New Constraints on Anisotropic Expansion from Supernovae Type Ia

• Published in: arXiv.org 2022-04
• Main Authors: Rahman, W, Trotta, R, Boruah, S S, Hudson, M J, van Dyk, D A
• Format: Article
• Language: English
• Subjects: Astronomical models; Bayesian analysis; Constraints; Deceleration; Dipoles; Galaxies; Isotropy; Red shift; Supernovae; Upper bounds
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2108.12497

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.