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

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 O(GeV) FOPT the PBHs could be similar to 10(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. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

This study proposes a novel mechanism for the formation of primordial black holes (PBHs) based on a first-order phase transition (FOPT). The research shows that if a fermion species acquires a huge mass in the true vacuum, the corresponding particles become trapped in the false vacuum. After the FOPT, these fermions are compressed into false vacuum remnants, forming non-topological solitons called Fermi-balls, which eventually collapse into PBHs due to the Yukawa attractive force. The derived PBH mass and abundance indicate that PBHs with a certain FOPT scale could account for all of dark matter, but if the FOPT occurs at a higher scale, PBHs would be typically overproduced, requiring additional dilution mechanisms to comply with current constraints.