The abundance of primordial black holes depends on the shape of the inflationary power spectrum

TL;DR

This paper demonstrates that the abundance of primordial black holes formed during the radiation era is highly sensitive to the shape of the inflationary power spectrum peak, with broad peaks producing significantly more black holes than narrow ones.

Contribution

It introduces a combined peak theory and numerical collapse analysis showing the dependence of primordial black hole abundance on the power spectrum shape, impacting dark matter models.

Findings

Broad peaks lead to exponentially higher PBH abundance than narrow peaks.

For PBHs to account for all dark matter, smaller amplitude spectra are needed with broad peaks.

PBH mass distribution is dominated by horizon-scale masses despite a broad spectrum.

Abstract

In this letter, combining peak theory and the numerical analysis of gravitational collapse in the radiation dominated era, we show that the abundance of primordial blacks holes, generated by an enhancement in the inflationary power spectrum, is extremely dependent on the shape of the peak. Given the amplitude of the power spectrum, we show that the density of primordial black holes generated from a narrow peak, is exponentially smaller than in the case of a broad peak. Specifically, for a top-hat profile of the power spectrum in Fourier space, we find that for having primordial black holes comprising all of the dark matter, one would only need a power spectrum amplitude an order of magnitude smaller than suggested previously whereas in the case of a narrow peak, one would instead need a much larger power spectrum amplitude, which in many cases would invalidate the perturbative analysis of cosmological perturbations. Finally, we show that, although critical collapse gives a broad mass spectrum, the density of primordial black holes formed is dominated by masses roughly equal to the cosmological horizon mass measured at horizon crossing.