Dangerous "naked" black holes may be hiding in the universe

According to mainstream physics, every object that swallows the matter of the universe is surrounded by an event horizon. Once you cross this boundary, you fall in and never get out.

But what if some black holes are "naked", i.e., they have no such boundary at all? As far as we know, singularity is always wrapped within the event horizon, but a more detailed look at the mathematics of general relativity shows that this is not the case.

If such "naked" black holes do exist and are scattered throughout our universe, how do we detect and find them? Scientists tell us one way: by looking at the halo around it.

Naked singularities

Black holes are the result of Einstein's mathematical theory of general relativity. These equations tell us that if a mass collapses to a volume too small, then its gravity will make it smaller and smaller until it is crushed into an infinitely small point. This point is called a singularity, which is a signal that the mathematics we use to describe spacetime has completely collapsed.

The gravitational force of a singularity is infinitely strong. Objects are pulled toward the singularity faster than the speed of light. Near the singularity, general relativistic physics is no longer able to predict the future trajectory of particles, which is one of the main ideas of physics. Without the ability to predict, physics would collapse.

Fortunately, as far as we know, all singularities are wrapped in event horizons. The event horizon is the distance from the singularity at which the gravitational force is strong enough to pull in anything. At that point, you would have to be traveling faster than the speed of light to escape. That's what makes a black hole black -- even light can't escape.

Since humans first discovered the existence of black holes, we've wondered if it's possible to form a singularity without a relevant horizon -- a so-called "naked singularity". This would be a very dangerous place because it would be the place where the laws of physics would break down and the rest of the universe would be perfectly accessible. At least in a conventional black hole, the singularity is safely wrapped under the event horizon, so that even though it is an extremely unknown place of physics, at least anything that happens is locked away from the rest of the universe.

Distorting a point

If bare singularities exist, they are certainly not common. We know of only one sure way to form a singularity, and that is when a giant star runs out of fuel and collapses on itself. When this happens, the singularity will naturally have an event horizon.

The existence of naked singularities is so disturbing to physicists that they speculate that perhaps nature doesn't allow them to exist at all. But so far, we don't have any evidence to support this idea.

Although, only under the most extreme conditions could naked singularities form. If a black hole is spinning, it can form a second horizon, located in the interior of the first horizon. The faster the black hole spins, the closer these horizons are to each other. If it spins fast enough, the math predicts that the event horizons can "cancel out" (the actual physics is of course much more complicated) and reveal a "naked" singularity.

So far, we have not found any black holes spinning fast enough to reveal their singularities, but beyond that, we usually cannot determine whether a random astrophysical object is an ordinary black hole or a naked singularity. Images provided by line-of-sight telescopes are an exception, but of course, we do not have enough such images.

Making a difference

And now, a theoretical physicist has faced this challenge head-on and begun to investigate whether a naked singularity might appear otherwise, especially when it is surrounded by a ring of matter. This ring is called an accretion disk and is a common feature around black holes (and potentially bare singularities). When gas and dust fall on a dense object, the material flattens into a disk and then pools down. This disk is very bright, proving the existence of black holes (in fact, this is how we know that the vast majority of black holes in the universe exist).

Most theoretical studies of naked singularities assume that the object exists in isolation, which doesn't hold in the real universe. But in this new work, the theorist examines the entire complex and finds a surprising result.

The accretion disk is not completely separate from the black hole (or naked singularity). The disk itself has its gravitational force, which distorts and deforms the dense object at its center. This distortion in turn affects the gravitational environment around the object, subtly altering the path of matter rotating inward.

This theorist found that bare singularities do behave slightly differently from normal black holes and that the accretion disk around a bare singularity may be much brighter than the light around a black hole. So far, our telescopes can't tell the difference, but future instruments could, perhaps an updated version of the Event Horizon Telescope could do so.

If a naked singularity could indeed be found in the real universe, it would be a major discovery in physics. We could point to a place in the sky where we know that the knowledge we have collapses. So a more detailed study of the environment around the confirmed naked singularities would reveal some of the deepest mysteries of the universe.