We still don’t know exactly what happens when black holes die.
Since Stephen Hawking discovered that black holes evaporatedid we know that they could possibly disappear from our universe. But our understanding of gravity and quantum mechanics is insufficient to describe the final moments of a black hole’s life.
Now, new research motivated by string theory suggests possible and equally odd fates for evaporating black holes: a leftover nugget that we could, in principle, access, or a singularity unobscured by an event horizon.
Related: What happens at the center of a black hole?
The importance of Hawking radiation
black holes are not strictly black. In pure general relativity, without other modifications or considerations of other physics, they remain black for eternity. Once one forms, it will just hang there and be a black hole forever. But in the 1970s, Hawking used the language of quantum mechanics to study what happens near the boundary of a black hole known as the event horizon.
He surprisingly found that a strange interaction between the quantum fields of our universe and the one-way barrier of the event horizon provided a way for energy to escape from the black hole. This energy takes the form of a slow but steady stream of radiation and particles that has come to be known as Hawking radiation. With every bit of energy that escapes, the black hole loses mass and shrinks, eventually disappearing from existence altogether.
The emergence of Hawking radiation created what is known as Black hole information paradox. All information describing material falling into a black hole crosses the event horizon and is never seen again. But the Hawking radiation itself carries no information, and yet the black hole eventually disappears. So where has all the information gone?
Related: Stephen Hawking was right: Black holes can evaporate, strange new study shows
Go beyond Einstein
The black hole information paradox is a giant, flashing neon sign for physicists that we don’t understand something. We may not understand the nature of quantum information, the nature of heaviness 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 it Einstein’s general theory of relativity.
After all, we already know that general relativity breaks down at the centers of black holes, which are tiny holes in spacetime known as singularities where density goes to infinity. The only way to correctly describe the singularity is with a quantum theory of gravity that correctly predicts how strongly gravity behaves on extremely small scales.
Unfortunately, we currently lack a theory on this quantum gravity. It would be nice to look at singularities directly, but as far as we understand general relativity, all singularities are trapped behind event horizons, making them inaccessible to us.
But by studying the process of Hawking radiation, maybe we can find a shortcut to approach a singularity and understand the crazy physics that’s happening there. As black holes evaporate, they get smaller and smaller, and their event horizons get uncomfortably close to the central singularities. In the final moments of black hole life, gravity becomes too strong and the black holes become too small for our current knowledge to properly describe. So if we can develop a better theory of gravity, we can use the final moments of Hawking radiation to test how the theory behaves.
There are many candidates for a quantum theory of gravity, with string theory being the most advanced. Although there are no known solutions to string theory, it is possible to take what we know about the theory’s general features and create modified versions of general relativity from it.
Related: How Stephen Hawking changed our understanding of black holes
These modified theories are not the “complete” correct replacement for general relativity, but they allow us to study how gravity might behave as it approaches the quantum limit ever closer. Recently, a team of theorists used such a theory, known as Einstein-Dilaton-Gauss-Bonnet gravity, to study the ultimate final states of evaporating black holes. They detailed their work in an article published in the preprint database arXiv (opens in new tab) in May.
The details of the team’s results are a bit hazy. This is because modified general relativity is not as well understood as regular general relativity, and solving the complicated math requires a variety of approximations and a lot of guesswork. Still, the researchers were able to paint a general picture of what’s going on.
One of the key features of Einstein-Dilaton-Gauss-Bonnet gravity is that black holes have a minimum mass, allowing theorists to study what happens when an evaporating black hole starts to reach that minimum mass.
Depending on the exact nature of the theory and the evolution of the black hole, in some cases the evaporation process leaves behind a microscopic nugget. This nugget would be missing an event horizon, so in principle you could fly your spaceship there and pick it up. While the nugget would be extremely exotic, it would at least retain all of the information that fell into the original black hole, solving 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” appear to be forbidden in normal general relativity, but if they exist they would be direct windows into the realm of quantum gravity.
It is still unclear whether Einstein-Dilaton-Gauss-Bonnet gravity is a valid route to quantum gravity. But findings like these help physicists shed light on one of the most complex scenarios in the universe and may provide clues on how to solve them.
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