Cosmological backreaction in higher-derivative gravity expansions

TL;DR

This paper investigates how inhomogeneities in the universe influence its large-scale dynamics within higher-derivative gravity theories, revealing that such effects can be significant even in the late universe and may hint at new physics.

Contribution

It extends Green and Wald's averaging framework to higher-derivative gravity, deriving an effective stress-energy tensor that accounts for cosmological backreaction effects.

Findings

Backreaction remains significant in the late universe with high-frequency perturbations.

Higher-derivative terms can produce an effective fluid potentially explaining cosmic acceleration.

The approach links cosmological backreaction to ultraviolet particle physics phenomena.

Abstract

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. This ensures that the perturbation length scales remain in fixed proportion to the length scale of the effective theory as the cosmological averaging scale is taken to be large. We find that backreaction from higher-derivative terms in the effective action can continue to be important in the late universe, given a source of sufficiently high-frequency metric perturbations. This backreaction might also provide a window on exotic particle physics in the far ultraviolet.