# Percent-Level Test of Isotropic Expansion Using Type Ia Supernovae

## TL;DR

This paper introduces a new non-parametric method to test the isotropy of the universe's expansion using Type Ia supernovae, setting a stringent upper limit on spatial variations in the Hubble parameter.

## Contribution

It presents a novel, robust statistical test for isotropy based on supernova data, improving constraints on spatial variations in cosmic expansion.

## Key findings

- Less than 1% rms spatial variation in Hubble parameter at 99.7% confidence
- The method effectively accounts for systematic effects
- Provides a new upper limit on isotropy deviations

## Abstract

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. For the sky smoothed with a Gaussian with fwhm=60 deg, less than 1% rms spatial variation in the Hubble parameter is allowed at 99.7% confidence.