Primordial black holes from a cosmic phase transition: The collapse of Fermi-balls

TL;DR

This paper introduces a new mechanism for primordial black hole formation involving a first-order phase transition that traps fermions into Fermi-balls, which then collapse into black holes potentially explaining dark matter.

Contribution

It presents a novel PBH formation process via Fermi-balls formed during a first-order phase transition, with detailed calculations of PBH mass and abundance.

Findings

PBHs of about 10^{17} g can form from a GeV-scale FOPT.

Such PBHs could account for all dark matter.

Higher scale FOPTs tend to overproduce PBHs, requiring dilution mechanisms.

Abstract

We propose a novel primordial black hole (PBH) formation mechanism based on a first-order phase transition (FOPT). If a fermion species gains a huge mass in the true vacuum, the corresponding particles get trapped in the false vacuum as they do not have sufficient energy to penetrate the bubble wall. After the FOPT, the fermions are compressed into the false vacuum remnants to form non-topological solitons called Fermi-balls, and then collapse to PBHs due to the Yukawa attractive force. We derive the PBH mass and abundance, showing that for a $\mathcal{O}({\rm GeV})$ FOPT the PBHs could be $\sim10^{17}$ g and explain all of dark matter. If the FOPT happens at higher scale, PBHs are typically overproduced and extra dilution mechanism is necessary to satisfy current constraints.