Primordial Black Hole Reformation in the Early Universe

TL;DR

This paper explores how primordial black holes can reform in the early universe during matter-dominated phases, potentially leading to observable gamma-ray and gravitational wave signatures that inform early universe physics.

Contribution

It introduces a novel scenario where initial light PBHs grow and collapse into heavier PBHs during early matter domination, linking early universe processes to observable signals.

Findings

Reformed PBHs can emit detectable gamma rays.

Early matter domination produces a stochastic gravitational wave background.

Scenario allows decoupling of PBH origins from initial formation mechanisms.

Abstract

Primordial black holes (PBH) can arise in a wide range of scenarios, from inflation to first-order phase transitions. Light PBHs, such as those produced during preheating, in bounce cosmologies, or at the GUT scale, could induce an early matter-dominated phase given a moderate initial abundance. During the early matter domination, the growth of initial PBH density perturbations can trigger collapse on horizon scales, producing much heavier PBHs. While the remaining original PBHs evaporate and reheat the Universe, these massive reformed PBHs survive for an extended period of time, producing potentially observable signatures at the present. We study this PBH reformation scenario and show that those reformed PBHs can emit significant quantities of gamma rays detectable by the next generation of experiments. The rapid reheating after matter domination generates a coincident stochastic gravitational wave background, which could be within the range of the upcoming CMB-S4 experiment. The PBH reformation scenario provides an intriguing possibility of decoupling the current PBH population and the initial formation mechanism from early Universe physics, while providing opportunities for observation through multi-messenger astronomy.