Cosmological Perturbations in the Projectable Version of Horava-Lifshitz Gravity

We consider linear perturbations about a homogeneous and isotropic cosmological background in the projectable version of Ho\v{r}ava-Lifshitz gravity. Starting from the action for cosmological perturbations, we identify the canonically normalized fluctuation variables. We find that - in contrast to what happens in the non-projectable version of the theory - the extra scalar cosmological perturbation mode is already dynamical at the level of linear perturbations. For values of the parameter $\lambda$ in the range $1/3 < \lambda < 1$,theextramodeisghost-like,forvalues$1 < \lambda$and$\lambda < 1/3$ it is tachyonic. This indicates a problem for the projectable version of Ho\v{r}ava-Lifshitz gravity.