Seven Hints for Primordial Black Hole Dark Matter (1711.10458v2)

Abstract: Seven observations point towards the existence of primordial black holes (PBH), constituting the whole or an important fraction of the dark matter in the Universe: the mass and spin of black holes detected by Advanced LIGO/VIRGO, the detection of micro-lensing events of distant quasars and stars in M31, the non-detection of ultra-faint dwarf satellite galaxies with radius below 15 parsecs, evidences for core galactic dark matter profiles, the correlation between X-ray and infrared cosmic backgrounds, and the existence of super-massive black holes very early in the Universe's history. Some of these hints are newly identified and they are all intriguingly compatible with the re-constructed broad PBH mass distribution from LIGO events, peaking on PBH mass $m_{\rm PBH} \approx 3 M_\odot$ and passing all other constraints on PBH abundances. PBH dark matter also provides a new mechanism to explain the mass-to-light ratios of dwarf galaxies, including the recent detection of a diffuse galaxy not dominated by dark matter. Finally we conjecture that between 0.1% and 1% of the events detected by LIGO will involve a PBH with a mass below the Chandrasekhar mass, which would unambiguously prove the existence of PBH.

• The paper demonstrates seven observational and theoretical hints that support primordial black holes as a dominant dark matter component.
• It utilizes LIGO merger rates, characteristic log-normal mass spectra, and observed low spins to challenge conventional particle dark matter models.
• The study links microlensing events and galactic core profiles to gravitational effects expected from primordial black holes, offering a coherent alternative framework.

Primordial Black Holes as Dark Matter: An Overview of Current Evidence

The paper "Seven Hints for Primordial Black Hole Dark Matter" by Sébastien Clesse and Juan García-Bellido investigates the possibility that primordial black holes (PBHs) serve as the constituents of dark matter, diverging from traditional particle-based models. This exploration is driven by the lack of direct evidence for particle candidates like WIMPs despite extensive searches. Several observations and theoretical predictions, as detailed in this paper, suggest that PBHs might be a viable alternative to account for the 85% of the Universe's matter density that dark matter supposedly represents.

Key Observational and Theoretical Insights

The authors argue for a PBH-dark matter model based on the following observations:

1. Black Hole Mergers and Mass Spectrum: LIGO's detection of black hole mergers with masses inconsistent with stellar origin and the inferred merger rates potentially align with a model of PBH dark matter. The reconstructed PBH mass spectrum, characterized by a log-normal distribution centred around a few solar masses, fits within the allowed range for being the dominant dark matter component.
2. Spin of Black Holes: The observed small spins of black holes from LIGO/Virgo's binary black hole mergers are consistent with what would be expected if PBHs had formed in the early universe, as opposed to arising from stellar origins.
3. Microlensing Events: Observations of microlensing events from stars in M31 and distant quasars suggest a 20% contribution of compact objects to dark matter, which is compatible with PBHs having masses in the sub-solar to solar range.
4. Dwarf Galaxy Dynamics: The absence of small-radius ultra-faint dwarf galaxies (UFDGs) with radii below 15 parsecs, combined with their dark matter profile, correlates with PBH-induced gravitational dynamics.
5. Core Galactic Dark Matter Profiles: The presence of cored rather than cuspy dark matter halo profiles in galaxies mirrors the effects of PBH interactions, where gravitational scattering can homogenize kinetic energy distribution.
6. Correlations in Cosmic Backgrounds: The strong spatial correlations between cosmic infrared and X-ray backgrounds could be attributed to PBHs, which might have influenced early star formation and cosmic reionization processes.
7. Existence of Early Supermassive Black Holes: The formation and observed abundance of supermassive black holes (SMBHs) in the early universe might be explained if PBHs acted as initial seeds, facilitating rapid accretion and growth.

Implications for Cosmology and Astrophysics

The proposal that PBHs constitute a significant fraction of dark matter has profound implications. The PBH model potentially alleviates several standing cosmological conundrums, including the missing satellites problem, the core-cusp issue, and the presence of early SMBHs. Moreover, it offers a coherent framework for understanding dwarf galaxy mass-to-light ratios.

Future Prospects

Despite the compelling nature of these findings, further investigation is required. This includes more detailed analyses of PBH mass distributions, dynamics within galaxy halos, and interactions with baryonic matter. Improved gravitational wave observations could potentially identify PBH characteristics, like sub-Chandrasekhar mass mergers, which would distinctly point to their primordial nature. Enhancements in microlensing surveys and cosmic background measurements will also be critical in testing the PBH hypothesis.

In conclusion, while the paper stops short of declaring PBHs as the definitive solution to the dark matter puzzle, it builds a strong case for them as feasible dark matter candidates. The exploration encourages a re-evaluation of dark matter paradigms and fosters a potential shift in understanding cosmic evolution. The road ahead involves rigorous testing of PBH predictions against forthcoming empirical data, which will either substantiate or constrain this provocative model.