Dark Energy and Modified Gravity in the Effective Field Theory of
  Large-Scale Structure
  We develop an approach to compute observables beyond the linear regime of
dark matter perturbations for general dark energy and modified gravity models.
We do so by combining the Effective Field Theory of Dark Energy and Effective
Field Theory of Large-Scale Structure approaches. In particular, we parametrize
the linear and nonlinear effects of dark energy on dark matter clustering in
terms of the Lagrangian terms introduced in a companion paper, focusing on
Horndeski theories and assuming the quasi-static approximation. The Euler
equation for dark matter is sourced, via the Newtonian potential, by new
nonlinear vertices due to modified gravity and, as in the pure dark matter
case, by the effects of short-scale physics in the form of the divergence of an
effective stress tensor. The effective fluid introduces a counterterm in the
solution to the matter continuity and Euler equations, which allows a
controlled expansion of clustering statistics on mildly nonlinear scales. We
use this setup to compute the one-loop dark-matter power spectrum.
