Primordial Black Hole Formation via Inverted Bubble Collapse

TL;DR

This paper introduces a new mechanism for primordial black hole formation via inverted bubble collapse during a phase transition, producing monochromatic PBHs that could explain microlensing observations.

Contribution

It proposes a novel PBH formation process involving inverted bubble collapse, distinct from traditional mechanisms, applicable to the singlet extension of the Standard Model.

Findings

Can produce PBHs up to 10^{-7} to 10^{-5} solar masses

Results in highly monochromatic PBH mass spectrum

Potentially explains microlensing events in OGLE and Subaru HSC data

Abstract

We propose a novel mechanism of primordial black hole (PBH) formation through inverted bubble collapse. In this scenario, bubbles nucleate sparsely in an incomplete first-order phase transition, such that they remain isolated and do not percolate or collide with each other due to the extremely low nucleation rate. This is followed by a bulk phase transition in the rest of the universe that inverts these pre-existing bubbles into false vacuum regions. These spherically symmetric false-vacuum bubbles subsequently collapse to form PBHs. Unlike conventional PBH formation mechanisms associated with domain wall collapse or bubble coalescence, our inverted bubble collapse mechanism naturally ensures spherical collapse. We demonstrate that, when applied to the singlet extension of the Standard Model, this mechanism can produce highly monochromatic PBHs with masses up to ${\cal O}(10^{-7}\,\text{-}\,10^{-5}) M_\odot$, which potentially explain the microlensing events observed in the OGLE and Subaru HSC data.