Has LIGO Detected Primordial Black Hole Dark Matter? -- Tidal Disruption in Binary Black Hole Formation

TL;DR

This paper investigates whether primordial black holes could account for dark matter by analyzing tidal effects in binary formation, concluding that current constraints limit their contribution to less than one percent, favoring alternative dark matter candidates.

Contribution

It introduces the concept of the loss zone to analyze tidal effects and demonstrates that primordial black holes are unlikely to be the dominant dark matter component based on LIGO data.

Findings

Tidal effects are not significant in binary formation for PBHs.

Microlensing constraints limit PBH dark matter fraction to below 1%.

Early-universe binary formation scenarios are more consistent with observations.

Abstract

The frequent detection of binary mergers of $\sim 30 M_{\odot}$ black holes (BHs) by the Laser Interferometer Gravitational-Wave Observatory (LIGO) rekindled researchers' interest in primordial BHs (PBHs) being dark matter (DM). In this work, we looked at PBHs distributed as DM with a monochromatic mass of $30 M_{\odot}$ and examined the encounter-capture scenario of binary formation, where the densest central region of DM halo dominates. Thus, we paid special attention to the tidal effect by the supermassive black hole (SMBH) present. In doing so, we discovered a necessary tool called loss zone that complements the usage of loss cone. We found that the tidal effect is not prominent in affecting binary formation, which also turned out insufficient in explaining the totality of LIGO's event rate estimation, especially due to a microlensing event constraining the DM fraction in PBH at the mass of interest from near unity to an order smaller. Meanwhile, early-universe binary formation scenario proves so prevailing that the LIGO signal in turn constrains the PBH fraction below one percent. Thus, people should put more faith in alternative PBH windows and other DM candidates.