Hints of FLRW Breakdown from Supernovae

TL;DR

This paper investigates anisotropies in the Hubble constant using supernova data, revealing a correlation with the CMB dipole and suggesting potential anisotropic expansion of the universe.

Contribution

It provides evidence of directional variation in H_0 from supernova observations, linking it to the CMB dipole, and proposes a method to probe anisotropic cosmic expansion.

Findings

Detected ~1 km/s/Mpc variation in H_0 at antipodal sky points.

Found correlation between higher H_0 and CMB dipole direction.

Observed stronger anisotropic signals at higher redshift supernovae.

Abstract

A 10\% difference in the scale for the Hubble parameter constitutes a clear problem for cosmology. Here, considering angular distribution of Type Ia supernovae (SN) within the Pantheon compilation and working within flat $\Lambda$CDM cosmology, we observe a correlation between higher $H_0$ and the CMB dipole direction, confirming our previous results for strongly-lensed quasars \cite{Krishnan:2021dyb}. Concretely, we record a $\sim 1$ km/s/Mpc variation in $H_0$ at antipodal points on the sky within the Pantheon sample, which is evident in the Low $z$ subsample ($z \lesssim 0.075$) and gets enhanced by higher redshift SN. Our work raises the possibility that we may be at the precision required to probe anisotropic Hubble expansions, while providing a concrete prediction for future inferences of $H_0$.