Gravastar

A gravastar is an object hypothesized in astrophysics by Pawel O. Mazur and Emil Mottola as an alternative to the black hole theory. It has usual black hole metric outside of the horizon, but de Sitter metric inside. On the horizon there is a thin shell of matter. The term "gravastar" is a portmanteau of the words "gravitational vacuum star". In the original formulation by Mazur and Mottola, gravastars contain a central region featuring a p = −ρ false vacuum or "dark energy", a thin shell of p = ρ perfect fluid, and a true vacuum p = ρ = 0 exterior. The dark energy-like behavior of the inner region prevents collapse to a singularity and the presence of the thin shell prevents the formation of an event horizon, avoiding the infinite blue-shift. The inner region has thermodynamically no entropy and may be thought of as a gravitational Bose–Einstein condensate. Severe red-shifting of photons as they climb out of the gravity well would make the fluid shell also seem very cold, almost absolute zero. In addition to the original thin shell formulation, gravastars with continuous pressure have been proposed. These objects must contain anisotropic stress. Externally, a gravastar appears similar to a black hole: It is visible by the high-energy radiation it emits while consuming matter, and by the Hawking radiation it creates. Astronomers search the sky for X-rays emitted by infalling matter to detect black holes. A gravastar would produce an identical signature. It is also possible, if the thin shell is transparent to radiation, that gravastars may be distinguished from ordinary black holes by different gravitational lensing properties as null geodesics may pass through. Mazur and Mottola suggest that the violent creation of a gravastar might be an explanation for the origin of our universe and many other universes, because all the matter from a collapsing star would implode "through" the central hole and explode into a new dimension and expand forever, which would be consistent with the current theories regarding the Big Bang.