Stellar center is dynamical in Hořava-Lifshitz gravity

In Ho r rava-Lifshitz gravity, regularity of a solution requires smoothness of not only the spacetime geometry but also the foliation. As a result, the regularity condition at the center of a star is more restrictive than in general relativity. Assuming that the energy density is a piecewise-continuous, non-negative function of the pressure and that the pressure at the center is positive, we prove that the momentum conservation law is incompatible with the regularity at the center for any spherically symmetric, static configurations. The proof is totally insensitive to the structure of higher spatial curvature terms and, thus, holds for any values of the dynamical critical exponent z. Therefore, we conclude that a spherically symmetric star should include a time-dependent region near the center. This supports the picture that Ho r rava-Lifshitz gravity does not recover general relativity at low energy but can instead mimic general relativity plus cold dark matter: the cold dark matter accretes toward a star and thus makes the stellar center dynamical. We also comment on the condition under which linear instability of the scalar graviton does not show up.