Exterior spacetime for stellar models in five-dimensional Kaluza-Klein gravity

It is well known that Birkhoff's theorem is no longer valid in theories with more than four dimensions. Thus, in these theories the effective four-dimensional picture allows the existence of different possible, non-Schwarzschild, scenarios for the description of the spacetime outside of a spherical star, contrary to four-dimensional general relativity. We investigate the exterior spacetime of a spherically symmetric star in the context of Kaluza-Klein gravity. We take a well-known family of static spherically symmetric solutions of the Einstein equations in an empty five-dimensional universe, and analyse possible stellar exteriors that are conformal to the metric induced on four-dimensional hypersurfaces orthogonal to the extra dimension. We show that all these nonSchwarzschild exteriors can continuously be matched with the interior of the star, indicating that the matching conditions at the boundary of a star do not require an unique exterior. Then, without making any assumptions about the interior solution, we prove the following statement: the condition that in the weak-field limit we recover the usual Newtonian physics singles out an unique exterior. This exterior is 'similar' to a Scharzschild vacuum in the sense that it has no effect on gravitational interactions. However, it is more realistic because instead of being absolutely empty, it is consistent with the existence of quantum zero-point fields (Wesson 1992 Phys. Essays, Orion 5 591). We also examine the question of how the deviation from the Schwarzschild vacuum exterior would affect the parameters of a neutron star. In the context of a model star of uniform density, we show that the general relativity upper limit M/R < 4/9 is significantly increased as we go away from the Schwarzschild vacuum exterior. We find that, in principle, the compactness limit of a star can be larger than 1/2, without being a black hole. The generality of our approach is also discussed.