Dark energy with non-adiabatic sound speed: initial conditions and
  detectability
  Assuming that the universe contains a dark energy fluid with a constant
linear equation of state and a constant sound speed, we study the prospects of
detecting dark energy perturbations using CMB data from Planck,
cross-correlated with galaxy distribution maps from a survey like LSST. We
update previous estimates by carrying a full exploration of the mock data
likelihood for key fiducial models. We find that it will only be possible to
exclude values of the sound speed very close to zero, while Planck data alone
is not powerful enough for achieving any detection, even with lensing
extraction. We also discuss the issue of initial conditions for dark energy
perturbations in the radiation and matter epochs, generalizing the usual
adiabatic conditions to include the sound speed effect. However, for most
purposes, the existence of attractor solutions renders the perturbation
evolution nearly independent of these initial conditions.
