A parametrisation of modified gravity on nonlinear cosmological scales
  Viable modifications of gravity on cosmological scales predominantly rely on
screening mechanisms to recover Einstein's Theory of General Relativity in the
Solar System, where it has been well tested. A parametrisation of the effects
of such modifications in the spherical collapse model is presented here for the
use of modelling the modified nonlinear cosmological structure. The formalism
allows an embedding of the different screening mechanisms operating in
scalar-tensor theories through large values of the gravitational potential or
its first or second derivatives as well as of linear suppression effects or
more general transitions between modified and Einstein gravity limits. Each
screening or suppression mechanism is parametrised by a time, mass, and
environment dependent screening scale, an effective modified gravitational
coupling in the fully unscreened limit that can be matched to linear theory,
the exponent of a power-law radial profile of the screened coupling, determined
by derivatives, symmetries, and potentials in the scalar field equation, and an
interpolation rate between the screened and unscreened limits. Along with
generalised perturbative methods, the parametrisation may be used to formulate
a nonlinear extension to the linear parametrised post-Friedmannian framework to
enable generalised tests of gravity with the wealth of observations from the
nonlinear cosmological regime.
