Late time anisotropy as an imprint of cosmological backreaction

Backreaction effects of the large-scale structure on the background dynamics have been claimed to lead to a renormalization of the background dynamics that may account for the late time acceleration of the cosmic expansion. This article emphasizes that generically the averaged flow is locally anisotropic, a property that can be related to observation. Focusing on perturbation theory, the spatially averaged shear, that characterizes the anisotropy of the flow, is computed. It is shown that this shear arising from backreaction differs from a homogeneous shear: its time evolution is different, and its amplitude is completely determined by the cosmological parameters and the matter power spectrum. It ranges within 2% and 37% at a redshift of order 0.5 so that the isotropy of the Hubble flow may allow us to constrain the backreaction approach to dark energy.