Horava Stars Revisited: New Phases of Incompressible Stars and Black Holes, and Buchdahl's Theorem
I study a particular exact solution for static stars in four-dimensional non-projectable Horava gravity, which has been proposed as a renormalizable gravity model without the ghost problem by abandoning Einstein's equal-footing treatment of space and time through anisotropic scaling with z > 1. Considering the spherically symmetric static black-hole solutions in z = 3 Horava gravity as the exterior spacetimes of stars, I obtain an exact solution for incompressible (i.e., uniform-density) static stars with an arbitrary cosmological constant and isotropic pressure, and lambda=1, in which Birkhoff's theorem holds. For a vanishing cosmological constant, I obtain a modified Buchdahl bound on the maximum compactness for uniform-density stars, which ranges from 4/9 to 1. By contrast, I find that Ultra-Compact Objects (UCOs) with compactness C > 1 also exist with negative pressure while, surprisingly, satisfying all four standard energy conditions. UCOs include the regular (non-singular) black-hole solutions with masses above the extremal black-hole mass. In these solutions, the matter is localized at the timelike core region bounded by the inner horizon, while their exterior metrics are unaffected by the core matter and identical to the corresponding vacuum Horava black-hole solutions. These long-sought regular black-hole solutions are essential manifestations of Birkhoff's theorem in Horava gravity. I also find negative-mass stars with positive pressure that violate all standard energy conditions. Finally, I prove Buchdahl's theorem in non-projectable Horava gravity. The proof uses a weight-monotonicity condition on the average density and Birkhoff's theorem, together with the usual assumption that average density is non-increasing.