Searching for anisotropy in the distribution of binary black hole mergers

TL;DR

This paper investigates the distribution of binary black hole mergers detected by gravitational waves to test for anisotropy, finding current data consistent with isotropy and projecting future sensitivity improvements.

Contribution

It introduces a spherical harmonic analysis of gravitational wave events to assess anisotropy, providing the first such constraints and future detection prospects.

Findings

Current binary black hole mergers are consistent with isotropy.

Future third-generation detectors will significantly improve anisotropy sensitivity.

Projected sensitivity could detect anisotropies at levels below 0.1%.

Abstract

The standard model of cosmology is underpinned by the assumption of the statistical isotropy of the Universe. Observations of the cosmic microwave background, galaxy distributions, and supernovae, among other media, support the assumption of isotropy at scales $\gtrsim 100$\,Mpc. The recent detections of gravitational waves from merging stellar-mass binary black holes provide a new probe of anisotropy; complementary and independent of all other probes of the matter distribution in the Universe. We present an analysis using a spherical harmonic model to determine the level of anisotropy in the first LIGO/Virgo transient catalog. We find that the ten binary black hole mergers within the first transient catalog are consistent with an isotropic distribution. We carry out a study of simulated events to assess the prospects for future probes of anisotropy. Within a single year of operation, third-generation gravitational-wave observatories will probe anisotropies with an angular scale of $\sim36^\circ$ at the level of $\lesssim0.1\%$.