The clustering of massive Primordial Black Holes as Dark Matter: measuring their mass distribution with Advanced LIGO (1603.05234v2)

Abstract: The recent detection by Advanced LIGO of gravitational waves (GW) from the merging of a binary black hole system sets new limits on the merging rates of massive primordial black holes (PBH) that could be a significant fraction or even the totality of the dark matter in the Universe. aLIGO opens the way to the determination of the distribution and clustering of such massive PBH. If PBH clusters have a similar density to the one observed in ultra-faint dwarf galaxies, we find merging rates comparable to aLIGO expectations. Massive PBH dark matter predicts the existence of thousands of those dwarf galaxies where star formation is unlikely because of gas accretion onto PBH, which would possibly provide a solution to the missing satellite and too-big-to-fail problems. Finally, we study the possibility of using aLIGO and future GW antennas to measure the abundance and mass distribution of PBH in the range [5 - 200] Msun to 10\% accuracy.

• The paper leverages gravitational wave merger rates from Advanced LIGO to constrain massive PBH populations as dark matter.
• It models the clustering of PBHs in compact sub-halos, potentially resolving cosmological puzzles like the missing satellite problem.
• Numerical results suggest a viable PBH mass spectrum of 5-200 solar masses that matches observed merging rates.

Primordial Black Holes as Dark Matter: Implications from Gravitational Wave Observations by Advanced LIGO

This paper explores the hypothesis that massive primordial black holes (PBHs) might constitute a significant portion, or even the totality, of dark matter (DM) in the universe. The paper builds on the detection of gravitational waves by Advanced LIGO, suggesting that these waves could provide empirical data to measure the mass distribution and clustering of such PBHs. The authors propose that PBHs, formed from high-density fluctuations in the early universe, could evade stringent constraints on dark matter if they are massive enough to avoid complete evaporation or observational microlensing constraints.

Numerical Results and Model Parameters

The rate of black hole mergers observed by Advanced LIGO provides new limits on PBH populations. The authors argue that if PBHs cluster similarly to the densities observed in ultra-faint dwarf galaxies, the merging rates would align with those predicted by LIGO. Specifically, they propose that these clusters of PBH, which might also solve the missing satellite and too-big-to-fail problems, would match the densities of substructures where typical star formation is unlikely.

The researchers calculate the potential merging rates by assuming PBH clusters and consider a broad spectrum of PBH masses, specifically in the range of 5 to 200 solar masses. It's suggested that a massive PBH DM model explaining a merging rate of 2 to 400 events per gigaparsec cubed per year could be viable. Numerical estimations of PBH populations indicate that densities comparable to dark matter densities in globular clusters are necessary for matching LIGO observations.

Implications for Dark Matter and Cosmology

This paper has multiple implications:

1. Clustering Model: The requirement that PBHs should cluster in compact sub-halos provides a mechanism for structure formation in early and subsequent cosmic epochs. This clustering could lead to the presence of massive PBHs early enough to seed supermassive black holes observed at galactic centers.
2. Broad PBH Mass Spectrum: Employing a broad mass spectrum allows the model to accommodate a variety of observable outcomes from LIGO and future GW detectors. This spectrum provides a basis for accommodating potential PBH populations while allowing the scenario of massive and less massive PBHs coexisting.
3. Astrophysical and Cosmological Problems: The proposed clustering of PBHs might naturally solve discrepancies like the missing satellite problem, where fewer than expected dwarf satellites are observed compared to $\Lambda$CDM model predictions.

Future Prospects

The extension of gravitational wave astronomy offers promising paths for testing these hypotheses further. As LIGO and other observatories increase their detection rates, it will allow significant refinement of PBH mass distribution and density concentrations. Upcoming missions, including the next generation of LIGO and Virgo-related experiments, could improve sensitivity and detection rates, providing insights into the nature of PBHs and DM composition. If gravitational wave data reveals extensive merging events, this could validate the proposed PBH frameworks within the cosmological landscape.

In conclusion, the paper presents a compelling framework for investigating PBHs as dark matter candidates through gravitational wave observations. This work aligns with the broader cosmological pursuit of understanding dark matter's nature and distribution, potentially resolving enduring cosmological problems through empirical analysis.