Black holes may not die the way we thought

We still don’t know exactly what happens when black holes die.

Since Stephen Hawking discovered that black holes evaporate, we knew that they could potentially disappear from our universe. But our understanding of gravity and quantum mechanics is not enough to describe the final moments of a black hole’s life.

Now a new string theory-based study suggests a possible and equally strange fate for evaporating black holes: a nugget of remnant that we could potentially access, or a singularity not shrouded in an event horizon.

Connected: What happens at the center of a black hole?

The Importance of Hawking Radiation

Black holes strictly speaking, not completely black. In pure general relativity, without any other modifications or other physics considerations, they remain black forever. Once formed, it will just hang there, becoming a black hole, forever. But in the 1970s, Hawking used the language of quantum mechanics to explore what happens near the edge of a black hole, known as event horizon.

He unexpectedly discovered that a strange interaction between the quantum fields of our universe and the one-sided barrier of the event horizon opens the way for energy to escape from a black hole. This energy takes the form of a slow but constant stream of radiation and particles that has come to be known as Hawking radiation. With every particle of energy that escapes, the black hole loses mass and thus shrinks, eventually disappearing entirely.

The advent of Hawking radiation created what is known as black hole information paradox. All information describing matter falling into a black hole crosses the event horizon and never appears again. But the Hawking radiation itself does not carry any information with it, and yet the black hole eventually disappears. So where did all the information go?

Connected: Stephen Hawking Was Right: Black Holes Can Evaporate, Strange New Research Shows

Go beyond Einstein

The black hole information paradox is a giant flashing neon sign for physicists that we don’t understand something. Perhaps we do not understand the nature of quantum information, the nature gravity or the nature of event horizons—or all three. The “simplest” approach to solving the black hole information paradox is to develop a new theory of gravity that goes beyond Einstein’s general theory of relativity.

After all, we already know that general relativity doesn’t work at the centers of black holes, which are tiny punctures in spacetime known as singularities where the density tends to infinity. The only way to correctly describe the singularity is to use the quantum theory of gravity, which correctly predicts how strong gravity behaves on extremely small scales.

Unfortunately we don’t have a theory at the moment quantum gravity. It would be nice to look at singularities directly, but as far as we understand through general relativity, all singularities are trapped behind the event horizon, making them inaccessible to us.

But by studying the process of Hawking radiation, we may be able to find a shortcut to get closer to the singularity and understand the crazy physics that goes on there. As black holes evaporate, they get smaller and smaller, with their event horizons uncomfortably close to central singularities. In the last moments of a black hole’s life, gravity becomes too strong and black holes become too small for us to properly describe them with our current knowledge. So, if we can develop a better theory of gravity, we can use the last moments of Hawking radiation to test how the theory behaves.

There are many candidates for a quantum theory of gravity, of which string theory is the most developed. Although there are no known solutions to string theory, it is possible to take what we know about the general features of the theory and use them to create modified versions of general relativity.

Connected: How Stephen Hawking changed our understanding of black holes

Radio data from the National Science Foundation’s Very Large Array object was used to create this image of Cygnus A, the brightest radio source in the sky outside our galaxy. (Image credit: NRAO/AUI)

Naked singularities

These modified theories are not a “complete” correct replacement for general relativity, but they allow us to explore how gravity might behave as it gets closer and closer to the quantum limit. Recently, a group of theorists used one such theory, known as Einstein-Dilaton-Gauss-Bonnet gravity, to investigate the final states of evaporating black holes. They detailed their work in an article posted to the arXiv preprint database. (will open in a new tab) in May.

The details of the command’s results are a bit blurry. This is because modified general relativity is not as well understood as regular general relativity and solving complex mathematics requires many approximations and many guesses. Nevertheless, the researchers were able to draw a general picture of what is happening.

One of the key features of Einstein-Dilaton-Gauss-Bonnet gravity is that black holes have a minimum mass, so theorists have been able to study what happens when an evaporating black hole starts to reach that minimum mass.

In some cases, depending on the exact nature of the theory and the evolution of the black hole, the evaporation process leaves behind a microscopic nugget. This nugget wouldn’t have an event horizon, so basically you could fly up to it in your spaceship and pick it up. Although the nugget would be extremely exotic, it would at least retain all the information that fell into the original black hole, thus resolving the paradox.

Another possibility is that the black hole reaches its minimum mass and loses its event horizon, but still retains a singularity. These “naked singularities” seem forbidden in ordinary general relativity, but if they existed, they would be a direct window into the realm of quantum gravity.

It is still unclear whether Einstein-Dilaton-Gauss-Bonnet gravity represents a valid path to quantum gravity. But results like this are helping physicists shed light on one of the most complex scenarios in the universe, and perhaps suggest how to solve it.

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