Constraints on modified gravity from Planck 2015: when the health of
  your theory makes the difference
  We use the effective field theory of dark energy (EFT of DE) formalism to
constrain dark energy models belonging to the Horndeski class with the recent
Planck 2015 CMB data. The space of theories is spanned by a certain number of
parameters determining the linear cosmological perturbations, while the
expansion history is set to that of a standard $\Lambda$CDM model. We always
demand that the theories be free of fatal instabilities. Additionally, we
consider two optional conditions, namely that scalar and tensor perturbations
propagate with subliminal speed. Such criteria severely restrict the allowed
parameter space and are thus very effective in shaping the posteriors. As a
result, we confirm that no theory performs better than $\Lambda$CDM when CMB
data alone are analysed. Indeed, the healthy dark energy models considered here
are not able to reproduce those phenomenological behaviours of the effective
Newton constant and gravitational slip parameters that, according to previous
studies, best fit the data.
