On the isotropy of SnIa absolute magnitudes in the Pantheon+ and SH0ES samples

TL;DR

This study tests the isotropy of Type Ia supernova absolute magnitudes in Pantheon+ and SH0ES samples across various distances, finding significant anisotropy transitions at small scales that could indicate local inhomogeneities or systematics.

Contribution

It introduces a hemisphere comparison method to analyze supernova data for anisotropy and identifies localized anisotropy transitions at small scales, which are rare in isotropic simulations.

Findings

Anisotropy levels are consistent with isotropic simulations overall.

Significant anisotropy transitions occur below 40 Mpc.

Rare anisotropy transitions suggest local inhomogeneities or systematics.

Abstract

We use the hemisphere comparison method to test the isotropy of the SnIa absolute magnitudes of the Pantheon+ and SH0ES samples in various redshift/distance bins. We compare the identified levels of anisotropy in each bin with Monte-Carlo simulations of corresponding isotropised data to estimate the frequency of such levels of anisotropy in the context of an underlying isotropic cosmological. We find that the identified levels of anisotropy in all bins are consistent with the Monte-Carlo isotropic simulated samples. However, in the real samples for both the Pantheon+ and the SH0ES cases we find sharp changes of the level of anisotropy occuring at distances less than $40Mpc$. For the Pantheon+ sample we find that the redshift bin $[0.005,0.01]$ is significantly more anisotropic than the other 5 redshift bins considered. For the SH0ES sample we find a sharp drop of the anisotropy level at distances larger than about $30Mpc$. These anisotropy transitions are relatively rare in the Monte-Carlo isotropic simulated data and occur in $2\%$ of the SH0ES simulated data and at about $7\%$ of the Pantheon+ isotropic simulated samples. This effect is consistent with the experience of an off center observer in a $30Mpc$ bubble of distinct physics or systematics.