Threshold for primordial black holes: Dependence on the shape of the cosmological perturbations

TL;DR

This paper investigates how the threshold for primordial black hole formation depends on the shape of initial cosmological perturbations, using a spherical symmetry approximation and gradient expansion to clarify the relation between perturbation shape and black hole formation criteria.

Contribution

It introduces a detailed analysis of the shape dependence of the primordial black hole formation threshold within the gradient expansion framework, clarifying the relation between local and averaged perturbation measures.

Findings

No universal threshold exists for black hole formation.

The mass excess of perturbations influences the formation criterion.

Shape of initial perturbations affects the abundance of primordial black holes.

Abstract

Primordial black holes may have formed in the radiative era of the early Universe from the collapse of large enough amplitude perturbations of the metric. These correspond to non linear energy density perturbations characterized by an amplitude larger than a certain threshold, measured when the perturbations reenter the cosmological horizon. The process of primordial black hole formation is studied here within spherical symmetry, using the gradient expansion approximation in the long wavelength limit, where the pressure gradients are small, and the initial perturbations are functions only of a time-independent curvature profile. In this regime it is possible to understand how the threshold for primordial black hole formation depends on the shape of the initial energy density profile, clarifying the relation between local and averaged measures of the perturbation amplitude. Although there is no universal threshold for primordial black hole formation, the averaged mass excess of the perturbation depends on the amplitude of the energy density peak, and it is possible to formulate a well-defined criterion to establish when a cosmological perturbation is able to form a black hole in terms of one of these two key quantities. This gives understanding of how the abundance of primordial black holes depends on the shape of the the inflationary power spectrum of cosmological perturbations.