Testing anisotropic Hubble expansion

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, H0, derived from differential zeropoint measurements of the Tully-Fisher distance estimator. We fit various models for directional variation in H0 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 H0 (i.e. ΔH0 = 2.10 ± 0.53 km s-1 Mpc-1 if H0 = 70 km s-1 Mpc-1) with a significance of 3.9σ. A model including this H0 dipole is only weakly favored relative to a model with a constant H0 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% H0 dipole anisotropy at 5.8σ confidence and distinguish it from the typical bulk flow predicted by ΛCDM over the volume of these surveys.