Primordial Black Holes from QCD Axion Bubbles

TL;DR

This paper proposes a novel scenario where early universe PQ symmetry breaking creates dense axion bubbles that can collapse into primordial black holes, potentially explaining dark matter, LIGO events, and supermassive black hole seeds.

Contribution

It introduces a mechanism linking PQ symmetry breaking to axion bubble formation and primordial black hole production, with mass predictions based on the axion decay constant.

Findings

PBH masses depend on the axion decay constant, reaching up to 10^4 solar masses.

Axion bubbles can produce dark matter, LIGO black hole events, and supermassive black hole seeds.

Axion miniclusters are more abundant and heavier than in previous models.

Abstract

We propose a scenario in which a strong Peccei-Quinn (PQ) symmetry breaking in the early universe results in large inhomogeneities of the initial QCD axion field value, leading to the formation of very dense axion bubbles. Some of the axion bubbles subsequently collapse into primordial black holes (PBHs). The spatially homogeneous part of the QCD axion explains dark matter of the universe, while the PBHs arising from the axion bubbles can explain the LIGO events or the seed of supermassive black holes. Interestingly, the mass of PBH is determined by the axion decay constant; for $f_a = 10^{17} (10^{16})$ GeV, the PBH mass is heavier than about $10 (10^4) M_\odot$. In addition, axion miniclusters are also formed from the axion bubbles more abundantly than PBHs, and their masses are expected to be heavier than in the usual scenario based on the spontaneous breaking of the PQ symmetry after inflation.