Title: Parametrizing modified gravity for cosmological surveys,
Abstract: One of the challenges in testing gravity with cosmology is the vast freedom
opened when extending General Relativity. For linear perturbations, one
solution consists in using the Effective Field Theory of Dark Energy (EFT of
DE). Even then, the theory space is described in terms of a handful of free
functions of time. This needs to be reduced to a finite number of parameters to
be practical for cosmological surveys. We explore in this article how well
simple parametrizations, with a small number of parameters, can fit observables
computed from complex theories. Imposing the stability of linear perturbations
appreciably reduces the theory space we explore. We find that observables are
not extremely sensitive to short time-scale variations and that simple, smooth
parametrizations are usually sufficient to describe this theory space. Using
the Bayesian Information Criterion, we find that using two parameters for each
function (an amplitude and a power law index) is preferred over complex models
for 86% of our theory space.