Effective field theory for perturbations in dark energy and modified
  gravity
  When recent observational evidence and the GR+FRW+CDM model are combined we
obtain the result that the Universe is accelerating, where the acceleration is
due to some not-yet-understood "dark sector". There has been a considerable
number of theoretical models constructed in an attempt to provide an
"understanding" of the dark sector: dark energy and modified gravity theories.
The proliferation of modified gravity and dark energy models has brought to
light the need to construct a "generic" way to parameterize the dark sector. We
will discuss our new way of approaching this problem. We write down an
effective action for linearized perturbations to the gravitational field
equations for a given field content; crucially, our formalism does not require
a Lagrangian to be presented for calculations to be performed and observational
predictions to be extracted. Our approach is inspired by that taken in particle
physics, where the most general modifications to the standard model are written
down for a given field content that is compatible with some assumed symmetry
(which we take to be isotropy of the background spatial sections).
