Evidence for lensing of gravitational waves from LIGO-Virgo

Understand

Recently, the LIGO-Virgo Collaboration (LVC) concluded that there is no evidence for lensed gravitational waves (GW) in the first half of the O3 run, claiming "We find the observation of lensed events to be unlikely, with the fractional rate at $\mu>2$ being $3.3\times 10^{-4}$".

• While we agree that the chance of an individual GW event being lensed at $\mu>2$ is smaller than $10^{-3}$, the number of observed events depends on the product of this small probability times the rate of mergers at high redshift.
• Observational constraints from the stochastic GW background indicate that the rate of conventional mass BBH mergers (8 < M (M$_{\odot}$) < 15) in the redshift range 1<z< 2 could be as high as O($10^7$) events per year, more than sufficient to compensate for the intrinsically low probability of lensing.
• To reach the LVC trigger threshold these events require high magnification, but would still produce up to 10 to 30 LVC observable events per year.