Primordial Black Hole Scenario for the Gravitational-Wave Event GW150914

TL;DR

The paper investigates whether GW150914 can be explained by mergers of primordial black holes with masses around 30 Msun, using the early-Universe PBH binary formation channel to compute the merger rate as a function of the PBH dark-matter fraction $f$. It finds that the rate would be too large if PBHs dominated DM but can match the LIGO range for subdominant PBHs with $f$ around $3×10^{-4}$, linking the explanation to existing cosmological constraints. The work highlights testable predictions, including potential CMB spectral distortions detectable by PIXIE and signatures in the binary mass distribution or stochastic gravitational-wave background, offering a falsifiable bridge between gravitational-wave sources and cosmological PBH constraints. If the PBH scenario is correct, these observations would provide a unique connection between GW observations and early-Universe physics, constraining or measuring the PBH contribution to dark matter.

Abstract

We point out that the gravitational-wave event GW150914 observed by the LIGO detectors can be explained by the coalescence of primordial black holes (PBHs). It is found that the expected PBH merger rate would exceed the rate estimated by the LIGO scientific Collaboration and Virgo Collaboration if PBHs were the dominant component of dark matter, while it can be made compatible if PBHs constitute a fraction of dark matter. Intriguingly, the abundance of PBHs required to explain the suggested lower bound on the event rate, $> 2$ events ${\rm Gpc}^{-3} {\rm yr}^{-1}$, roughly coincides with the existing upper limit set by the nondetection of the cosmic microwave background spectral distortion. This implies that the proposed PBH scenario may be tested in the not-too-distant future.

Figures (1)

• Figure 1: Event rate of mergers of $30~M_\odot-30~M_\odot$ PBH binaries as a function of the PBH fraction in dark matter $f=\Omega_{\rm BH}/\Omega_{\rm DM}$. The red line is the case for $\alpha=\beta=1$, which we have employed throughout the calculations. The blue dotted line is the case for $\alpha=0.4,~\beta=0.8$ suggested in Ioka:1998nz. The event rate estimated by the LIGO-Virgo Collaboration is shown as the shaded region colored orange. The black solid line at $f \approx 3\times 10^{-4}$ is the upper limit on $f$ from the nondetection of the CMB spectral distortion obtained in Ricotti:2007au.