Primordial Black Holes as Dark Matter: Recent Developments (2006.02838v3)

Abstract: Although the dark matter is usually assumed to be some form of elementary particle, primordial black holes (PBHs) could also provide some of it. However, various constraints restrict the possible mass windows to $10^{16}$ - $10^{17}\,$g, $10^{20}$ - $10^{24}\,$g and $10$ - $10^{3}\,M_{\odot}$. The last possibility is contentious but of special interest in view of the recent detection of black-hole mergers by LIGO/Virgo. PBHs might have important consequences and resolve various cosmological conundra even if they have only a small fraction of the dark-matter density. In particular, those larger than $10^{3}\,M_{\odot}$ could generate cosmological structures through the seed or Poisson effect, thereby alleviating some problems associated with the standard cold dark-matter scenario, and sufficiently large PBHs might provide seeds for the supermassive black holes in galactic nuclei. More exotically, the Planck-mass relics of PBH evaporations or stupendously large black holes bigger than $10^{12}\,M_{\odot}$ could provide an interesting dark component.

• The paper demonstrates that PBHs in specific mass ranges, from asteroid to intermediate levels, may satisfy dark matter constraints while aligning with observational limits.
• It employs varied astrophysical data—including microlensing, gamma-ray, and gravitational-wave observations—to critically evaluate the feasibility of primordial black holes as dark matter candidates.
• The study emphasizes that an extended PBH mass function, supported by recent LIGO/Virgo findings, could reconcile theoretical predictions with diverse cosmological observations.

An Examination of "Primordial Black Holes as Dark Matter: Recent Developments"

The paper by Bernard Carr and Florian Kühnel explores the hypothesis that primordial black holes (PBHs) could constitute a significant component of dark matter. This idea presents an alternative to the predominant assumption that dark matter is made up of undetected elementary particles. The authors' comprehensive review encompasses constraints on this hypothesis, potential cosmological impacts, and recent observations that might support the presence of PBHs.

Key Constraints and Potential Mass Windows

The authors outline the mass ranges where PBHs could potentially account for a sizeable portion of dark matter while satisfying observational constraints:

1. Asteroid Mass Range ($10^{16} - 10^{17}$ g): Constraints from microlensing and dynamical effects place limits on PBHs in this range. However, the faintness of their gravitational impact allows the hypothesis that they could still account for all the dark matter in this mass range.
2. Sublunar Mass Range ($10^{20} - 10^{24}$ g): Observational limits are weaker here, making it plausible for PBHs to contribute significantly if not entirely to dark matter.
3. Intermediate Mass Range ($10 - 10^3\,M_{\odot}$): This range has been of particular interest due to connections with LIGO/Virgo detections of black-hole mergers. While some constraints are in place, an extended PBH mass function may still allow a considerable contribution.
4. Supermassive Range ($10^5 - 10^9\,M_{\odot}$): Though not a region where PBHs could serve as dark matter, these massive objects can seed the formation of supermassive black holes observed in galactic centers.

Each mass window presents unique implications, requiring PBHs to satisfy various astrophysical observations and constraints, such as microlensing, gamma-ray emissions, and gravitational-wave backgrounds.

Implications and Theoretical Considerations

Theoretical models suggest that PBHs could arise from fluctuations in the early Universe, possibly due to phenomena like the QCD phase transition. This idea is compelling as it naturally ties the existence of PBHs to the known thermal history of the Universe, suggesting specific epochs more conducive to PBH formation.

In the intermediate mass range, PBHs offer potential explanations for the properties of gravitational-wave sources observed by LIGO/Virgo. Their presence could also impact galaxy formation via seeding supermassive black holes or influencing early structure formation through the Poisson effect or direct seeding.

Observational Support and Future Developments

Recent detection of gravitational waves from black-hole mergers revived interest in the PBH hypothesis, especially considering the mass range overlaps with that of potential PBHs. Despite this, the feasibility of PBHs constituting a significant dark matter component relies heavily on extended mass functions enabling them to escape existing constraints.

Future observational advancements might clarify PBH roles in cosmology, particularly through enhanced gravitational-wave observatories, stronger gamma-ray constraints, and improved microlensing surveys. An interdisciplinary approach employing both theoretical modeling and observational efforts holds promise for validating or refuting the hypothesis that PBHs serve as a dark matter component.

Conclusion

The paper advocates for a nuanced examination of primordial black holes as dark matter candidates, stressing the notion that a comprehensive view of their potential mass spectrum and formation history is imperative. While many challenges and constraints lie in the hypothesis's path, continued exploration might elucidate the intriguing prospect that PBHs play a cosmologically significant role, either alongside or in competition with dark matter particles.