How to optimally parametrize deviations from General Relativity in the
  evolution of cosmological perturbations
  The next generation of weak lensing surveys will trace the growth of large
scale perturbations through a sequence of epochs, offering an opportunity to
test General Relativity (GR) on cosmological scales. We review in detail the
parametrization used in MGCAMB to describe the modified growth expected in
alternative theories of gravity and generalized dark energy models. We
highlight its advantages and examine several theoretical aspects. In
particular, we show that the same set of equations can be consistently used on
super-horizon and sub-horizon linear scales. We also emphasize the sensitivity
of data to scale-dependent features in the growth pattern, and propose using
Principal Component Analysis to converge on a practical set of parameters which
is most likely to detect departures from GR. The connection with other
parametrizations is also discussed.
