Neutron stars and extra dimensions

Abstract

The authors study the consequences of the existence of hidden extra dimensions for the structure of neutron stars, considering the simplest Kaluza-Klein model with one circular extra dimension and employing the equation of state for cold, non-interacting neutrons. The physics depends on the choice of description for the matter; if a four-dimensional fluid description is used then the extra dimension has no effect. If a five-dimensional fluid description is used, the extra dimension is forced to have a spatial variation, which corresponds to the star carrying a scalar charge. This requires the attachment at the star boundary of a more general solution than the Schwarzschild one, where the extra dimension varies in size even outside the star; the authors demonstrate how this can be carried out. The maximum mass for the neutron star is reduced by the presence of the extra dimension; for the natural generalisation to five dimensions of the four-dimensional perfect fluid description this is unacceptably large, though alternative five-dimensional fluid descriptions can lead to only a small effect. Finally, they discuss the possible role of the scalar charge in gravitational collapse. Unlike some conventional scalar charges, it cannot be freely radiated away as its presence is intrinsically connected to that of the neutron fluid.