A star like a Matryoshka doll: new theory for gravastars

The interior of black holes remains an enigma for science. In 1916, German physicist Karl Schwarzschild proposed a solution to Albert Einstein’s equations of general relativity, in which the center of a black hole consists of a singularity, a point where space and time do not exist. no longer exist. Here, the theory says, all physical laws, including Einstein’s theory of general relativity, no longer apply; the principle of causality is suspended.

This is a big nuisance for science: after all, it means that no information can escape from a black hole beyond the so-called event horizon. This could be a reason why Schwarzschild’s solution did not attract much attention outside the theoretical realm, that is, until the first candidate for a black hole was discovered in 1971, followed by the discovery of the black hole at the center of our Milky Way in 1971. the 2000s, and finally the first image of a black hole, captured by the Event Horizon Telescope Collaboration in 2019.

In 2001, Pawel Mazur and Emil Mottola proposed a different solution to Einstein’s field equations that led to objects they called gravitational condensate stars, or gravistars. Unlike black holes, gravistars have several advantages from a theoretical astrophysical point of view.

On the one hand, they are almost as compact as black holes and also exhibit gravity on their surface that is essentially as strong as that of a black hole, thus resembling a black hole for all practical purposes. In contrast, gravistars do not have an event horizon, that is, a boundary within which no information can be sent, and their core does not contain a singularity.

Instead, the center of a gravastar is made up of exotic (dark) energy that exerts negative pressure on the enormous gravitational force compressing the star. The surface of a gravastar is represented by a very thin skin of ordinary matter, the thickness of which is close to zero.

Theoretical physicists Daniel Jampolski and Professor Luciano Rezzolla from Goethe University Frankfurt have presented a solution to the field equations of general relativity that describes the existence of a gravastar inside another gravastar. They gave this hypothetical celestial object the name “nestar” (from the English “nested”). The study is published in Classical and quantum gravity.

Daniel Jampolski, who discovered the solution as part of his undergraduate thesis supervised by Luciano Rezzolla, says: “The nestar is like a matryoshka doll. Our solution to the field equations allows us to obtain a whole series of nested gravastars. » While Mazur and Mottola posit that the gravastar has an almost infinite thin skin made of normal matter, the nestar’s shell composed of matter is a bit thicker: “It’s a bit easier to imagine that something like that could exist. »

Luciano Rezzolla, professor of theoretical astrophysics at Goethe University, explains: “It is great that even 100 years after Schwarzschild presented his first solution to Einstein’s field equations from the theory of general relativity, it is still possible to find new solutions. It’s a bit like finding a gold coin on a path that has been explored by many others before. Unfortunately, we still have no idea how such a gravastar could be created. But even if nestars don’t exist, exploring the mathematical properties of these solutions ultimately helps us better understand black holes.