New Constraints on Anisotropic Expansion from Supernovae Type Ia
  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.
