Dark energy from inhomogeneities in R^2 gravity

We study the effective stress-energy tensor induced by cosmological inhomogeneity in Einstein gravity plus curvature-squared correction, as it appears in the Starobinsky model of early universe inflation. We use Green and Wald's mathematically precise framework for averaging over classical fluctuations of short-wavelength. By ensuring that the leading non-linear terms from the fluctuations of the Einstein terms and the corrections both contribute in a certain ``weak'' limit, we derive a diffeomorphism invariant effective stress-energy tensor whose trace is non-vanishing and of the right sign to potentially account for the current acceleration of the universe. Moreover, we show that novel inhomogeneities in the dark sector would provide the right magnitude if the fundamental scale for these inhomogeneities is that of the weak interactions.

Organised by Alfio Bonanno