Consistency of Pantheon+ supernovae with a large-scale isotropic universe

TL;DR

This study tests the isotropy of the universe using Pantheon+ supernova data, finding no significant large-scale anisotropy and confirming consistency with an isotropic universe, especially at higher redshifts.

Contribution

It provides the first comprehensive analysis of Pantheon+ supernovae for anisotropy, showing the universe's large-scale isotropy and characterizing low-redshift anisotropic signals.

Findings

Full Pantheon+ data is consistent with isotropy.

A dipole anisotropy appears at low redshift ($z_c \,\leq\, 0.1$).

The low-redshift dipole is stable and not due to local motion.

Abstract

We investigate the possible anisotropy of the universe using the most up-to-date type Ia supernovae, i.e. the Pantheon+ compilation. We fit the full Pantheon+ data with the dipole-modulated $\Lambda$CDM model, and find that it is well consistent with a null dipole. We further divide the full sample into several subsamples with different high-redshift cutoff $z_c$. It is shown that the dipole appears at $2\sigma$ confidence level only if $z_c\leq 0.1$, and in this redshift region the dipole is very stable, almost independent of the specific value of $z_c$. For $z_c=0.1$, the dipole amplitude is $D=1.0_{-0.4}^{+0.4}\times 10^{-3}$, pointing towards $(l,b)=(334.5_{\ -21.6^{\circ}}^{\circ +25.7^{\circ}},16.0_{\ -16.8^{\circ}}^{\circ +27.1^{\circ}})$, which is about $65^{\circ}$ away from the CMB dipole. This implies that the full Pantheon+ is consistent with a large-scale isotropic universe, but the low-redshift anisotropy couldn't be purely explained by the peculiar motion of the local universe.