Dark Energy or Modified Gravity? An Effective Field Theory Approach

TL;DR

This paper develops an Effective Field Theory framework for analyzing cosmic acceleration, unifying dark energy and modified gravity models, and providing tools to connect theoretical parameters with cosmological observations.

Contribution

It extends the EFT approach to cosmological perturbations, enabling model-independent constraints on dark energy and modified gravity using observational data.

Findings

Formulated general EFT of cosmological perturbations around a Lambda-CDM background.

Derived relations between EFT parameters and observable quantities like sound speed and Newtonian constant.

Provided a framework for constraining models with cosmological observations.

Abstract

We take an Effective Field Theory (EFT) approach to unifying existing proposals for the origin of cosmic acceleration and its connection to cosmological observations. Building on earlier work where EFT methods were used with observations to constrain the background evolution, we extend this program to the level of the EFT of the cosmological perturbations - following the example from the EFT of Inflation. Within this framework, we construct the general theory around an assumed background which will typically be chosen to mimic Lambda-CDM, and identify the parameters of interest for constraining dark energy and modified gravity models with observations. We discuss the similarities to the EFT of Inflation, but we also identify a number of subtleties including the relationship between the scalar perturbations and the Goldstone boson of the spontaneously broken time translations. We present formulae that relate the parameters of the fundamental Lagrangian to the speed of sound, anisotropic shear stress, effective Newtonian constant, and Caldwell's varpi parameter emphasizing the connection to observations. It is anticipated that this framework will be of use in constraining individual models, as well as for placing model-independent constraints on dark energy and modified gravity model building.