Primordial Black Holes from the QCD Axion.

Francesc Ferrer,Giuliano Panico,Oriol Pujolas,Eduard Masso,Fabrizio Rompineve

doi:10.1103/physrevlett.122.101301

Abstract

We propose a mechanism to generate primordial black holes (PBHs) that is independent of cosmological inflation and occurs slightly below the QCD phase transition. Our setup relies on the collapse of long-lived string-domain wall networks and is naturally realized in QCD axion models with domain wall number N_{DW}>1 and Peccei-Quinn symmetry broken after inflation. In our framework, dark matter is mostly composed of axions in the meV mass range along with a small fraction, Ω_{PBH}≳10^{-6}Ω_{CDM} of heavy M∼10^{4}-10^{7} M_{⊙} PBHs. The latter could play a role in alleviating some of the shortcomings of the standard cosmological model on subgalactic scales. The scenario might have distinct signatures in ongoing axion searches as well as gravitational wave observatories.

Highlights

We propose a mechanism to generate primordial black holes (PBHs) that is independent of cosmological inflation and occurs slightly below the QCD phase transition
Introduction.—The recent detection of gravitational waves emitted by the merging of relatively heavy black holes [M ≳ Oð10ÞM⊙] [1] has revived interest in the proposal that the dark matter (DM) of the Universe comprises primordial black holes (PBHs) [2,3,4,5,6,7]
There are constraints on the abundance of PBHs for almost all viable masses, a small relic abundance of heavy (M ≳ 105 M⊙) PBHs may play an important role in the generation of cosmological structures and alleviate shortcomings of the standard cold dark matter (CDM) scenario on subgalactic scales [9,10]

Summary

Primordial Black Holes from the QCD Axion

[8]), a small relic abundance of heavy (M ≳ 105 M⊙) PBHs may play an important role in the generation of cosmological structures and alleviate shortcomings of the standard cold dark matter (CDM) scenario on subgalactic scales [9,10]. Such PBHs could shed light on the origin of the supermassive black holes (BHs) in the centers of most galaxies, some of which were already in place at very early times [11,12]. For our purposes the essential features can be captured by focusing on the closed walls that arise in the network [22]

Published by the American Physical Society

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Findings

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