Mergers of primordial black holes in extreme clusters and the $H_0$ tension

TL;DR

This paper proposes that collisions of primordial black holes in dense clusters produce gravitational waves and alter the early universe's expansion, potentially resolving the H_0 tension by shortening the sound horizon scale.

Contribution

It introduces a cosmological model where primordial black hole collisions impact early universe dynamics, offering a novel solution to the H_0 tension problem.

Findings

Approximately 10% of cluster mass converts to gravitational waves during collisions.

The model predicts a 10% higher matter density at recombination.

Shortening of the sound horizon scale helps resolve the H_0 tension.

Abstract

We consider a cosmological model with dark matter in the form of $\sim10^{-12}M_\odot$ primordial black holes in dense weakly relativistic clusters with masses $18-560M_\odot$. It is shown that during the multiple collisions of the black holes the $\sim10$\% of the initial cluster mass can be transformed into gravitational waves in the time interval from recombination to the redshifts $z\geq 10$. At the recombination epoch, the density of matter was larger by $\sim10$\% and, accordingly, the universe expansion rate was higher. This leads to a shortening of the sound horizon scale, as is necessary to solve the "$H_0$ tension" problem.