A gradient expansion for cosmological backreaction

Abstract

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 $\sim 10$ 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.

32 pages, 8 figures; version2: Updated version with new figure 5 showing Ω_X, indicating more clearly the potential relevance of backreaction. Discussion amended to reflect this. Matches published version

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Cited by in corpus (5)

• Lagrangian theory of structure formation in relativistic cosmology II: average properties of a generic evolution model
• Local and non-local measures of acceleration in cosmology
• Zel'dovich approximation and General Relativity
• Initial conditions for cold dark matter particles and General Relativity
• Relativistic cosmological perturbation scheme on a general background: scalar perturbations for irrotational dust