Primordial black hole formation for an anisotropic perfect fluid: Initial conditions and estimation of the threshold

TL;DR

This paper studies how anisotropic pressure in a radiation fluid affects the initial conditions and threshold for primordial black hole formation, potentially impacting their abundance in the universe.

Contribution

It introduces a covariant approach to model anisotropic effects on cosmological perturbations and estimates how the PBH formation threshold varies with anisotropy.

Findings

Anisotropy modifies the initial shape of perturbations.

The threshold for PBH formation depends on the anisotropy amplitude.

Variation in the threshold could significantly alter PBH abundance.

Abstract

This work investigates the formation of primordial black holes within a radiation fluid with an anisotropic pressure. We focus our attention on the initial conditions describing cosmological perturbations in the super horizon regime, using a covariant form of the equation of state in terms of pressure and energy density gradients. The effect of the anisotropy is to modify the initial shape of the cosmological perturbations with respect to the isotropic case. Using the dependence of the threshold $\delta_\mathrm{c}$ for primordial black holes with respect to the shape of cosmological perturbations, we estimate here how the threshold is varying with respect to the amplitude of the anisotropy. If this variation is large enough it could lead to a significant variation of the abundance of PBHs.