We might have to come up with a new way to explain the universe.
Researchers from the University of Cambridge and Queen Mary University of London have shown how a weirdly-shaped black hole could cause Einstein’s general theory of relativity, the foundation of many fields of physics, to break down. However, this object could only exist in a universe with five or more dimensions.
Using the COSMOS supercomputer, the researchers successfully simulated a black hole shaped like a very thin ring, which develops bulges that are connected by strings that become thinner and thinner over time. These strings eventually become so thin that they break off into a series of miniature black holes.
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Ring-shaped black holes were theorized to exist by physicists back in 2002, but this is the first time that one has been successfully simulated. If this type of black hole were to form, it would lead to the appearance of what is known as a naked singularity — causing the equations behind the theory of general relativity to break down.
You may not even realize how fundamental general relativity is to to our current understanding of gravity. Physicists use it to estimate the age of the stars and the universe, and we use it everyday for the GPS signals we need for directions and navigation.
The theory of relativity states that objects warp their surrounding spacetime, and what we call gravity is the effect of this warp. Amazingly, it has been 100 years since the theory was first published, and it has passed every test thrown its way except for one major limitation: the existence of singularities.
A singularity is a point where gravity is so strong that space, time and the laws of physics break down. General relativity does predict the existence of singularities at the center of black holes, but only those that are surrounded by an event horizon — a point of no return where the gravitation pull is so strong not even light can escape.
“As long as singularities stay hidden behind an event horizon, they do not cause trouble and general relativity holds — the 'cosmic censorship conjecture’,” said study co-author Markus Kunesch, a PhD student at Cambridge's Department of Applied Mathematics and Theoretical Physics (DAMTP) in a press release.
But what if a singularity existed outside an event horizon? If it did, it would represent an object that has collapsed to an infinite density, which causes the laws of physics to break down. Theoretical physicists have hypothesized that these naked singularities might exist, but in higher dimensions.
“If naked singularities exist, general relativity breaks down,” said co-author Saran Tunyasuvunakool, also a PhD student from DAMTP. “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.”
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We currently think of the universe as existing in three dimensions, plus the fourth dimension of time, and together they are referred to as spacetime. However, Einstein’s theory does not state how many dimensions there are in the universe, so physicists have been studying it in higher dimensions to see if the cosmic censorship still holds.
What the researchers found was that if a ring-shaped black hole is thin enough, it can lead to the formation of naked singularities. Only a very thin black ring becomes unstable enough to form bulges connected by thinner and thinner strings, eventually breaking off and forming a naked singularity.
“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,” said Tunyasuvunakool. “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.”
If the cosmic censorship conjecture is disproved, we would have to come up with another way to explain the universe. No easy task — however, one possibility is quantum gravity, which approximates Einstein's equations far from a singularity but provides a description of new physics close to the singularity.