Harvard Astronomy School Dean: We may have detected the naked singularity

There is a singularity at the center of a black hole, where the curvature of space-time is so great that the known laws of physics generally fail. But if the si...

Jan 07,2021 | Janice

There is a singularity at the center of a black hole, where the curvature of space-time is so great that the known laws of physics generally fail. But if the singularity is not imprisoned inside the black hole, these "naked singularities" could have unpredictable effects on the outside world. While some theories have tried to circumvent the existence of naked singularities in the past, in this article Avi Loeb, dean of the Harvard School of Astronomy and founder of the Harvard Black Hole Project, suggests that we may have detected naked singularities but just not been aware of them. Perhaps future theories will allow for the existence of naked singularities.

The region around a black hole where not even light can escape is called the event horizon. Just like the slang saying about Las Vegas (whatever happens in Vegas stays in Vegas), what happens within the event horizon stays within the event horizon, and no light, matter, or information can escape from it.

The event horizon of a black hole not only traps this information, but also prevents the widespread failure of the laws of physics in the universe. All matter entering a black hole ends up in a singularity, where the curvature of space-time is so great that Einstein's equations of general relativity fail. Fortunately, the event horizon "wraps around" the singularity, thus separating our cosmic space from it. Therefore, although existing theories still do not explain what happens at the singularity and where the matter entering the black hole goes, this does not affect our knowledge of the universe beyond the event horizon.

However, is it possible that there is a "naked" singularity that is not wrapped in the horizon - a "naked singularity". If a naked singularity does exist, then the existing laws of physics will fail in the space-time we live in, and we will lose the ability to predict future events using general relativity.

In order to avoid this crisis, physicist Roger Penrose proposed the cosmic censorship hypothesis in 1969, which postulates that there is a "supervisor" in the universe who prohibits the emergence of a naked singularity. If this theory holds true, then the regions where general relativity fails are restricted, and outside of these regions, general relativity remains valid.

However, is the existence of cosmic supervision really necessary? We know that Einstein's system of equations for general relativity is flawed in that it is missing an important variable in modern quantum mechanics. Although a complete theory of quantum gravity is still missing, in the future this theory may be able to solve the problem of the singularity and thus provide a theory that can explain the whole spacetime. In this theory, the existence of cosmic supervision may not be required.

In fact, we now have a strong clue that such a theory probably does exist, as Stephen Hawking made a major prediction in 1974 that black holes would evaporate. According to quantum theory, pairs of "imaginary particles" can appear anywhere and anytime. Normally, such a pair of positive and negative particles would quickly annihilate, but if the pair is created near the edge of the horizon, it might result in one particle appearing inside the horizon and the other outside the horizon, and the outside particle could escape with its energy. The escape of the particles would cause the black hole to shrink until it disappears completely, and those escaping particles are called "Hawking radiation".

But then, the evaporating black hole is a naked singularity - because the event horizon does not cover the black hole completely. However, for black holes with masses greater than three suns, the amount of Hawking radiation that can be emitted is too weak. And for some small-mass primordial black holes born at the beginning of the universe, their horizons are too small and have therefore evaporated over the long history of the universe. At present, we have not detected any evidence of black hole evaporation.

In addition, mathematicians and philosophers are debating whether the cosmic supervision in general relativity should be limited to a "reasonable" spacetime range. Meanwhile, computers have been used to solve Einstein's system of equations to find bare singularities under extreme conditions.

At a recent presentation at the Black Hole Initiative at Harvard University, a philosopher suggested that cosmic supervision theory is the most exciting question in modern general relativity research. At the end of his talk, only one question arose for me: "Math and philosophy are arguing here, but astronomers are able to search for the existence of naked singularities in the universe. If we observe these naked singularities, is there still a need for cosmic oversight of their existence?"

I think we should appreciate the opportunity to find them and use them to explore new areas of physics more than theories that avoid naked singularities. Of course, the key to this is "what does a naked singularity look like", for which we do not yet know the answer. Given the great curvature of spacetime at the singularity, the naked singularity might be inside a powerful fireball of energy that would release energetic particles. Have we ever observed such a fireball? Is there a possibility that we have detected the naked singularity and just misunderstood its nature and therefore failed to recognize it?

In the record, one such fireball is a gamma-ray burst. This astronomical phenomenon is usually accompanied by the formation of a black hole or the collision of neutron stars, but it is not associated with a naked singularity. Another possibility is a fast radio burst, but we still don't know what it is.

We need to keep our eyes open. If a bare singularity could really be detected with a telescope, it would be a great opportunity to test quantum gravity theories such as string theory. The problem, however, is that the various existing quantum theories of gravity do not predict what a naked singularity would look like. Therefore, observers should be the leaders in this direction, including searching for gravitational waves released by the Big Bang, a naked singularity, even though it is very challenging.

This article was written by Abraham Loeb, dean of the Harvard Astronomy School, founding director of the Harvard Black Hole Program, and director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. His main research areas are black holes, the future of the universe, and extraterrestrial life.

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