Searching for a subpopulation of primordial black holes in LIGO/Virgo gravitational-wave data

TL;DR

This study investigates the presence of primordial black holes in LIGO/Virgo gravitational-wave data using hierarchical Bayesian analysis, comparing astrophysical and primordial formation models to identify potential subpopulations.

Contribution

It introduces a novel hierarchical Bayesian framework that combines multiple formation models with primordial black hole populations to constrain their fraction in gravitational-wave data.

Findings

Primordial black holes are statistically favored over some astrophysical models.

A dominant stable-mass-transfer formation channel reduces the need for primordial black holes.

Further data and reduced uncertainties are needed to confirm primordial black hole contributions.

Abstract

With several dozen binary black hole events detected by LIGO/Virgo to date and many more expected in the next few years, gravitational-wave astronomy is shifting from individual-event analyses to population studies. Using the GWTC-2 catalog, we perform a hierarchical Bayesian analysis that for the first time combines several state-of-the-art astrophysical formation models with a population of primordial black holes (PBHs) and constrains the fraction of a putative subpopulation of PBHs in the data. We find that this fraction depends significantly on the set of assumed astrophysical models. While a primordial population is statistically favored against certain competitive astrophysical channels, such as globular clusters and nuclear stellar clusters, a dominant contribution from the stable-mass-transfer isolated formation channel drastically reduces the need for PBHs, except for explaining the rate of mass-gap events like GW190521. The tantalizing possibility that black holes formed after inflation are contributing to LIGO/Virgo observations could only be verified by further reducing uncertainties in astrophysical and primordial formation models, and it may ultimately be confirmed by third-generation interferometers.