The Effective Field Theory of Dark Energy
  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.
