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