Like the surface of an ocean, the fabric of the Universe is constantly rippling with an infinitude of gravitational waves, propagating outward across the cosmos.
We can count, in triple digits, the number of these ripples we've detected to date – and most of them have been linked to collisions between black holes, the densest objects in the Universe.
But how do we know that every collision attributed to black holes is what it looks like? And what of the gravitational wave signatures we are yet to detect?
What if there's something else out there that can make the Universe sing? What if colliding wormholes can make a similar splash – and how would we recognize it when we find it?
For one particular kind of wormhole, at least, new simulations suggest collisions would be profoundly strange – and could leave distinctive signatures in the fabric of spacetime.
According to a preprint authored by physicist Nikita M. Shirokov of Moscow Institute of Physics and Technology and machine learning engineer Ilya Nachevsky of the Higher School of Economics in Russia, several things can happen when binary Ellis-Bronnikov wormholes interact – and most of them are extremely weird.
When you think of wormholes, you probably think first of science fiction – jump gates, hyperspace, tunnels through the cosmic backrooms that provide a shortcut from one region to another.
But, like many of the ideas presented in Star Trek and The Expanse, wormholes are firmly rooted in real scientific theory – a mathematical possibility emerging from Einstein's theory of general relativity, which describes gravity as the curvature of spacetime.
In theory, spacetime could be curved in such a way that two otherwise distant regions are connected by a tunnel, or throat – that's a wormhole.
Whether such a structure could actually exist in the Universe remains an open question.
You've probably heard the term Einstein-Rosen bridge. That's a type of wormhole that is non-traversable. A different model is the Ellis-Bronnikov wormhole – a traversable passage held open by something called a phantom scalar field.
This hypothetical field has unusual negative-energy properties that prevent the throat from pinching shut.
Traversable means something could go through it, but it does not mean stable, so whether a human should go through it is another matter.
This is particularly true for Ellis-Bronnikov wormholes.
Although the phantom scalar field can hold a wormhole's throat open, even a small disturbance can upset the delicate balance. The throat may collapse, forming a black hole, or begin expanding in a runaway process called inflation.
So you'd expect that if two of these volatile structures somehow cross paths, something pretty spectacularly destructive might happen.
To find out what those fireworks might look like, Shirokov and Nachevsky ran three-dimensional simulations of pairs of Ellis-Bronnikov wormholes, varying their initial separation, trajectories, and the properties of the phantom scalar fields that hold them open.
The results were surprisingly diverse.
Depending on the configuration, the wormholes could attract or repel one another. Some passed close enough to interact before flying apart, their throats expanding dramatically in the process.
Others plunged towards each other – but instead of merging into one larger wormhole, something far stranger happened.
A common apparent horizon – similar to a black hole's event horizon – formed around both throats, enclosing them together inside a developing black hole.
This is where things got even weirder – because that apparent horizon then did something that black hole horizons aren't supposed to do.
It shrank.
As the phantom scalar field fell into the collapsing region of space, the horizon lost about 28 percent of its area.
Under ordinary circumstances, the surface area of a black hole's event horizon can only increase or stay the same, according to the black hole area theorem proposed by Stephen Hawking in 1971.
But that theorem relies on assumptions about the behavior of energy that the phantom scalar field violates. Its exotic negative-energy properties allow the simulated horizon to shrink rather than grow.
This doesn't contradict the area theorem, since the simulation involves an apparent horizon rather than an event horizon, and a field that violates the theorem's assumptions. But it's still a striking result.
Now, at this point, you're probably wondering what all this has to do with gravitational waves.
When massive objects such as black holes spiral inward toward a collision, their changing motion sends ripples through spacetime, like a boat's wake on the ocean. That's what gravitational waves are.
Wormholes, at least the massive Ellis-Bronnikov wormholes modeled in this study, should also generate gravitational waves as they move and interact.
But the researchers found that the resulting signals would be quite different from those produced by a pair of black holes spiraling toward each other.
The spiraling black hole pair has a distinctive signal called a chirp that grows stronger and higher in frequency as the two objects draw closer together.
By contrast, the researchers' simulations of wormhole encounters produced short, sharp bursts of gravitational waves.
The signal's precise shape and strength depended on whether the wormholes plunged toward each other or merely grazed each other in passing.
Interestingly, some of the closest flybys produced particularly strong gravitational-wave bursts, even without the formation of a common apparent horizon.
This doesn't mean that a gravitational-wave burst without a chirp is evidence of wormholes. Black holes can also produce such signals under certain circumstances.
One particularly unusual signal, GW190521, was detected in 2019 and has since been investigated as a possible signature of exotic spacetime physics.
That doesn't mean it was produced by a wormhole, and the new simulations don't establish a connection to that event. But it illustrates why understanding the possible signatures of hypothetical objects is worthwhile.
There's another catch, though. The researchers note that wormholes formed in the early Universe would produce signals stretched by cosmic expansion to frequencies far below those detectable by gravitational-wave observatories such as LIGO and the planned LISA mission.
For these particular signals to be detectable, the wormholes would need to form much later in cosmic history – and scientists don't currently know how that could happen.
Still, the Universe has surprised us before. And now, if we somehow do manage to record the signal of an Ellis-Bronnikov wormhole collision, we'll have a better idea of what we might be looking at.
The research, which is yet to be peer-reviewed, is available on preprint server arXiv.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.