Non-linear Dynamics and Primordial Black Hole Formation During Kination

TL;DR

This paper uses numerical relativity to study how large scalar inhomogeneities evolve during the kination epoch, exploring their potential to form primordial black holes and reheat the universe, revealing different behaviors on sub- and super-horizon scales.

Contribution

It provides the first detailed numerical analysis of non-linear scalar perturbations during kination, comparing their evolution to perturbative predictions and assessing black hole formation.

Findings

Perturbative results hold in the deep sub-horizon limit.

Super-horizon perturbations exhibit complex non-linear behavior.

Primordial black hole formation is possible from collapsing scalar inhomogeneities.

Abstract

We investigate the effects of large scalar inhomogeneities during the kination epoch, a period in which the universe's dynamics are dominated by the kinetic energy of a scalar field, by fully evolving the Einstein equations using numerical relativity. By tracking the non-linear growth of scalar perturbations with both sub-horizon and super-horizon initial wavelengths, we are able to compare their evolution to perturbative results. Our key findings show that in the deep sub-horizon limit, the perturbative behaviour remains valid, whereas in the super-horizon regime, non-linear dynamics exhibit a much richer phenomenology. Finally, we discuss the possibility of primordial black hole formation from the collapse of such perturbations and assess whether this process could serve as a viable mechanism to reheat the universe in the post-inflationary era.