Black holes are strange. Predicted as a result of Einstein’s general theory of relativity, they contain an outer region known as the event horizon – from which nothing, not even light, can escape. Furthermore, they are predicted to have an infinitely dense point where our understanding of physics breaks down and nothing makes sense.
That’s before we even get into the black hole information paradox. If a black hole has mass (and they have a lot of it), then it should have a temperature according to the first law of thermodynamics, and according to the second law of thermodynamics it should radiate heat. Stephen Hawking showed that black holes should emit radiation – now called Hawking radiation – created at the boundary of the black hole.
“Hawking then pointed out a paradox. If a black hole can evaporate, some of the information it contains is lost forever,” explains French astrophysicist Jean-Pierre Luminet in a 2016 review. “The information contained in the thermal radiation emitted by the black hole is degraded, it does not recapitulate the information about the matter previously absorbed by the black hole. The irreversible loss of information contradicts one of the basic postulates of quantum mechanics. According to the Schrödinger equation, physical systems that change over time cannot create nor destroy information, a property known as unitarity.”
In short, we are probably missing something. Physicists and mathematicians have been trying to come up with ideas that would eliminate these problems and ended up with some pretty strange results. Some have even suggested that the universe could be holographic, with the universe we know and love actually being the result of interactions at the infinitely distant frontier. We told you black holes are weird.
And yet we have definitely observed objects that appear to have the properties of black holes, including (but by no means limited to) the image of the black hole M87*. But what if they don’t exist at all?
One idea is that black holes are actually “gravastars,” a portmanteau of gravity, vacuum, and stars. First proposed in 2002 by Pawel O. Mazur and Emil Mottola, the idea is that at some point during the collapse of a large star, intense gravity transforms its mass into a new Bose-Einstein condensate (BEC)-like state.
BEC occurs when atoms are cooled to such low energy states that they begin to behave as a single “super atom”. For gravastars, the team proposed that when a star collapses to the point of the event horizon, its mass transforms into a new state that exerts an outward pressure and prevents the star from collapsing into a physics-defying singularity. In gravastars, this highly warped (but familiar) space-time is surrounded by an ultra-thin, ultra-cold, ultra-dark and virtually indestructible shell.
“Because this new form of matter is very durable but somewhat flexible, like a bubble, anything trapped by its intense gravity and smashed into it would be destroyed and then assimilated into the Gravastar envelope,” Mottola said in a statement. after the first article about gravastars.
One of the main attractions of gravastars is the removal of chaotic event horizons and singularities. But while they are interesting as an idea, they also have to explain what we observe, and we have certainly observed objects that look like black holes.
“This shadow is not caused by the trapping of light at the event horizon, but by a slightly different phenomenon called ‘gravitational redshift’, which causes light to lose energy as it moves through a region with a strong gravitational field,” João Luís Rosa, Professor of Physics at the University of Gdańsk in Poland, and the author of the new gravastar study told Live Science. “Indeed, when light emitted from regions near these alternative objects reaches[es] our telescopes, most of its energy would be lost to the gravitational field, causing this shadow to appear.”
As with black holes, things get messy when spin is added, and there are (controversial) suggestions that gravastars would be unstable when spinning. And they’re a little weird too (hey, this is the universe we’re talking about). There are suggestions that the interiors of gravastars could contain a number of more powerful shells, known as nestars.
They are not perfect and a lot of work needs to be done to model how they work. It is also possible that both black holes and gravastars exist. The big problem is that they’re difficult to tell apart, although some models suggest they should emit very different gravitational radiation, allowing us to know whether we’re looking at gravastars or traditional black holes, and all the headaches that entails.
The new study by Rosa and colleagues is published in Physical Review D.