Cosmological backreaction in higher-derivative gravity expansions

Understand

We calculate a general effective stress-energy tensor induced by cosmological inhomogeneity in effective theories of gravity where the action is Taylor-expandable in the Riemann tensor and covariant derivatives of the Riemann tensor.

• This is of interest as an effective fluid that might provide an alternative to the cosmological constant, but it also applies to gravitational waves.
• We use an adaptation of Green and Wald's weak-averaging framework, which averages over perturbations in the field equation where the perturbation length scales are small compared to the averaging scale.
• In this adaptation, the length scale of the effective theory, $1/M$, is also taken to be small compared with the averaging scale.