Evidence for anisotropy of cosmic acceleration

Observations reveal a “bulk flow” in the local Universe which is faster and extends to much larger scales than are expected around a typical observer in the standard ΛCDM 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 Light-curve Analysis 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 cosmic microwave background dipole, which falls exponentially with redshift z : q 0 = q m + q d . n̂ exp(- z / S ). The best fit to data yields q d = −8.03 and S = 0.0262 (⇒ d ∼ 100 Mpc), rejecting isotropy ( q d = 0) with 3.9 σ statistical significance, while q m = −0.157 and consistent with no acceleration ( q m = 0) at 1.4 σ . 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.

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