Does small scale structure significantly affect cosmological dynamics?
  The large-scale homogeneity and isotropy of the universe is generally thought
to imply a well defined background cosmological model. It may not. Smoothing
over structure adds in an extra contribution, transferring power from small
scales up to large. Second-order perturbation theory implies that the effect is
small, but suggests that formally the perturbation series may not converge. The
amplitude of the effect is actually determined by the ratio of the Hubble
scales at matter-radiation equality and today - which are entirely unrelated.
This implies that a universe with significantly lower temperature today could
have significant backreaction from more power on small scales, and so provides
the ideal testing ground for understanding backreaction. We investigate this
using two different N-body numerical simulations - a 3D Newtonian and a 1D
simulation which includes all relevant relativistic effects. We show that while
perturbation theory predicts an increasing backreaction as more initial
small-scale power is added, in fact the virialisation of structure saturates
the backreaction effect at the same level independently of the equality scale.
This implies that backreaction is a small effect independently of initial
conditions. Nevertheless, it may still contribute at the percent level to
certain cosmological observables and therefore it cannot be neglected in
precision cosmology.
