Stars shine because atoms fuse in their interiors, releasing energy. When a very massive star has exhausted its nuclear fuel, radiation pressure can no longer provide sufficient counterforce to gravity. The star then collapses under its own mass until only a single point remains: the singularity.

While the formation of a black hole appears plausible, black holes themselves continue to pose major challenges for science. How can 10 billion solar masses concentrate at a single tiny point? How can spacetime be curved infinitely at that point, the singularity? At this stage, the laws of physics break down, making it impossible to predict what happens. Moreover, black holes conceal all information from observation: Everything, including light, disappears irretrievably beyond the event horizon.

Filled with dark energy
It is therefore possible that black holes are in fact entirely different objects, such as ultra-compact stars, which cannot be seen because of their intense gravity and are therefore also called gravastars. In addition to ordinary matter present in their outer layers, they would be filled with dark energy, which exerts an outward pressure and stabilizes their mass, which wants instead to collapse. Gravastars are easier for physicists to accept than black holes because they do not possess a singularity or an event horizon and yet are almost as massive and compact as black holes. What had remained unclear, however, was how such gravastars could form in practice.

The theoretical physicists Daniel Jampolski and Professor Luciano Rezzolla have now presented, for the first time, a dynamic solution to the field equations of Albert Einstein's general relativity describing the collapse of a star that could lead to the formation of such a gravastar. The solution showed that the collapse may trigger the creation of a mini-universe inside the collapsing matter, not very different from the Big Bang from which our universe emerged. Like our own universe, its expansion is driven by dark energy. The findings are published in the journal Physical Review D.

In this way, the expansion of the new universe counteracts the gravitational forces and halts the collapse of the star before a black hole can form. In this process, an equilibrium is established between the expanding mini-universe and the collapsing matter, and this equilibrium is what leads to a stable gravastar. With this solution to general relativity, the Frankfurt physicists have provided the first answer to a question that scientists have been debating for 25 years: How do gravastars form during the collapse of ordinary matter?