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

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 $z_c\le 0.1$, 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.

11 pages, 6 figures

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• Testing the cosmological principle on gigaparsec scales
• Revisiting the equation of state of dark energy from DESI BAO with SNe Ia and CMB
• An effective $\mathbf{\Lambda }$-Szekeres modelling of the local Universe with Cosmicflows-4