A new solution to Einstein's equations suggests that under certain conditions the collapse of a massive star will not end in a black hole, but in the formation of a gravistar - an extremely dense body with an expanding, universe-like region at its center. The researchers emphasize that black holes are still considered the simplest and most plausible result of gravitational collapse
What happens when a massive star runs out of nuclear fuel and collapses under gravity? According to the conventional explanation, if the core mass is large enough, the collapse leads to the formation of a black hole. However, two theoretical physicists from Goethe University in Frankfurt offer an alternative mathematical solution: During the collapse, a tiny, expanding region, a kind of “mini-universe,” may form in the center of the star, which will stop the collapse and leave behind a compact body called a gravistar.
The solution developed by Daniel Yampolsky and Prof. Luciano Razzola describes for the first time a dynamic process in which ordinary matter collapsing under its own gravity can reach the state of a gravistar. The study was published in the journal Physical Review D.
This is not an astronomical observation or proof that gravitystars exist. It is a mathematical model built on the basis of general relativity and under certain, well-tuned conditions.
When the pressure in the star's core disappears
Stars produce energy through nuclear fusion in their cores. The energy released creates outward pressure that balances the force of gravity, which pulls the star's material inward.
When a massive star exhausts its available nuclear fuel, the supporting pressure weakens. The star may explode as a supernova, while its core continues to collapse. Depending on its mass, it can become a neutron star or a black hole.
According to general relativity, a collapse that creates a black hole eventually leads to a singularity – a region where the density and curvature of space-time become unobstructed, and classical theory can no longer provide a complete physical description. Surrounding the black hole is the event horizon, a boundary beyond which even light cannot return to an external observer.
The general assumption is not that black holes themselves are questionable. They explain a wide range of observations, including stellar orbits, radiation from accretion disks, gravitational waves, and images from the Event Horizon Telescope. The question is whether all the dense objects we identify as black holes necessarily contain singularities and event horizons.
Gravestar instead of black hole
One of the theoretical alternatives that has been proposed is a gravistar – short for gravitational vacuum star.
From the outside, a Gravestar would look a lot like a black hole: it would be massive, compact, and extremely small, with enormous gravity nearby. However, according to the model, it would have no singularity or true event horizon.
The exterior of the Gravstar would be made of dense matter, while inside it would be a region dominated by a vacuum energy with negative pressure. This component is mathematically similar to dark energy: instead of pulling in, it creates an effect that pushes space to expand.
The idea of gravastars was proposed as early as the early 2000s. However, for almost a quarter of a century, a fundamental problem remained: it was possible to write equations describing a stable gravastar, but it was not clear how such an object could actually form from the collapse of an ordinary star.
A tiny “big bang” at the heart of the star
Yampolsky and Rezola started from a simple, classical model of gravitational collapse: a uniform sphere of pressureless matter, sometimes called “dust,” collapsing under its own gravity. This is the model also used in Oppenheimer and Snyder’s 1939 description of the collapse.
According to the new solution, at a very advanced stage of the collapse, a tiny region of de Sitter space can form at the center of the sphere – a space containing vacuum energy and expanding at an accelerated rate. The region essentially starts from zero size and expands within the collapsing material.
The researchers compare the process to a miniature Big Bang: instead of all matter continuing to collapse towards a singularity, a new inner region begins to expand.
As the region expands, it encounters the collapsing envelope of material. Its expansion slows as it approaches the Schwarzschild radius—the size at which an event horizon would form if the collapse had continued. Under the right conditions, an equilibrium is established between the inner region pushing outward and the material being pulled inward.
The result is a stable gravastar: a body that looks almost exactly like a black hole from the outside, but contains an expanding region inside rather than a singularity.
Not every crashing star can become a Gravestar.
The solution doesn't allow any star to randomly choose between a black hole and a gravastar. The researchers found that the process only occurs under well-tuned conditions.
One of the conditions is related to the initial compactness of the star – the ratio of its mass to its size. According to the model, if the initial compactness is higher than a certain value, the collapse into a black hole cannot be stopped.
The researchers found a threshold value of 3/8 in the compactness index they used. Above this threshold, a black hole forms, while below it, under the right conditions, an expanding de Sitter region and eventually a gravistar may form.
This finding is important because it shows that the model does not completely replace black holes. It only offers another possible path in a limited part of the space of conditions.
Simple model vs. real star
The new solution is a theoretical step, but it is still far from a complete description of a collapsing star in nature. The sphere is a uniform model and does not exert normal pressure. Real stars consist of layers of different densities, rotate, contain magnetic fields, and emit particles and radiation.
The origin of the vacuum energy region is also not fully explained. The model assumes that under extreme density conditions a de Sitter region can form, but additional physics is needed to explain what microscopic process would cause this.
Yampolsky, who discovered the solution as part of his master's thesis under the supervision of Rezola, believes that the process can only begin at a very late stage, when the star has already shrunk to almost the state of a black hole. At such densities, he says, physical effects that are not yet known may emerge.
Black holes remain the leading explanation
Razzola emphasizes that the search for gravastars does not express opposition to the existence of black holes. He says that black holes are still the most natural and simplest solution to the fate of gravitational collapse.
The goal of the research is to see if Einstein's equations allow for other results, and what would be required to create them. It will then be necessary to examine whether such gravistars remain stable when rotation, pressure, inhomogeneities, and other perturbations are added to the model.
Another challenge will be to identify observational differences between a gravastar and a black hole. Since the two objects may appear almost identical from the outside, the differences may only be detected in tiny details of gravitational waves, the motion of nearby matter, or the behavior of the object after a collision.
The study does not show that a new universe is actually born inside a dying star. It shows that under certain mathematical conditions, general relativity allows for a scenario in which the collapse of the star is halted by an expanding inner region—without the need for a singularity or event horizon.
FAQ
Have researchers discovered a new universe inside a star?
No. The study presents a mathematical solution to the equations of general relativity. No gravity star has been observed, and no direct evidence of a miniature universe inside a star has been found.
What is Gravestar?
A gravastar is a theoretical, highly compressed object that resembles a black hole from the outside, but does not contain a singularity or event horizon. Inside it is supposed to be a region of vacuum energy with pressure acting against collapse.
Can any massive star create a gravastar?
No. According to the model, this is only possible under well-tuned conditions and only if the initial compactness of the star is below a certain threshold. In other cases, the collapse ends in a black hole.
Does the research disprove the existence of black holes?
No. The researchers emphasize that black holes remain the simplest and most plausible explanation for the results of gravitational collapse. The gravistar is proposed as another theoretical possibility.
Tags:
Gravestar, black holes, gravitational collapse, massive stars, universe, mini-universe, general relativity, Albert Einstein, singularity, event horizon, vacuum energy, dark energy, de Sitter space, Luciano Razzola, Daniel Yampolsky, Goethe University Frankfurt
Alternative text for the image:
Simulation of an expanding mini-universe inside a gravity star forming a Gravastar
Scientific source:
Daniel Jampolski and Luciano Rezzolla, "Formation of gravastars", Physical Review D 113, L121502, 2026. (arXiv)
Notes:
The scientific paper presents the collapse of a uniform “dust” ball according to the Oppenheimer–Snyder model. The gravistar is formed only under well-tuned conditions, through the formation of a de Sitter region with an initial size of zero. Its expansion slows down near the Schwarzschild radius, and the researchers found a maximum initial compactness of 3/8; above which the collapse into a black hole is inevitable. (arXiv)
The Goethe University statement emphasizes that this is the first dynamical solution describing how a gravastar might form from the collapse of ordinary matter. Razzola explicitly notes that black holes remain the most natural and simplest solution, and the study explores a possible exotic alternative – not a refutation of the black hole model. (uni-frankfurt.de)
More of the topic in Hayadan:
One response
Wow this could solve the question of how the Big Bang was created if it is confirmed!!