Primordial Black Holes from the QCD Axion

Area of Science:

• Cosmology
• Particle Physics
• Astrophysics

Background:

• Primordial black holes (PBHs) are potential dark matter candidates, but their formation mechanisms are often linked to cosmic inflation.
• QCD axion models with broken Peccei-Quinn symmetry and domain wall number N_{DW}>1 offer a rich framework for early universe physics.

Purpose of the Study:

• To propose a new mechanism for generating PBHs independent of cosmic inflation.
• To explore the implications of this mechanism within QCD axion models.
• To investigate the potential role of these PBHs in dark matter composition and addressing cosmological model shortcomings.

Main Methods:

• Investigating the collapse of long-lived string-domain wall networks.
• Analyzing the formation of PBHs slightly below the QCD phase transition.
• Modeling dark matter composition including axions and PBHs.

Main Results:

• A mechanism for PBH generation is proposed, occurring below the QCD phase transition and independent of inflation.
• The model predicts dark matter composed of meV-mass axions and a fraction of heavy PBHs (M~10^4-10^7 M☉).
• These PBHs could potentially resolve some small-scale challenges in the standard cosmological model.

Conclusions:

• The proposed string-domain wall collapse mechanism provides an alternative route to PBH formation.
• The model offers a compelling explanation for dark matter, combining axions and PBHs.
• Distinct observational signatures are predicted for axion searches and gravitational wave detectors.