Anisotropic distance ladder in Pantheon+supernovae
Ruairí Mc Conville, Eoin Ó Colgáin
Abstract
We decompose $\mathrm{Pantheon}+$ Type Ia supernovae (SN) in hemispheres on the sky finding angular variations up to $4\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$, corresponding to a statistical significance up to $1.9\ensuremath{\sigma}$, in the Hubble constant ${H}_{0}$ 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. ${H}_{0}$ is larger in a hemisphere encompassing the CMB dipole direction. The variations we see exceed the errors on the recent SH0ES determination, ${H}_{0}=73.04\ifmmode\pm\else\textpm\fi{}1.04\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$, 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.