[Paper Review] GW190425, GW190521 and GW190814: Three candidate mergers of primordial black holes from the QCD epoch

Sébastien Clesse, J. García-Bellido|arXiv (Cornell University)|Jul 13, 2020

Pulsars and Gravitational Waves Research34 citations

TL;DR

The paper argues that the GW190425, GW190521, and GW190814 events can be explained by mergers of primordial black holes with a QCD-epoch imprinted mass distribution, formed via tidal capture in dense halos or early-universe binaries, and discusses consistency with dark matter constraints.

ABSTRACT

The two recent gravitational-wave events GW190425 and GW190814 from the third observing run of LIGO/Virgo have both a companion which is unexpected if originated from a neutron star or a stellar black hole, with masses $[1.6-2.5]~M_\odot$ and $[2.5-2.7]~M_\odot$ and merging rates $ 460^{+1050}_{-360} $ and $ 7^{+16}_{-6}$ events/yr/Gpc$^3$ respectively, at 90\% c.l.. Moreover, the recent event GW190521 has black hole components with masses 67 and $91~M_\odot$, and therefore lies in the so-called pair-instability mass gap, where there should not be direct formation of stellar black holes. The possibility that all of these compact objects are Primordial Black Holes (PBHs) is investigated. The known thermal history of the Universe predicts that PBH formation is boosted at the time of the QCD transition, inducing a peak in their distribution at this particular mass scale, and a bump around $30-50~M_\odot$. We find that the merging rates inferred from GW190425, GW190521 and GW190814 are consistent with PBH binaries formed by capture in dense halos in the matter era or in the early universe. At the same time, the rate of black hole mergers around $30~M_\odot$ and of sub-solar PBH mergers do not exceed the LIGO/Virgo limits. Such PBHs could explain a significant fraction, or even the totality of the Dark Matter, but they must be sufficiently strongly clustered in order to be consistent with current astrophysical limits.

Motivation & Objective

• Motivate the PBH scenario as an explanation for unexpected GW events with masses in the lower and upper mass gaps.
• Show how a QCD-era PBH mass distribution features a proton-peak (~2–3 Msun) and a pion-bump (~30–50 Msun) and how this matches observed masses.
• Assess whether PBH binary formation channels (tidal capture in dense halos vs early-universe binaries) can reproduce the observed merger rates without violating existing constraints.
• Discuss implications for PBH dark matter scenarios and clustering requirements to satisfy microlensing and other limits.

Proposed method

• Adopt a PBH mass distribution imprinted by the QCD transition with a proton-peak at 2–3 Msun and a pion-bump at 30–50 Msun.
• Compute the merger rate distribution for PBH binaries formed by tidal capture in dense halos: dτ/dln m1 dln m2 = R_clust. f(m1) f(m2) (m1+m2)^{10/7}/(m1 m2)^{5/7} yr^{-1} Gpc^{-3}.
• Explore early-Universe binary formation via Poisson-distributed proximity at formation: dτ/dln m1 dln m2 ≈ 1.6×10^6 Gpc^{-3} yr^{-1} f(m1) f(m2) f_sup × [(m1+m2)/Msun]^{-32/37}[(m1 m2)/(m1+m2)^2]^{-34/37} with suppression factors.
• Introduce suppression factors f_sup and f_PBH to account for binary disruption by matter fluctuations, nearby PBHs, and PBH clusters, and discuss their mass- and f_PBH-dependence.
• Compare predicted rates with GW190425, GW190814, and GW190521 fits and with LIGO/Virgo limits to evaluate viability of PBH scenario.

Experimental results

Research questions

• RQ1 Can a QCD-transition-imprinted PBH mass function explain the masses of GW190425, GW190814, and GW190521?
• RQ2 Do PBH binaries formed by tidal capture in dense halos or by early-Universe formation produce merger rates compatible with LIGO/Virgo observations and limits?
• RQ3 What clustering and suppression factors are required for PBHs to remain consistent with microlensing and other astrophysical constraints?
• RQ4 To what extent can PBHs constitute all or a significant fraction of dark matter without violating current bounds?

Key findings

• The PBH mass distribution with a QCD proton-peak and a pion-bump yields component masses compatible with GW190425, GW190814, and GW190521.
• Merging rates from PBH binaries formed in dense halos can be around 20 yr^{-1} Gpc^{-3} for primary masses >5 Msun and q>0.2, consistent with GW190425, GW190521, GW190814 within 90% c.l. when clustering factor R_clust ~ 400–450.
• PBHs could explain a significant fraction or all of dark matter only if sufficiently strongly clustered to evade microlensing and other limits.
• Early-Universe binaries alone cannot explain all three events and large-mass rates without additional clustering or channel considerations.
• Sub-solar PBH mergers are compatible with current limits, with predicted rates up to O(10^3) yr^{-1} Gpc^{-3} depending on mass, but remain consistent with observational bounds when extended mass functions and clustering are considered.