Dark Energy or Modified Gravity? An Effective Field Theory Approach

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 and identify the parameters of interest for constraining dark energy and modified gravity models with observations.