Evidence for anisotropy of cosmic acceleration

Abstract

Observations reveal a `bulk flow' in the local Universe which is faster and extends to much larger scales than is expected around a typical observer in the standard $\Lambda$CDM cosmology. This is expected to result in a scale-dependent dipolar modulation of the acceleration of the expansion rate inferred from observations of objects within the bulk flow. From a maximum-likelihood analysis of the Joint Lightcurve Analysis (JLA) catalogue of Type Ia supernovae we find that the deceleration parameter, in addition to a small monopole, indeed has a much bigger dipole component aligned with the CMB dipole which falls exponentially with redshift $z$: $q_0=q_{\mathrm{m}}+{\vec{q}}_{\mathrm{d}}.\hat{n}\exp (-z/S)$. The best fit to data yields $q_{\mathrm{d}}=-8.03$ and $S=0.0262(\Rightarrow d\sim 100\mathrm{Mpc})$, rejecting isotropy ($q_{\mathrm{d}}=0$) with $3.9\sigma$ statistical significance, while $q_{\mathrm{m}}=-0.157$ and consistent with no acceleration ($q_{\mathrm{m}}=0$) at $1.4\sigma$. Thus the cosmic acceleration deduced from supernovae may be an artefact of our being non-Copernican observers, rather than evidence for a dominant component of `dark energy' in the Universe.

7 pages, 4 figures, 4 tables; Title changed, Bayesian information criterion values added, conclusions strengthened; Results can be reproduced using code at: https://github.com/rameez3333/Dipole_JLA ; Accepted for publication in Astronomy & Astrophysics Letters