A new type of astrophysical object proposed by Emil Mottola of Los Alamos National Laboratory and Pawel Mazur of the University of South Carolina as an alternative to black holes. They are said to consist of a shell of ultradense matter supported by some strange springy space within. Up close, they would look just like what one might expect a black hole to look like, a cold, dark body lit only by the matter raining down on it and being torn to shreds by the immense gravity. The difference is that a gravastar doesn't have an event horizon or central singularity where the laws of physics break down the way a black hole is supposed to. Also known as quasi-black hole objects (QBHO's), or gravitational condensate stars. They were first described in Mottola and Mazur's paper: Gravitational Condensate Stars, which was first submitted for peer review on September 11, 2001. You can get a preprint of the paper from the Los Alamos Preprint Server at http://xxx.lanl.gov/abs/gr-qc/0109035. As of this writing the paper has been submitted to Physical Review Letters and is currently undergoing peer review.
Mottola and Mazur have used arguments based on current theories of general relativity and quantum gravity to show that there is an alternative state for matter whose gravity is so great that not even the exclusion principle repulsion between neutrons can keep it from collapsing indefinitely. The theory they have proposed says that when an object undergoing gravitational collapse, say, a massive star out of nuclear fuel, space-time itself undergoes a phase transition, similar to the formation of a Bose-Einstein Condensate. Just when an event horizon is about to form around the object undergoing gravitational collapse, the theory goes, the immense gravitational forces would distort the fluctuations in space-time (the quantum foam) so greatly that a radical change in the nature of space-time would occur, similar to the formation of a Bose-Einstein condensate (see CapnTrippy's writeup on that node for some recent experimental evidence for how this repulsive effect might happen).
This would create a condensate bubble surrounded by a shell of ultradense, ultracold matter, like a stationary shock wave of gravitational energy where a black hole's event horizon should be. According to Mazur and Mottola's calculations this strange space-time within the gravastar would exert an outward repulsive force, so matter inside the shell would bounce back toward the shell, while matter outside would continue to rain down on it.
Furthermore Mazur and Mottola have shown that gravastars represent a stable solution to Albert Einstein's equations of general relativity, just like black holes, but without the mathematical anomalies and paradoxes that black holes suffer from.
A star with fifty or so solar masses would undergo violent gravitational collapse when its nuclear fuel was exhausted, and form a gravastar roughly 300 km in diameter, the same size as the event horizon of the expected black hole but the shell of ultradense matter (which would be so dense that a teaspoonful of that matter would weigh a hundred billion tons or so) would only be roughly 10-30 km thick, with a temperature about a few billionths of a degree above absolute zero, thus emitting hardly any radiation at all, just like a black hole.
These strange objects could be a possible source for the many violent high-energy astrophysical phenomena that have been observed, such as X-ray and gamma-ray bursts, and, just like black holes, a profound explanation for many outstanding problems in cosmology.
Mottola further proposed that if one would imagine a gravastar whose interior would be large enough to contain the entire visible universe, the outward pressure that the gravitational condensate exerts matches the pressure that seems to be accelerating the expansion of the universe, i.e. the outward pressure being exerted is just like the effect of Einstein's proposed cosmological constant. So the universe might actually be on the inside of one huge cosmic gravastar! Mottola states: "We might be able to entertain the really radical notion that we -- and everything we see in the Universe -- could be inside such an object." As of this writing Mottola and Mazur are working out the details of this extension of their theory.
It's still a big open question, and many loose ends need to be tied up in the theory. For one thing, they have no clear idea of what the strange stuff inside the shell really is, and how exactly this gravitational condensate is formed, and whether or not it is actually possible for such a thing to occur in stellar gravitational collapse. It's a very very new theory...
To be honest, I don't really quite understand the Mazur and Mottola paper as much as I would like, so if anyone is out there who's read the paper and who feels they understand it better has any corrections to make to this wu, please /msg me.