Redshift Dependence of H 0 Dipole in Pantheon+ Supernovae

We examine the dipole structure in the local cosmic expansion rate H 0 using the Pantheon+ compilation of Type Ia supernovae. We reconstruct directional maps of H 0 across the sky within the low-redshift regime, z min < z < 0.2, evaluated in the CMB rest frame. To perform a tomographic analysis, we choose 10 different values for z min in [0.015, 0.045] range. We find a dipole amplitude A dip=1.16 plus or minus0.28 km/s/Mpc for the lowest bin, monotonically decreasing to A dip =0.35 plus or minus0.52 km/s/Mpc, as we increase z min. Since A dip is positive-definite, by construction A dip > 0 even in an isotropic universe, thus to assess the statistical significance, we compare to A dip generated from 1000 Monte Carlo simulations based on the CDM model and the same redshift ranges. Our results reveal a 2-3 sigma dipole pattern (depending on how the significance level is computed) for z min 0.032. For redshift thresholds where the signal remains statistically significant (>2 sigma), the inferred dipole direction points close to Shapley supercluster and CMB dipole directions. As z min increases, the dipole amplitude diminishes in line with expectations of a higher redshift isotropic Universe, but the difference in maximal antipodal H 0 determinations, H 0 max, retain a signal of an anisotropy that disappears in the dipole ansatz. The decreasing statistical significance of all estimators with increasing z min suggests that any H 0 dipole is a low-redshift feature.