Primordial Black Holes as a dark matter candidate (2007.10722v3)

Abstract: The detection of gravitational waves from mergers of tens of Solar mass black hole binaries has led to a surge in interest in Primordial Black Holes (PBHs) as a dark matter candidate. We aim to provide a (relatively) concise overview of the status of PBHs as a dark matter candidate, circa Summer 2020. First we review the formation of PBHs in the early Universe, focusing mainly on PBHs formed via the collapse of large density perturbations generated by inflation. Then we review the various current and future constraints on the present day abundance of PBHs. We conclude with a discussion of the key open questions in this field.

• The paper presents a comprehensive analysis of primordial black hole formation and viability as dark matter using updated numerical simulations.
• It evaluates observational constraints from evaporation, microlensing, gravitational waves, and dynamical effects on PBHs.
• The study highlights unconstrained PBH mass windows and advocates further surveys to refine dark matter candidate assessments.

Primordial Black Holes as a Dark Matter Candidate

The paper, authored by Anne M. Green and Bradley J. Kavanagh, provides a thorough analysis of the role of primordial black holes (PBHs) as a viable candidate for dark matter (DM). This discussion is timely in light of recent gravitational wave observations, which have rekindled interest in PBHs, originally considered for this role since their theoretical prediction.

Formation and Characteristics of PBHs

PBHs are hypothesized to form from high density perturbations in the early Universe, predominantly during the radiation-dominated era. The paper revisits seminal calculations by Carr and shows enhancements due to recent numerical simulations. The threshold dynamics for PBH formation and the mass spectrum are critically evaluated, highlighting the dependence on the curvature perturbation and the density contrast exceeding a critical value. Notably, PBHs must have formed with masses above $5 \times 10^{14} \, \text{g}$ to avoid evaporation via Hawking radiation within the age of the Universe.

Observational Constraints

The authors discuss multiple observational constraints on PBHs, essential in limiting their abundance and mass spectrum allowed by current astrophysical data. The constraints are categorized as follows:

1. Evaporation: For PBHs with initial masses less than about $5 \times 10^{14} \, \text{g}$, gamma-ray observations limit their contribution to DM. Observations from extragalactic backgrounds and CMB anisotropies also tightly constrain these tiny PBHs.
2. Stellar Microlensing: Surveys such as MACHO, EROS, and OGLE constrain PBHs with masses around $10^{-7}M_{\odot}$ to $30M_{\odot}$, markedly ruling out their dominant presence in the Milky Way halo.
3. Gravitational Waves: Mergers of PBH binaries could produce significant gravitational wave signatures, especially in the context of the LIGO-Virgo detections. The stochastic gravitational wave background adds another layer of restriction.
4. Accretion and Dynamical Effects: PBH interactions with stars and accretion processes exert additional constraints from both theoretical predictions and empirical observations, including CMB power spectra constraints and wide binary dynamics.

Implications and Future Directions

The authors discuss the importance of PBH mass function and clustering—PBHs with a non-monochromatic mass function or formed in clusters could evade current constraints due to different gravitational interactions and observable effects. The paper emphasizes that despite strong constraints, PBHs in the sub-lunar mass range and asteroid mass range remain largely unconstrained by existing observations. Future surveys and space missions offer the potential to explore these regions further.

The theoretical implications extend to inflationary models where the production of PBH-forming density perturbations could be high enough, if non-Gaussianity or specific inflationary dynamics are realized. This coupling of cosmological inflation models with PBH scenarios presents an exciting frontier.

Conclusion

This paper provides an insightful update on the status of PBHs as a plausible dark matter candidate, integrating theoretical advances with a breadth of observational constraints. While certain mass windows are effectively closed, others remain viable for PBHs contributing to the dark matter density. The research suggests that continued advancements in observational capabilities and theoretical models could shed more light on this enduring question in cosmology.