An Isotropy Measurement with Gravitational Wave Observations

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.