A gradient expansion for cosmological backreaction

Journal of Cosmology and Astroparticle Physics (Impact Factor: 5.81). 12/2011; 2012(03). DOI: 10.1088/1475-7516/2012/03/026
Source: arXiv


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 instantaneous effects of backreaction in this model could be as large as ~ 10% of the background. This suggests that a proper understanding of the cumulative effects of backreaction could be crucial for precision cosmology and any future exploration of the dark sector.

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    • "It as also proved useful in studying the backreaction of cosmic structure formation, see [18] and arXiv:1203.2796v2 [astro-ph.CO] 18 Oct 2012 more recently [19]. In most of past works the gradient expansion has been applied directly on the Einstein equations. "
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    • "Developed in [25] [53] and used in [35] to study backreaction, this technique—contrary to perturbation theory—seems to take better care of the impact of the small-scale nonlinear effects on the larger ones. A recent study [29] has shown that, already for gradient expansion quantities of the 4 th -level, the effects of backreaction grows to 5–10% of the background. A realization of this iterative strategy is the subject of forthcoming work. "
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