Cosmological backreaction has been suggested as an explanation of dark energy and is heavily disputed since. We combine cosmological perturbation theory with Buchert’s nonperturbative framework, calculate the relevant averaged observables up to second order in the comoving synchronous gauge, and discuss their gauge dependence. With the help of an integrability condition, the leading second order contributions follow from the first order calculation. We focus on the onset of cosmological backreaction, as a perturbative treatment is necessarily restricted to the era when the effect is still small. We demonstrate that the leading contributions to all averaged physical observables are completely specified on the boundary of the averaged domain. For any finite domain, these surface terms are nonzero in general and thus backreaction is for real. We map the backreaction effect on an effectively homogeneous and isotropic fluid. The generic effective equation of state is not only time dependent, but also depends on scale.

• Received 22 June 2007

DOI:https://doi.org/10.1103/PhysRevD.76.083011