The statistics of primordial black holes in a radiation dominated Universe -- recent and new results

TL;DR

This paper reviews the non-linear statistics of primordial black holes formed during a radiation-dominated era, presenting new insights into their mass spectrum, abundance, and the limitations of common approximation methods.

Contribution

It introduces a power-law mass spectrum based on critical collapse exponents and demonstrates the inaccuracies of the Press-Schechter approximation in PBH abundance predictions.

Findings

Mass spectrum follows a power law up to a cutoff.

Abundance for narrow spectra is exponentially suppressed.

Press-Schechter approximation yields incorrect PBH abundance estimates.

Abstract

We review the non-linear statistics of Primordial Black Holes that form from the collapse of over-densities in a radiation dominated Universe. We focus on the scenario in which large over-densities are generated by rare and Gaussian curvature perturbations during inflation. As new results, we show that the mass spectrum follows a power law determined by the critical exponent of the self-similar collapse up to a power spectrum dependent cut-off, and that the abundance related to very narrow power spectra is exponentially suppressed. Related to this, we discuss and explicitly show that the Press-Schechter approximation, as well as the statistics of mean profiles, lead to wrong conclusions for the abundance and mass spectrum. Finally, we clarify that the transfer function in the statistics of initial conditions for primordial black holes formation (the abundance) does not play a significant role.