Black holes continue to mystify scientists. In 1916, German physicist Karl Schwarzschild proposed a solution to Albert Einstein’s general relativity equations suggesting that the center of a black hole contains a “singularity” — a point where space and time cease to exist. According to theory, within the singularity all physical laws, including Einstein’s general relativity, break down. Causality no longer applies.
This poses problems for science since it means no information can escape from within a black hole’s “event horizon”. This may explain why Schwarzschild’s solution received little attention until the first candidate black hole was discovered in 1971, followed by discoveries of the black hole at the center of the Milky Way in the 2000s and the first image of a black hole by the Event Horizon Telescope in 2019.
In 2001, Pawel Mazur and Emil Mottola proposed an alternative solution to Einstein’s field equations describing objects they called gravastars or gravitational condensation stars. Unlike black holes, gravastars have advantages from a theoretical astrophysics perspective. Gravastars are almost as compact as black holes and have similarly strong gravitational pull at their surfaces, resembling black holes in practical terms.
However, gravastars lack an event horizon boundary from which information cannot escape. Their cores do not contain singularities, but are formed from “exotic dark energy” producing negative pressure counterbalancing immense gravitational compression. Gravastars have an extremely thin outer “skin” of normal matter approaching zero thickness.
More recently, theorists Daniel Jampolsky and Luciano Rezzolla from Goethe University Frankfurt presented a solution describing a gravastar nested within another gravastar, which they termed a nestar. Jampolsky notes the nestar is like a matryoshka doll with their field equation solution allowing for series of nested gravastars. While Mazur and Mottola proposed infinitesimally thin gravastar skins, nestars have slightly thicker envelopes made of normal matter, making their existence somewhat more conceivable.
Rezzolla remarked that even 100 years after Schwarzschild’s breakthrough, new solutions continue emerging, like finding a gold coin along an already explored path, though nestars’ formation remains unknown. While nestars may not exist, exploring mathematical properties of solutions helps advance understanding of mysteries within black holes.
Sources
Goethe Universität Frankfurt am Main | Daniel Jampolski, Luciano Rezzolla, Nested solutions of gravitational condensate stars. Classical and Quantum Gravity vol.41, no.6, DOI 10.1088/1361-6382/ad2317
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