New Constraints on Anisotropic Expansion from Supernovae Type Ia

TL;DR

This study uses improved peculiar velocity models and advanced statistical methods to test for anisotropic expansion with Type Ia supernovae data, finding no evidence for anisotropy and supporting isotropic acceleration.

Contribution

It introduces a novel peculiar velocity correction method and Bayesian hierarchical modeling to constrain anisotropic expansion using supernova data.

Findings

No evidence for anisotropic expansion was found.

Tight upper bounds on dipole amplitude were established.

An accelerating universe is strongly favored over a decelerating one.

Abstract

We re-examine the contentious question of constraints on anisotropic expansion from Type Ia supernovae (SNIa) in the light of a novel determination of peculiar velocities, which are crucial to test isotropy with supernovae out to distances $\lesssim 200/h$ Mpc. We re-analyze the Joint Light-Curve Analysis (JLA) Supernovae (SNe) data, improving on previous treatments of peculiar velocity corrections and their uncertainties (both statistical and systematic) by adopting state-of-the-art flow models constrained independently via the 2M$++$ galaxy redshift compilation. We also introduce a novel procedure to account for colour-based selection effects, and adjust the redshift of low-$z$ SNe self-consistently in the light of our improved peculiar velocity model. We adopt the Bayesian hierarchical model \texttt{BAHAMAS} to constrain a dipole in the distance modulus in the context of the $\Lambda$CDM model and the deceleration parameter in a phenomenological Cosmographic expansion. We do not find any evidence for anisotropic expansion, and place a tight upper bound on the amplitude of a dipole, $|D_\mu| < 5.93 \times 10^{-4}$ (95\% credible interval) in a $\Lambda$CDM setting, and $|D_{q_0}| < 6.29 \times 10^{-2}$ in the Cosmographic expansion approach. Using Bayesian model comparison, we obtain posterior odds in excess of 900:1 (640:1) against a constant-in-redshift dipole for $\Lambda$CDM (the Cosmographic expansion). In the isotropic case, an accelerating universe is favoured with odds of $\sim 1100:1$ with respect to a decelerating one.