An Isotropy Measurement with Gravitational Wave Observations

TL;DR

This paper uses gravitational wave data from the third LVK observing run to measure the anisotropy of merging compact binaries across the sky, providing the tightest constraints to date on their distribution.

Contribution

It introduces a method to constrain the anisotropy of binary merger rates using GW data, improving previous limits and analyzing the distribution's isotropy with new observational data.

Findings

Constrained the variability of merger rate density to less than 16% at 90% confidence.

Tightened anisotropy constraints to less than 3.5% with larger correlation length.

Found no strong evidence against isotropy in the distribution of mergers.

Abstract

We constrain the distribution of merging compact binaries across the celestial sphere using the GWTC-3 catalog from the LIGO-Virgo-KAGRA Collaborations' (LVK) third observing run. With 63 confident detections from O3, we constrain the relative variability (standard deviation) of the rate density across the sky to be $\lesssim 16\%$ at 90\% confidence assuming the logarithm of the rate density is described by a Gaussian random field with correlation length $\geq 10^\circ$. This tightens to $\lesssim 3.5\%$ when the correlation length is $\geq 20^\circ$. While the new O3 data provides the tightest constraints on anisotropies available to-date, we do not find overwhelming evidence in favor of isotropy, either. A simple counting experiment favors an isotropic distribution by a factor of $\mathcal{B}^\mathrm{iso}_\mathrm{ani} = 3.7$, which is nonetheless an improvement of more than a factor of two compared to analogous analyses based on only the first and second observing runs of the LVK.