Lensing by primordial black holes: constraints from gravitational wave observations

TL;DR

This paper investigates how primordial black holes could lens gravitational waves from binary black hole mergers, proposing methods to detect or constrain their abundance based on GW data analysis.

Contribution

It introduces a novel approach to detect or constrain primordial black holes as gravitational wave lenses, providing potential measurements of their masses and improved dark matter fraction limits.

Findings

GW lensing by PBHs can be detected with careful data analysis

Non-detection constrains PBH dark matter fraction to very low levels

Potential to measure PBH masses with relative errors less than 30%

Abstract

Primordial black holes (PBHs) have been proposed to explain at least a portion of dark matter. Observations have put strong constraints on PBHs in terms of the fraction of dark matter which they can represent, $f_{\rm PBH}$, across a wide mass range -- apart from the stellar-mass range of $20M_\odot\lesssim M_{\rm PBH}\lesssim 100M_\odot$. In this paper, we explore the possibility that such PBHs could serve as point-mass lenses capable of altering the gravitational-wave (GW) signals observed from binary black hole (BBH) mergers along their line-of-sight. We find that careful GW data analysis could verify the existence of such PBHs based on the $fitting~factor$ and odds ratio analyses. When such a lensed GW signal is detected, we expect to be able to measure the redshifted mass of the lens with a relative error $\Delta M_{\rm PBH}/M_{\rm PBH}\lesssim0.3$. If no such lensed GW events were detected despite the operation of sensitive GW detectors accumulating large numbers of BBH mergers, it would translate into a stringent constraint of $f_{\rm PBH}\lesssim 10^{-2}-10^{-5}$ for PBHs with a mass larger than $\sim10M_\odot$ by the Einstein Telescope after one year of running, and $f_{\rm PBH}\lesssim 0.2$ for PBHs with mass greater than $\sim 50M_\odot$ for advanced LIGO after ten years of running.