Cosmological perturbations in $k$-essence model
  Subhorizon approximation is often used in cosmological perturbation theory. In this paper, however, it is shown that the subhorizon approximation is not always a good approximation at least in case of $k$-essence model. We also show that the sound speed given by $k$-essence model exerts a huge influence on the time evolution of the matter density perturbation, and the future observations could clarify the differences between the $\Lambda$CDM model and $k$-essence model. 
Kazuharu Bamba, Jiro Matsumoto, and Shin'ichi Nojiri
hep-th astro-ph.CO gr-qc