted merger rates [224][225][226][227]. Merger rates may also be increased (leading to stronger constraints) in scenarios where PBHs are formed with large initial clustering (as discussed in section 2.1.5) [83,228], though reference [229] argue that clustering instead should weaken constraints. Given these outstanding problems of PBH clustering and binary survival, in figure 2 we show constraints [166][167][168] that assume no clustering and no disruption of the binaries 18 .…”

PBH Formation from Spherically Symmetric Hydrodynamical Perturbations: A Review

“…Soon after this remarkable discovery, it was suggested that the constituents of the black hole merger could have had a primordial origin [19,45]. Later on, several analyses of the data signals suggested that the population of black holes detected could have a primordial origin [27,[46][47][48][49][50] (see also the references therein). On the other hand, recent results from the NANOGrav experiment [51] have also been connected with PBHs [26,[52][53][54][55][56][57][58][59].…”

Tracking the origin of black holes with the stochastic gravitational wave background popcorn signal

“…We note that the normalization of PBH merger rate has nothing to do with the fraction of CDM as our merger rate model described in Eq. ( 7) depends on the combination R clust f tot 2 PBH , which is the known degeneracy between clustering and abundance of PBHs (Raidal et al 2017;Clesse & Garcia-Bellido 2020;Vaskonen & Veermäe 2020;Young & Byrnes 2020;Trashorras et al 2021;Atal et al 2020;De Luca et al 2020b).…”

“…They can take masses well below Chandrasekhar mass, with the only constraint being that PBHs lighter than 10 15 g would have already evaporated by today. Besides their mass function, PBHs also differ in their merger rate, which is expected to grow with redshift, unlike that of ABHs, which follow the star formation rate (Cholis 2017;Raidal et al 2017;Vaskonen & Veermäe 2020;Atal et al 2020;De Luca et al 2020a;.…”