Primordial black holes from an interrupted phase transition

TL;DR

This paper introduces a novel mechanism for primordial black hole formation during early Universe reheating, involving an interrupted phase transition that leaves behind density perturbations collapsing into black holes.

Contribution

It proposes a new primordial black hole formation process triggered by a first-order phase transition during reheating, with detailed estimates of black hole abundance based on bubble nucleation.

Findings

Primordial black holes can form from incomplete phase transitions during reheating.

The black hole abundance depends on the bubble nucleation rate at maximum temperature.

The mechanism allows for potentially large primordial black hole populations.

Abstract

We propose a new mechanism of primordial black hole formation via an interrupted phase transition during the early matter-dominated stage of reheating after inflation. In reheating, induced by the decay of a pressureless fluid dominating the Universe at the end of inflation, dubbed as reheaton, the temperature of the radiation bath typically increases, reaching a maximum temperature $T_{\rm max}$, and then decreases. We consider a first-order phase transition induced by the increase of the temperature that is aborted as $T_{\rm max}$ is higher than the critical temperature but not sufficiently high for the bubble nucleation rate to overcome the expansion of the Universe. Although bubbles never fully occupy the space, some may be nucleated and expand until the temperature once again decreases to the critical temperature. We argue that these bubbles shrink and disappear as the temperature drops further, leaving behind macroscopic spherical regions with positive density perturbations. These perturbed regions accrete the surrounding matter (reheatons) and eventually collapse into primordial black holes whose mass continues to grow until the onset of radiation domination. We estimate the abundance of these primordial black holes in terms of the bubble nucleation rate at $T_{\rm max}$, and demonstrate that the abundance can be significantly large from a phenomenological perspective.