Anisotropy in Pantheon+ supernovae

TL;DR

This study detects significant dipolar anisotropies in the local universe's expansion rate using Pantheon+ supernova data, challenging the assumption of isotropy and suggesting local bulk flows influence cosmic acceleration measurements.

Contribution

It introduces a maximum likelihood analysis of supernova data across multiple frames revealing anisotropies in the Hubble and deceleration parameters, indicating local effects impact cosmological inferences.

Findings

Significant dipolar variation in Hubble rate exceeding 1.5 km/s/Mpc.

Redshift-dependent dipolar modulation of the deceleration parameter at > 5 sigma.

Cosmic acceleration likely influenced by local bulk flows, not just a cosmological constant.

Abstract

We employ Maximum Likelihood Estimators to examine the Pantheon+ catalogue of Type Ia supernovae for large scale anisotropies in the expansion rate of the Universe. The analyses are carried out in the heliocentric frame, the CMB frame, as well as the Local Group frame. In all frames, the Hubble expansion rate in the redshift range 0.023 < z < 0.15 is found to have a statistically significant dipolar variation exceeding 1.5 km/s/Mpc, i.e. bigger than the claimed 1% uncertainty in the SH0ES measurement of the Hubble parameter H_0. The deceleration parameter too has a redshift-dependent dipolar modulation at > 5 sigma significance, consistent with previous findings using the SDSSII/SNLS3 Joint Lightcurve Analysis catalogue. The inferred cosmic acceleration cannot therefore be due to a Cosmological Constant, but is likely a general relativistic effect due to the anomalous bulk flow in our local Universe.