The Effective Field Theory of Dark Energy

TL;DR

This paper develops a comprehensive effective field theory framework for dark energy and modified gravity, unifying various models and analyzing their perturbations, stability, and observational signatures.

Contribution

It introduces a universal EFT formalism for single-field dark energy and modified gravity models, extending previous inflationary approaches to cosmology.

Findings

The formalism simplifies background evolution to three operators.

It identifies key operators affecting scalar-tensor theories.

The paper demonstrates stability and sound speed properties of models.

Abstract

We propose a universal description of dark energy and modified gravity that includes all single-field models. By extending a formalism previously applied to inflation, we consider the metric universally coupled to matter fields and we write in terms of it the most general unitary gauge action consistent with the residual unbroken symmetries of spatial diffeomorphisms. Our action is particularly suited for cosmological perturbation theory: the background evolution depends on only three operators. All other operators start at least at quadratic order in the perturbations and their effects can be studied independently and systematically. In particular, we focus on the properties of a few operators which appear in non-minimally coupled scalar-tensor gravity and galileon theories. In this context, we study the mixing between gravity and the scalar degree of freedom. We assess the quantum and classical stability, derive the speed of sound of fluctuations and the renormalization of the Newton constant. The scalar can always be de-mixed from gravity at quadratic order in the perturbations, but not necessarily through a conformal rescaling of the metric. We show how to express covariant field-operators in our formalism and give several explicit examples of dark energy and modified gravity models in our language. Finally, we discuss the relation with the covariant EFT methods recently appeared in the literature.