Primordial black hole formation with full numerical relativity

TL;DR

This study uses full numerical relativity to investigate primordial black hole formation in a matter-dominated universe, revealing two main formation mechanisms and significant post-formation mass accretion.

Contribution

It provides a detailed numerical analysis of PBH formation mechanisms, including direct collapse and post-collapse accretion, challenging previous criteria like the hoop conjecture.

Findings

Two primary formation mechanisms identified: direct collapse and post-collapse accretion.

Initial black hole mass around 10^{-2} H^{-1} M_{Pl}^2.

Post-formation, PBHs rapidly grow in mass through ambient accretion.

Abstract

We study the formation of black holes from subhorizon and superhorizon perturbations in a matter dominated universe with 3+1D numerical relativity simulations. We find that there are two primary mechanisms of formation depending on the initial perturbation's mass and geometry -- via $\textit{direct collapse}$ of the initial overdensity and via $\textit{post-collapse accretion}$ of the ambient dark matter. In particular, for the latter case, the initial perturbation does not have to satisfy the hoop conjecture for a black hole to form. In both cases, the duration of the formation the process is around a Hubble time, and the initial mass of the black hole is $M_\mathrm{BH} \sim 10^{-2} H^{-1} M_\mathrm{Pl}^2$. Post formation, we find that the PBH undergoes rapid mass growth beyond the self-similar limit $M_\mathrm{BH}\propto H^{-1}$, at least initially. We argue that this implies that most of the final mass of the PBH is accreted from its ambient surroundings post formation.