FRANKFURT. 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 ten billion solar masses concentrate on 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 nor 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 two 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 has shown 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 has emerged. Like our own universe, its expansion is driven by dark energy. 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?

Room for new physics

Daniel Jampolski, who discovered the solution in his master’s thesis supervised by Luciano Rezzolla, explains: “The Big Bang of the emerging universe can unfold once the star has already collapsed almost to the point of becoming a black hole.” The unresolved behavior of extremely compressed matter leaves room for new physics: “It is easier to imagine that the Big Bang occurs only at a very late stage, when matter has already been compressed to an extreme degree, thereby giving rise to new effects.”

Rezzolla, Professor of Theoretical Astrophysics at Goethe University, adds: “Looking for alternatives to black holes should not suggest a skepticism towards black holes, which still represent the most natural and simplest solution to the fate of gravitational collapse. However, as scientists in general, and as theoretical physicists in particular, it is essential to maintain an unbiased approach towards what we do not know and hence explore both the accepted wisdom and the more exotic interpretations. History teaches us that it is not unusual for the latter to become the former.”