Anisotropic distance ladder in Pantheon+supernovae

We decompose Pantheon+ Type Ia supernovae (SN) in hemispheres on the sky finding angular variations up to 4 km/s/Mpc, corresponding to a statistical significance up to 1.9σ, in the Hubble constant H0 both in the SH0ES redshift range 0.0233<z<0.15 and in extended redshift ranges. The variations are driven largely by variations in absolute magnitude from SN in Cepheid hosts but are reinforced by SN in the Hubble flow. H0 is larger in a hemisphere encompassing the CMB dipole direction. The variations we see exceed the errors on the recent SH0ES determination, H0=73.04±1.04 km/s/Mpc, but are not large enough to explain early versus late Universe discrepancies in the Hubble constant. Nevertheless, the Cepheid-SN distance ladder is anisotropic at current precision. The anisotropy may be due to a breakdown in the cosmological principle, or mundanely due to a statistical fluctuation in a small sample of SN in Cepheid host galaxies.