The fate of black holes could be to evaporate to reveal gravitational singularities otherwise concealed by the inviolable barrier represented by the event horizon or to assume a stable form comparable to the most suggestive objects predicted by Einstein's General Relativity, the wormholes. That is one of the conclusions reached by a study conducted by a group of researchers from the Department of Physics of Sapienza University of Rome and the National Institute of Nuclear Physics (INFN), in collaboration with a colleague from the Danish Niels Bohr Institute, who, through complex numerical simulations, explored for the first time, within the framework of a modified theory of general relativity, the possible outcome of the evaporation of black holes, a phenomenon predicted by the famous theoretical physicist Stephen Hawking. The result, published in the journal Physical Review Letter, highlights the importance of numerical simulations (numerical relativity) in providing new explanations for the fate of black holes while suggesting the possibility of new dark matter candidates formed at the end of their evaporation in the early moments of the universe.

Although their strong gravitational field regime allows neither matter nor light to break free from their dark grip, due to quantum effects, black holes evaporate by continuously emitting thermal radiation. As described in 1974 by Stephen Hawking, this evaporation would involve the narrowing of a black hole's event horizon, a process not yet observed, the final stage of which is one of the great mysteries of theoretical physics. The dissolution of the black hole may not be the only possible outcome of the evaporation, which could change dramatically depending on the gravitational conditions during the process.

"The shrinkage of a black hole," says Fabrizio Corelli, a researcher at the Department of Physics of Sapienza associated with INFN and first author of the study, "could result in the event horizon approaching the gravitational singularity inside it, and thus towards regions of spacetime of increasing curvature. It is therefore inevitable that, during Hawking's evaporation, gravitational effects related to the high curvature of spacetime would gradually become more and more relevant, to the point of changing the final stage of evaporation. This is why it is particularly interesting to study these phenomena in a modified theory of gravity such as the one we are considering."

By imposing appropriate corrections to General Relativity and resorting to complex numerical simulations, the researchers were, therefore, able to obtain for the first time some possible end states for the evaporation process of black holes. Among the results discussed in the article in Physical Review Letter is one that suggests the appearance of singularities outside their event horizons. However, this scenario is at odds with Roger Penrose's so-called "cosmic censorship" principle, which assumes that the singularity must be relegated to the interior of the black hole and cannot be in direct communication with the outside world. A second alternative instead concerns the transformation of black holes into wormholes, structures capable of connecting different points in spacetime, predicted on the basis of certain exotic solutions to the equations of General Relativity, but never observed so far, whose characteristics could make it possible to explain the still elusive nature of dark matter.

"The results of this study," concludes Paolo Pani of the Department of Physics of Sapienza and INFN researcher, "show that the evaporation of a black hole in theories with high curvature corrections to General Relativity could violate cosmic censorship. Indeed, the simulations show that during the evaporation process, singularities could escape from the black hole. If confirmed, this would imply the need for a quantum gravitation theory to explain black holes' fate. However, the fate of Hawking's evaporation may be the formation of a wormhole, an object without a singularity and without an event horizon that does not evaporate further, thus fulfilling Penrose's conjecture. If confirmed, in this scenario, primordial black holes, formed in the first moments of the universe, would evaporate to a stable configuration, thus becoming perfect candidates to explain dark matter".

References:

What is the fate of Hawking evaporation in gravity theories with higher curvature terms? - Fabrizio Corelli, Marina De Amicis, Taishi Ikeda, Paolo Pani - Physical Review Letter  https://doi.org/10.1103/PhysRevLett.130.091501

Further Information

Paolo Pani
Dipartimento di Fisica
paolo.pani@uniroma1.it