Towards a fully consistent parameterization of modified gravity
  There is a distinct possibility that current and future cosmological data can
be used to constrain Einstein's theory of gravity on the very largest scales.
To be able to do this in a model-independent way, it makes sense to work with a
general parameterization of modified gravity. Such an approach would be
analogous to the Parameterized Post-Newtonian (PPN) approach which is used on
the scale of the Solar System. A few such parameterizations have been proposed
and preliminary constraints have been obtained. We show that the majority of
such parameterizations are only exactly applicable in the quasistatic regime.
On larger scales they fail to encapsulate the full behaviour of typical models
currently under consideration. We suggest that it may be possible to capture
the additions to the `Parameterized Post-Friedmann' (PPF) formalism by treating
them akin to fluid perturbations.
