Primordial black holes from the QCD epoch: Linking dark matter, baryogenesis and anthropic selection

TL;DR

This paper proposes a scenario where primordial black holes formed during the QCD epoch could account for dark matter and baryogenesis, linking their formation to inflationary fluctuations and anthropic selection, with testable observational predictions.

Contribution

It introduces a novel link between PBH formation, baryogenesis, and anthropic selection, suggesting PBHs as dark matter candidates and explaining baryon-to-dark matter ratios.

Findings

PBHs formed at the quark-hadron epoch can explain dark matter and baryon asymmetry.

The scenario predicts a PBH mass function extending to high masses, consistent with LIGO/Virgo observations.

Testable predictions include specific features in the PBH mass spectrum and related cosmological signals.

Abstract

If primordial black holes (PBHs) formed at the quark-hadron epoch, their mass must be close to the Chandrasekhar limit, this also being the characteristic mass of stars. If they provide the dark matter (DM), the collapse fraction must be of order the cosmological baryon-to-photon ratio $\sim 10^{-9}$, which suggests a scenario in which a baryon asymmetry is produced efficiently in the outgoing shock around each PBH and then propagates to the rest of the Universe. We suggest that the temperature increase in the shock provides the ingredients for hot spot electroweak baryogenesis. This also explains why baryons and DM have comparable densities, the precise ratio depending on the size of the PBH relative to the cosmological horizon at formation. The observed value of the collapse fraction and baryon asymmetry depends on the amplitude of the curvature fluctuations which generate the PBHs and may be explained by an anthropic selection effect associated with the existence of galaxies. We propose a scenario in which the quantum fluctuations of a light stochastic spectator field during inflation generate large curvature fluctuations in some regions, with the stochasticity of this field providing the basis for the required selection. Finally, we identify several observational predictions of our scenario that should be testable within the next few years. In particular, the PBH mass function could extend to sufficiently high masses to explain the black hole coalescences observed by LIGO/Virgo.