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 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.
