Probing the Anisotropic Universe with Gravitational Waves

TL;DR

This paper investigates potential anisotropy in the universe using gravitational wave data, performing hemispherical analyses of H0 measurements to test for directional variations, with current results consistent with isotropy but highlighting the need for more data.

Contribution

First hemispherical decomposition of standard siren H0 measurements to probe universe anisotropy using gravitational waves.

Findings

Maximum H0 in CMB dipole direction for dark sirens

Bright siren data reverses the anisotropy indication

Current data is consistent with an isotropic universe

Abstract

Modern cosmology rests on the working assumption that the Universe is isotropic and homogeneous at large scales. Here, we document a number of anomalous observations pointing to an anisotropic Universe in a direction consistent with the CMB dipole. If physical, this should be confirmed by any cosmological probe with sufficient data quality. To that end, we perform the first hemispherical decomposition of bright and dark standard siren $H_0$ determinations, finding that the maximum of the $H_0$ posterior is larger in the CMB dipole direction for dark sirens, but including the bright siren posterior reverses this conclusion. As may be expected, the limited number of GW observations is consistent with an isotropic Universe, but this can change going forward. It is imperative to repeat these tests as GW sources become more numerous and better localised.