The death of black holes could be very different than we thought

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That black holes, celestial bodies so compact that the intensity of their gravitational fields prevent any form of matter or radiation from escaping, fascinate scientists and amateurs. Among the many mysteries they perpetuate, the course of their deaths is one of the most intriguing. Stephen Hawking had demonstrated the probability of the evaporation and disappearance of black holes, but unfortunately contradicted the general theory of relativity: it is the paradox of information. Recently, a team of researchers exploited a theory known as “Einstein-dilaton-Gauss-Bonnet gravity” to study the final states of evaporating black holes. Their discovery, which has yet to be confirmed, sheds new light on these celestial bodies.

A black hole is a celestial object whose gravity is so intense that it swallows everything beyond its “event horizon,” including light. The event horizon of a black hole represents the limit beyond which nothing can appear. These celestial bodies are an essential part of the structure of the universe. The strong gravity generated by black holes (or by the singularity more precisely) occurs because matter has been compressed into a small space. This can happen at the end of a star’s life, making many black holes the result of dying stars. By the way, there are black holes at the center of most galaxies, including ours. But we still don’t know exactly how these objects die.

Stephen Hawking demonstrated the possibility of black hole evaporation, a phenomenon he called Hawking radiation. In other words, it is radiation that any black hole would emit due to the laws of quantum mechanics, causing it to evaporate by losing mass, angular momentum if the black hole is rotating, and electrical charge if it is charging.

But according to general relativity, this phenomenon is impossible because nothing that enters the black hole’s event horizon can leave it. This theory nevertheless predicts the existence of points of infinite density where the laws of physics break down: the singularities. It is what lies at the center of black holes, where all matter in the star is concentrated. So what happens when the black hole evaporates? Scientists have used a particular theory to study these end states. Their results are available at arXiv, pending peer review.

A new theory to understand the death of black holes

The appearance of Hawking radiation thus created the black hole information paradox. In March 2022, scientists hypothesized that black holes would not forever swallow the information that fell into them: rather, it would imprint itself in their gravitational field.

Moreover, by studying the process of Hawking radiation, it would be possible to understand the physics of a singularity that potentially contains the information absorbed by the black hole. As black holes evaporate, they become smaller and smaller, and their event horizons approach central singularities. In the final moments of black hole life, gravity becomes too strong and black holes become too small to be described and understood through the prism of current knowledge. It will therefore be necessary to develop a better theory of gravity.

Relativity and quantum mechanics certainly do not work very well together, but using the different elements that these two theories offer is one possible way. There are many candidates for a quantum theory of gravity, starting with modified general relativity. The study’s researchers looked at the theory known as Einstein-dilaton-Gauss-Bonnet gravity.

The unexpected death of black holes

The details of the team’s results are unfortunately a bit fuzzy. This is because modified general relativity is not as well understood as classical general relativity, and solving the equations relies on a lot of guesswork. However, scientists have been able to describe the death of a black hole according to the nature and evolution of the latter.

You should know that one of the main features of the Einstein-dilaton-Gauss-Bonnet theory of gravity is that black holes have minimal mass, so the authors were able to study what happens when an evaporating black hole begins to reach this minimum mass.

Thus, on the one hand, the evaporation process may leave a residual “microscopic nugget”, devoid of an event horizon. The authors believe that, in theory, it would then be possible to recover this “lump” containing all the information that fell into the original black hole, thus solving the information paradox. On the other hand, the black hole could reach its minimum mass and shed its event horizon while retaining its singularity. These “naked singularities” seem forbidden in general relativity, but if they exist, they would be direct windows into the realm of quantum gravity.

Pending confirmation that Einstein-dilaton-Gauss-Bonnet gravity can represent a valid path to quantum gravity, results like those revealed in this study will help physicists develop valid scenarios for how holes evolve.

Source: arXiv

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