Ring-shaped black holes in 5D universe could produce naked singularities

Black holes existing in a five-dimensional universe could produce a "naked singularity", physicists have said. The scientists successfully simulated a black hole shaped like an extremely thin ring, which eventually became so stretched it broke up into a series of miniature black holes.

The team, from the University of Cambridge and Queen Mary University of London, published their findings in the journal Physical Review Letters. Theoretical physicists first "discovered" ring-shaped black holes almost 15 years ago, but their dynamics have never before been successfully simulated using supercomputers. Should these black holes actually exist, they could cause Einstein's general theory of relativity to break down.

General relativity (at the moment) underpins our understanding of the universe and has so far passed every test – including the recent discovery of gravitational waves. However, one of its major limitations is the existence of singularities.

A singularity is a point in space where gravity is so intense, the laws of physics break down. The theory predicts these singularities exist at the centre of black holes, surrounded by an event horizon. This means they cannot be observed from the outside of the black hole, thus they cannot be tested.

Markus Kunesch, one of the study authors, said: "As long as singularities stay hidden behind an event horizon, they do not cause trouble and general relativity holds – the 'cosmic censorship conjecture' says that this is always the case. As long as the cosmic censorship conjecture is valid, we can safely predict the future outside of black holes. Because ultimately, what we're trying to do in physics is to predict the future given knowledge about the state of the universe now."

However, what the latest study did was to look at what would happen if a singularity existed outside of a black hole. If this was to happen, it would be an object that had collapsed into an infinite density and a state that functions outside of the laws of physics. Previous theories have said this naked singularity could exist in higher dimensions.

We perceive the universe in three dimensions with a fourth dimension of time. Together this is spacetime. However, string theory (in theoretical physics) says there could be up to 11 dimensions. The general theory of relativity does not say how many dimensions there are, so scientists have studied it in higher dimensions to see if this "cosmic censorship" still works.

The latest study indicates that ring-shaped black holes in five dimensions could break up to produce naked singularities. They performed a full simulation of Einstein's theory in higher dimensions to confirm the black hole rings are unstable and that eventually, they could form bulges that eventually break up – like a trickle of water breaking up into droplets.

Study co-author Saran Tunyasuvunakool said: "If naked singularities exist, general relativity breaks down. And if general relativity breaks down, it would throw everything upside down, because it would no longer have any predictive power – it could no longer be considered as a standalone theory to explain the universe.

"The better we get at simulating Einstein's theory of gravity in higher dimensions, the easier it will be for us to help with advancing new computational techniques – we're pushing the limits of what you can do on a computer when it comes to Einstein's theory. But if cosmic censorship doesn't hold in higher dimensions, then maybe we need to look at what's so special about a four-dimensional universe that means it does hold."