Reviving primordial black hole formation in slow first-order phase transitions

TL;DR

This paper revisits primordial black hole formation during slow first-order phase transitions, demonstrating that an early matter-dominated era caused by reheating allows small overdensities to collapse, reviving a previously dismissed mechanism.

Contribution

It shows that slow reheating after supercooled transitions enables primordial black hole formation despite previous gauge-related objections.

Findings

Reheating can be slow enough to induce an early matter-dominated era.

Small overdensities can grow and collapse into black holes during this era.

The mechanism remains viable despite earlier gauge-related concerns.

Abstract

Large curvature perturbations generated during slow first-order phase transitions are a promising source of primordial black holes. However, recent analyses suggested that the mechanism is ruled out once the density contrast and the formation threshold are evaluated in the same gauge. In this work, we show that this mechanism remains viable: after a supercooled transition, reheating can be sufficiently slow that the Universe enters an early matter-dominated era, during which even small overdensities grow and collapse into primordial black holes.