Primordial black hole production during first-order phase transitions

TL;DR

This paper introduces a novel, model-independent mechanism for primordial black hole formation during first-order phase transitions in the early universe, linking PBHs, gravitational waves, and dark matter.

Contribution

It proposes a new PBH production mechanism during first-order phase transitions, with implications for dark matter and gravitational wave observations.

Findings

PBHs can form during phase transitions due to postponed vacuum decay.

The PBH mass function is nearly monochromatic.

PBHs from phase transitions can account for dark matter and observed GW signals.

Abstract

Primordial black holes (PBHs) produced in the early Universe have attracted wide interest for their ability to constitute dark matter and explain the compact binary coalescence. We propose a new mechanism of PBH production during first-order phase transitions (PTs) and find that PBHs are naturally produced during PTs model-independently. Because of the randomness of the quantum tunneling, there always exists some probability that the vacuum decay is postponed in a whole Hubble volume. Since the vacuum energy density remains constant while radiation is quickly redshifted in the expanding Universe, the postponed vacuum decay then results in overdense regions, which finally collapse into PBHs as indicated by numerical simulations. Utilizing this result one can obtain mutual predictions and constraints between PBHs and GWs from PTs. The predicted mass function of PBHs is nearly monochromatic. We investigate two typical cases and find that 1) PBHs from a PT constitute all dark matter and GWs peak at $1$Hz, 2) PBHs from a PT can explain the coalescence events observed by LIGO-Virgo collaboration, and meanwhile GWs can explain the common-spectrum process detected by NANOGrav collaboration.