Did LIGO detect dark matter? (1603.00464v2)

Abstract: We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses $20\,M_\odot \lesssim M_{\rm bh} \lesssim 100\, M_\odot$ where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will rapidly spiral inward due to emission of gravitational radiation and ultimately merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the $2-53$ Gpc$^{-3}$ yr$^{-1}$ rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have no optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next generation experiments will be invaluable in performing these tests.

• The paper demonstrates that mergers of primordial black holes via gravitational wave emission could explain LIGO’s observed black hole merger rates.
• The study employs NFW dark matter profiles and halo mass functions to derive conservative merger rate estimates that align with observed data.
• The research suggests that confirming PBH merger signatures could redefine dark matter models and influence future gravitational wave observations.

Analyzing the Potential Detection of Primordial Black Hole Dark Matter by LIGO

The paper "Did LIGO detect dark matter?" explores an innovative hypothesis regarding the possibility that the black holes detected by LIGO could be primordial black holes (PBHs), potentially comprising dark matter. The paper focuses on black hole masses within the range of 20 to 100 solar masses, a range not entirely dismissed by current astrophysical constraints, suggesting that PBHs may serve as viable dark matter candidates.

Key Hypothesis and Methodology

The researchers propose that pairs of primordial black holes within galactic halos might become gravitationally bound after radiating energy via gravitational waves during close encounters. These bounded systems would lose energy rapidly through continued gravitational wave emission, eventually leading to a merger, observable by LIGO as seen in the celebrated GW150914 event.

Estimating Merger Rates

The crux of the paper lies in calculating whether the rate of such PBH mergers aligns with the rates LIGO has observed for similarly-massed black hole binaries. Initially, the authors use conservative estimates to evaluate expected merger rates, incorporating density, velocity dispersion, and spatial distribution of dark matter within halos. Despite inherent uncertainties in phase-space structures, estimates closely align with LIGO's observed range of 2-53 Gpc$^{-3}$ yr$^{-1}$ for black hole merger rates, sparking the notion that events like GW150914 could indeed be manifestations of PBH mergers.

Detailed Analysis

For a more refined calculation, the paper employs NFW profiles for dark matter density and several halo mass functions. Concentration-mass relations derived from simulations guide these assessments to estimate the merger rates across different halo masses. The research acknowledges that smaller halos, characterized by higher density and lower velocity dispersion, could substantially enhance merger rates due to gravitational capture dynamics prevalent in such environments.

Observational Implications

The prospect of identifying dark matter through PBH mergers has profound implications for astrophysics and cosmology. Differences in spatial distribution, ellipticity, and lack of electromagnetic or neutrino counterparts distinguish PBH mergers from those originating in stellar environments. Future gravitational wave observatories could validate these distinctions, employing mass spectrum discrepancies and ellipticity in waveforms.

The potential realization of PBHs as dark matter candidates would precipitate a paradigm shift in dark matter research, demanding a reinterpretation of its cosmic role and distribution. Expansions in gravitational wave observational bandwidth by future missions such as the Einstein Telescope and DECIGO could offer pivotal insights into the spectral properties and masses of these primordial entities.

Conclusion

The work articulates a thought-provoking argument for PBHs as significant dark matter constituents, grounded in the interpretation of LIGO observations. If corroborated by further gravitational wave analyses, this research could point to a substantial fraction of dark matter existing as primordial black holes, redefining current cosmological models. Future detections with refined instruments could further reinforce or challenge this emerging hypothesis, underpinning the evolving narrative of dark matter research.