Merger rate of primordial black-hole binaries

By Y. Ali-Haimoud, E. Kovetz, Marc Kamionkowski Nyu, Jhu. 2017

324 citations recorded by the source corpus.

Primordial black holes (PBHs) have long been a candidate for the elusive dark matter (DM), and remain poorly constrained in the ~20-100 Msun mass range. PBH binaries were recently suggested as the possible source of LIGO's first detections. In this paper, we thoroughly revisit existing estimates of the merger rate of PBH binaries. We compute the probability distribution of orbital parameters for PBH binaries formed in the early Universe, accounting for tidal torquing by all other PBHs, as well as standard large-scale adiabatic perturbations. We then check whether the orbital parameters of PBH binaries formed in the early Universe can be significantly affected between formation and merger. Our analytic estimates indicate that the tidal field of halos and interactions with other PBHs, as well as dynamical friction by unbound standard DM particles, do not do significant work on nor torque PBH binaries. We estimate the torque due to baryon accretion to be much weaker than previous calculations, albeit possibly large enough to significantly affect the eccentricity of typical PBH binaries. We also revisit the PBH-binary merger rate resulting from gravitational capture in present-day halos, accounting for Poisson fluctuations. If binaries formed in the early Universe survive to the present time, as suggested by our analytic estimates, they dominate the total PBH merger rate. Moreover, this merger rate would be orders of magnitude larger than LIGO's current upper limits if PBHs make a significant fraction of the dark matter. As a consequence, LIGO would constrain ~10-300 Msun PBHs to constitute no more than ~1% of the dark matter. To make this conclusion fully robust, though, numerical study of several complex astrophysical processes - such as the formation of the first PBH halos and how they may affect PBH binaries, as well as the accretion of gas onto an extremely eccentric binary - is needed.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/1bdf52a1e78e19012d534df2a319526e8ae0fbab

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In Astronomy and Astrophysics

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Citations in the corpus

• Two populations of LIGO-Virgo black holes (2020, methodology): “However, the observed merger rate alone constrains the allowed O(30)M PBH DM abundance to be marginal [8, 11, 12].”
• Binary Black Hole Population Properties Inferred from the First and Second Observing Runs of Advanced LIGO and Advanced Virgo (2018, background): “…a primordial BH population in the early universe(Carr & Hawking 1974; Bird et al. 2016; Carr et al. 2016; Inayoshi et al. 2016; Sasaki et al. 2016; Ali-Haïmoud et al. 2017; Clesse & García-Bellido 2017; Inomata et al. 2017; Ando et al. 2018; Chen & Huang 2018), where their mass spectrum is…”
• Primordial Black Holes as Dark Matter: Recent Developments (2020, methodology): “Indeed, the LIGO/Virgo results have already been used to constrain the PBH dark-matter fraction [47], although the limit is sensitive to the predicted merger rate, which is very model-dependent [48].”
• Primordial black holes with an accurate QCD equation of state (2018, background): “1% of DM is made up of PBHs in the LIGO mass range [12, 13, 56, 57] (assuming that the PBHs are not initially clustered).”
• Primordial black holes as dark matter candidates (2021, background): “[173] inferred that the binary merger rate from gravitational capture in present-day halos should be subdominant if binaries formed in the early Universe survive until the present.”