Gravastar

In astrophysics, a gravastar (a blend word of "gravitational vacuum star") is an object hypothesized in a 2001 paper by Pawel O. Mazur and Emil Mottola as an alternative to the black hole theory. It has the usual black hole metric outside of the horizon, but de Sitter metric inside. A typical gravastar is as big as London, but weighing ten solar masses. On the horizon there is an ultra-thin, incredibly tight shell of entirely new, unique exotic matter. This solution to the Einstein equations is stable and has no singularities. Instead, a gravastar is filled either with dark energy or with vacuum energy, but also vacuum, only the inside one is 10⁴⁴ times denser than the outside. As a bonus, further theoretical considerations of gravastars include the notion of a nestar (a second gravastar "nested" within the first one).