Percent-Level Test of Isotropic Expansion Using Type Ia Supernovae

Understand

We propose and implement a novel, robust, and non-parametric test of statistical isotropy of the expansion of the universe, and apply it to around one thousand type Ia supernovae from the Pantheon sample.

• We calculate the angular clustering of supernova magnitude residuals and compare it to the noise expected under the isotropic assumption.
• We also test for systematic effects and demonstrate that their effects are negligible or are already accounted for in our procedure.
• We express our constraints as an upper limit on the rms spatial variation in the Hubble parameter at late times.