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.
