Evidence for anisotropy of cosmic acceleration (Colin et al.)

Vol. 631
In section 1. Letters to the Editor

Evidence for anisotropy of cosmic acceleration

by J. Colin, R. Mohayaee, M. Rameez, and S. Sarkar 2019, A&A, 631, L13

In our real Universe, there are “peculiar motions” due to the local inhomogeneity and anisotropy of surrounding structures. The deceleration parameter inferred from previous supernovae (SNe) Ia datasets has a redshift, and indeed directional, dependence. This was interpreted to be indicative of local anisotropy in the matter distribution, that is, our being located in an asymmetric void. In this new work involving a maximum likelihood analysis of the Joint Light-curve Analysis (JLA) catalog of Type Ia supernovae, the authors find that the deceleration parameter, in addition to a small monopole, certainly has a much bigger dipole component that is aligned with the cosmic microwave background (CMB) dipole which falls exponentially with redshift z. As a result, the model-independent evidence for uniform acceleration of the Hubble expansion rate from the largest public catalog of Type Ia supernovae is only 1.4 sigma. This is in contrast to earlier claims that acceleration is established by SNe Ia at > 6 sigma in the framework of the LambdaCDM model. Moreover there is a significant, 3.9 sigma, indication for a dipole component that is aligned with the CMB dipole. This is to be expected if the apparent acceleration is an artefact of our being located in a local bulk flow that extends out far enough to include most of the supernovae studied. This possibility must be taken seriously given the observational evidence that there is no convergence to the CMB frame as far out as redshift z ~ 0.1, which includes half of the known SNe Ia.