Testing anisotropic Hubble expansion
Paula Boubel, Matthew Colless, Khaled Said, L. Staveley‐Smith
Abstract
Abstract The cosmological principle asserting the large-scale uniformity of the Universe is a testable assumption of the standard cosmological model. We explore the constraints on anisotropic expansion provided by measuring directional variation in the Hubble constant, H 0 , derived from differential zeropoint measurements of the Tully-Fisher distance estimator. We fit various models for directional variation in H 0 using the Tully-Fisher dataset from the all-sky Cosmicflows-4 catalog. The best-fit dipole variation has an amplitude of 0.063 ± 0.016 mag in the direction ( ℓ , b ) = (142 ± 30°,52 ± 10°). If this were due to anisotropic expansion it would imply a 3% variation in H 0 (i.e. Δ H 0 = 2.10 ± 0.53 km s -1 Mpc -1 if H 0 = 70 km s -1 Mpc -1 ) with a significance of 3.9 σ . A model including this H 0 dipole is only weakly favored relative to a model with a constant H 0 and a bulk motion of the volume sampled by Cosmicflows-4 consistent with the standard ΛCDM cosmology. However, we show from simulations that the expected Tully-Fisher data from the WALLABY and DESI surveys should allow detection of a 1% H 0 dipole anisotropy at 5.8 σ confidence and distinguish it from the typical bulk flow predicted by ΛCDM over the volume of these surveys.