Black holes or gravastars?

As I explained in a previous post, our two fundamental physical theories, general relativity and quantum mechanics, predict infinities that physicists don’t like. General relativity does this in gravitational singularities: the Big Bang and black holes. Quantum mechanics, in vacuum energy and the quantities that must be renormalized in quantum field theory.

Until a little time ago, the theory of black holes, formulated by Subrahmanyan Chandrasekhar in 1930, stated the following: when a star 30 to 70 times more massive than the sun undergoes a supernova explosion, it expels most of its mass, but a part of it (at least 3.8 times more massive than the sun) collapses to such a point that it occupies zero volume, and so it will have an infinite density.

In 2001, physicists Pawel Mazur and Emil Mottola attempted to combine general relativity and quantum mechanics to eliminate the infinities associated with black holes. According to their theory, when a massive star collapses, and according to the previous hypotheses would have given rise to a black hole, the introduction of quantum effects would prevent the total collapse of matter, which would give rise to a different object: a gravastar, a gravitational vacuum condensate star.

For the time being, the existence of the gravastars is a theoretical speculation that gives rise to three possibilities:

• Gravastars do not exist. Although they are theoretically possible, they may not happen.
• Perhaps all the objects we have so far considered black holes are not black holes, but gravastars.
• Finally, perhaps both gravastars and black holes exist in the universe, if both theories predicting the existence of these two types of objects are applicable in practice.

What differentiates a Gravastar from a black hole?

1. Black holes have a very large mass (several solar masses) concentrated at one point. In gravastars, this matter would be concentrated in a spherical surface of very small thickness, inside which there would be a bubble of empty space of De Sitter type, which is compatible with general relativity. There would, therefore, be no singularity. Although its matter would tend to concentrate, De Sitter space tends to expand, so the two effects could balance.
2. Black holes have an event horizon, the distance from the center, at which the escape velocity would be equal to the speed of light. Closer to the center, not even light could escape a black hole (hence its name). Gravastars, however, would have no event horizon.
3. Black holes have no internal structure. Since all their matter has collapsed in one geometric point, the rest of the black hole is empty unless it receives an additional supply of matter. Gravastars, however, could have several internal structures compatible with their existence. In an article published in February 2024 in the journal Classical and Quantum Gravity, a doctoral student (Daniel Jampolsky) and his director (Luciano Rezolla) show that the internal structure of a gravastar could consist of several alternating layers of matter and De Sitter vacuum, which would give it a structure similar to a matryoshka, one of those Russian dolls that contain several dolls inside, increasingly smaller. The authors of the article call these entities “nestar” (nested gravastar).

Will we be able to discover whether what we have until now considered black holes are actually gravastars, nestars, or anything else that comes to mind? I doubt it. To do this we should be able to investigate the inside of the alleged black holes, and I don’t think that we will be in a position to do so until many thousands of years have passed, if ever. But physicists insist on formulating mathematical theories that no one can test, or unvalidated simulations, and presenting them as if they were part of science. They certainly have a lot of fun doing this, but why should we pay them to do it?

The same post in Spanish

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Manuel Alfonseca