Bounded excursion stable gravastars and black holes

Dynamical models of prototype gravastars were constructed in order to study their stability. The models are the Visser-Wiltshire three-layer gravastars, in which an infinitely thin spherical shell of stiff fluid divides the whole spacetime into two regions, where the internal region is de Sitter, and the external is Schwarzschild. It is found that in some cases the models represent the "bounded excursion" stable gravastars, in which the thin shell is oscillating between two finite radii, while in other cases they collapse until the formation of black holes. In the phase space, the region for the ``bounded excursion" gravastars is very small in comparison to that of black holes, but not empty. Therefore, although the existence of gravastars cannot be excluded from such dynamical models, our results do indicate that, even if gravastars indeed exist, they do not exclude the existence of black holes.