For 100 years since it was published, Einstein’s General Theory of Relativity has mostly passed every kind of test thrown at it. ‘Mostly’ is a keyword here. Because it implies that there are a few scenarios where the theory doesn’t hold up. And one such case is the existence of singularities.
A singularity is defined as a point where the force of gravity is too overwhelming that the laws of physics break down. General relativity says that an example of such singularity exists at the center of a black hole. It also predicts that a black hole is surrounded by what is referred to as an ‘event horizon’ — a.k.a. the ‘point of no return’ because anything that passes through it gets devoured by the black hole, and nothing (not even light) can escape from gravity’s super-strong pull.
This necessarily implies that it is virtually impossible to observe a black hole and its event horizon from the outside or anywhere near it because whatever is being used will just get sucked in, right? There’s actually a term for this — cosmic censorship conjecture. It simply means that a singularity will always be cloaked from view. This also suggests that outside a black hole, the singularity within will not have any effect, and general relativity’s predictions will remain intact. In short, a singularity can’t possibly form outside a black hole.
Still, there’s a name given — should they happen to be real — to a singularity that exists outside a black hole: naked singularity. Predictions about the existence of such have been done before, but they are typically based on a five-dimensional universe. Which is like saying that it’s impossible for a naked singularity to exist, at least in the universe we know.
So does that mean general relativity will remain uncontested? Maybe. Maybe not. Because according to research recently published in the journal Physical Review Letters, it’s not just possible for a naked singularity to exist in an extraordinary (or maybe fictional?) universe; it can exist in a real universe like ours too.
Based on simulations done by physicists Toby Crisford and Jorge Santos from the University of Cambridge, a four-dimensional universe (three spatial dimensions plus time as the fourth dimension just like ours) can host a naked singularity. If it happens to be saddle-shaped that is.
General relativity does allow for universes of different shapes. And a saddle-shaped one, also called Anti-de Sitter space, is one of those possible shapes.
One of the distinguishing features of a saddle-shaped universe is a point of no return where light doesn’t get trapped, but instead gets reflected back. As described by Crisford to Phys.org: “It’s a bit like having a spacetime in a box. At the boundary, the walls of the box, we have the freedom to specify what the various fields are doing, and we use this freedom to add energy to the system and eventually force the formation of a singularity.”
On the other hand, co-author Santos also says that introducing charged particles in their simulation would probably diminish the formation of the naked singularity. “If true, it could imply a connection between the cosmic censorship conjecture and the weak gravity conjecture, which says that any consistent theory of quantum gravity must contain sufficiently charged particles. In Anti-de Sitter space, the cosmic censorship conjecture might be saved by the weak gravity conjecture,” he explains.
It’s not going to be easy to prove the existence of such an extremely curved saddle-shaped universe, though. Nonetheless, just being possible, is already enough to disrupt what we think we know. And that gives scientists so much more to ponder on, which could hopefully lead to more explanations eventually, instead of more questions.