Formation and clustering of primordial black holes in Brans-Dicke theory

TL;DR

This paper explores how primordial black holes form and cluster in Brans-Dicke gravity, showing that scalar field dynamics influence their formation probability, clustering, and potential contribution to gravitational wave events.

Contribution

It introduces corrections to the density perturbation threshold in Brans-Dicke theory and links scalar field inhomogeneities to black hole clustering and gravitational wave signals.

Findings

Corrections to the collapse threshold depend on scalar field evolution.

Inhomogeneous scalar fields can lead to black hole clustering.

Cluster formation increases the likelihood of black hole mergers and gravitational wave events.

Abstract

The formation of primordial black holes in the early universe in the Brans-Dicke scalar-tensor theory of gravity is investigated. Corrections to the threshold value of density perturbations are found. Above the threshold, the gravitational collapse occurs after the cosmological horizon crossing. The corrections depend in a certain way on the evolving scalar field. They affect the probability of primordial black holes formation, and can lead to their clustering at large scales if the scalar field is inhomogeneous. The formation of the clusters, in turn, increases the probability of black holes merge and the corresponding rate of gravitational wave bursts. The clusters can provide a significant contribution to the LIGO/Virgo gravitational wave events, if part of the observed events are associated with primordial black holes.