A gradient expansion for cosmological backreaction

We address the issue of cosmological backreaction from non-linear structure formation by constructing an approximation for the time evolved metric of a dust dominated universe based on a gradient expansion. Our metric begins as a perturbation of a flat Friedmann-Robertson-Walker state described by a nearly scale invariant, Gaussian, power-law distribution, and evolves in time until non-linear structures have formed. After describing and attempting to control for certain complications in the implementation of this approach, this metric then forms a working model of the universe. We numerically calculate the evolution of the average scale factor in this model and hence the backreaction. We argue that, despite its limitations, this model is more realistic than previous models that have confronted the issue of backreaction. We find that the \emph{instantaneous} effects of backreaction in this model could be as large as $\sim10$ of the background. This suggests that a proper understanding of the \emph{cumulative} effects of backreaction could be crucial for precision cosmology and any future exploration of the dark sector.

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