Cosmological backreaction from perturbations

We write the averaged Einstein equations in a form suitable for use with Newtonian gauge linear perturbation theory and track the size of the modi. cations to standard Robertson-Walker evolution on the largest scales as a function of redshift for both Einstein-de Sitter and Lambda CDM cosmologies. In both cases the effective energy density arising from linear perturbations is of the order of 10(-5) times the matter density, as would be expected, with an effective equation of state w(eff) approximate to -1/19. Employing a modified Halofit code to extend our results to quasilinear scales, we find that, while larger, the deviations from Robertson-Walker behaviour remain of the order of 10(-5).