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The interior of black holes remains a mystery to science. In 1916, German physicist Karl Schwarzschild drew a solution to Albert Einstein’s general relativity equations, where the center of a black hole consists of a so-called singularity, the point at which space and time do not exist.

Here, the theory goes, all the laws of physics, including Einstein’s theory of general relativity, no longer apply; The principle of causality is suspended. This poses a big problem for science, because it means that no information can escape from the black hole beyond the so-called event horizon. This may be why Schwarzschild’s solution did not attract much attention outside the theoretical world, that is, until the discovery of the first black hole candidate in 1971, followed by the discovery of the black hole at the center of our Milky Way Galaxy in the 2000s. And finally, the first image of a black hole was taken by the Event Horizon Telescope Collaboration in 2019.

In 2001, Paul Mazur and Emil Mottola proposed another solution to Einstein’s field equations, giving rise to objects they called gravitationally condensing stars or tomb stars. Unlike black holes, gravastars have several advantages from a theoretical astrophysical perspective. On the one hand, they are almost as compact as black holes, and they also exhibit surface gravity as strong as a black hole’s. Gravastars, on the other hand, have no event horizon, i.e. boundaries within which no information can be transmitted, and their core does not contain a singularity.

Instead, the gravastar’s center consists of strange (dark) energy that exerts negative pressure on the massive gravitational force pressing down on the star. The surface of the Gravastar is represented by a thin skin of ordinary matter, the thickness of which is close to zero. Theoretical physicists Daniel Jampolski and Prof. Luciano Rizzola of Goethe University Frankfurt has now provided a solution to the field equations of general relativity that describe the existence of a gravastar inside another star. They called this hypothetical celestial body “nestar” (from the English word “nested”). The research was published in Classical and Quantum Gravity.

“The star is like a Matryoshka doll,” says Daniel Jampolski, who discovered the solution as part of his bachelor’s thesis supervised by Luciano Rizzola. “Our solution of the field equations allows for a whole series of overlapping dangerous stars.” While Mazur and Mottola argue that the gravastar has a very thin shell made up of ordinary matter, the matter-made shell of the gravastar is slightly thicker: “It’s a little easier to imagine that such a thing could exist.”

“It is remarkable that even 100 years after Schwarzschild provided his first solution to Einstein’s field equations from general relativity, new solutions can still be found,” explains Luciano Rizzola, professor of theoretical astrophysics at Goethe University. “It’s a bit like finding a coin.” gold along a path that has been explored by many others. Unfortunately, we still have no idea how such a crater formed. But even if stars didn’t exist, exploring their mathematical properties helps with these solutions. Eventually, they will allow us to improve Understanding black holes.

**source: **Psyg.org/Goethe-Universität Frankfurt am Main