When matter gets swallowed by a black hole, it could fall into another universe contained inside the black hole, or get trapped inside a wormhole-like connection to a second black hole, a new study suggests.
What's inside a black hole is one of the biggest mysteries in physics. The theory that predicted black holes in the first place – general relativity – says that all the matter inside them gets squashed into a central point of infinite density called a singularity. But then, "things break down mathematically", says Christian Böhmer of University College London, in the UK. "We would like to see the singularity removed."
Many researchers believe that some kind of new, overarching theory that unites gravity and quantum effects will resolve the problem. String theory is the most popular of these alternatives.
But Böhmer and colleague Kevin Vandersloot of the University of Portsmouth in the UK use a rival approach called loop quantum gravity, which defines space-time as a network of abstract links that connect tiny chunks of space.
Loop quantum gravity has been used before to tackle the singularity that would seem to have occurred at the origin of our universe. It suggests that instead of a big bang, an earlier universe could have collapsed and then exploded outward again in a "big bounce".
A similar repulsiveness appeared when the loop quantum approach was previously applied to the inside of a black hole with particular properties. Those studies suggested there was a repulsive boundary that blocked matter from clumping together in the singularity.
But Böhmer and Vandersloot wanted to see what happened if they applied loop quantum gravity to black holes in general. Because loop quantum gravity equations cannot be solved exactly for the inside of every black hole, the researchers used computers to approximate what would happen to the infalling matter.
"We were very surprised about the results," Böhmer says. Instead of a boundary around the singularity, they got two other kinds of solutions – both bizarre – that replaced the singularity.
Böhmer realised that one set of answers looked like a so-called 'Nariai universe' – a mathematical model of a universe allowed by general relativity in which the universe expands in only one spatial direction. (Our observed universe appears to be a "de Sitter space" instead because it expands in all three dimensions, so that distant galaxies move away from us no matter where we look in the sky.)
"The interior becomes a universe of its own," Böhmer says. Instead of matter falling into a singularity, it would travel forever through this Nariai universe, which it would experience as infinite in size – even though it fits inside a black hole of finite size.
The other set of solutions they came up with were for a tunnel-like connection between the mouths of two black holes. The tunnel is reminiscent of a wormhole, a hypothetical feature of space-time that connects two distance points via a shortcut. In this case, it's not clear yet what would happen to matter inside, but it could oscillate back and forth inside of the two-mouthed black hole.
The new study is a "significant step forward," says Carlo Rovelli of the Centre for Theoretical Physics in Marseille, France.
"The idea of applying loop quantum gravity to resolve the singularity at the centre of a black hole was started some time ago," he told New Scientist. "But it is now reaching a stage of maturity, where one can indeed compute concretely how quantum space-time in the centre of a black hole could actually look."
But one physicist contacted by New Scientist who did not want to be quoted by name says the new work may not actually do away with the problem of singularities in black holes. He says a Nariai universe is inherently unstable, so it would eventually either collapse or become a de Sitter universe – which would itself harbour black holes.
If that is so, then black holes may contain their own universes, but those universes would likely contain their own black holes, which could contain their own universes … in an infinite loop.