Gravitational Waves Could Help Us Detect the Universe's Hidden Dimensions

gravitational wave
Artist impression of a black hole merger that produced a gravitational wave. LIGO

Updated | Gravitational waves might be used to uncover hidden dimensions in the universe. By looking at these ripples in spacetime, researchers at the Max Planck Institute for Gravitational Physics in Germany say we could work out what impact hidden dimensions would have on them, and use this information to find these effects.

The discovery of gravitational waves was announced in February 2016. Scientists used the Laser Interferometer Gravitational-wave Observatory (LIGO) detectors to find fluctuations in spacetime created by a pair of colliding black holes. Scientists can now use this information to see the universe in a whole new way—potentially even one day tracing waves that came from the Big Bang.

At present, our models of the universe are incomplete. They cannot explain many of the things we observe in the universe, so many physicists believe we are missing something—and that something could be the presence of extra dimensions.

If scientists were to find evidence of extra dimensions, they could start answering some of the most fundamental unknowns of the universe, like what dark matter is and why the universe is expanding at an accelerating rate.

Gravitational waves are ripples in spacetime caused by extremely energetic events. These events, like merging black holes, would release so much energy they would disrupt the way spacetime moves, creating 'waves' that would propagate out from the source—similar to the way a pebble thrown into a pond creates ripples moving outwards.

Gravitational waves were first predicted by Albert Einstein over 100 years ago, but until now we have not been able to find them. By the time the ripples reach us, they are so tiny that detecting them requires hugely sensitive equipment. This is what LIGO was able to do.

In the latest study, which appears on the preprint server, David Andriot and Gustavo Lucena Gómez look at how gravitational waves move through the known dimensions—three representing space and another for time. They then investigate what effects extra dimensions might have on the four dimensional waves we see.

"If there are extra dimensions in the universe, then gravitational waves can walk along any dimension, even the extra dimensions," Lucena Gómez told New Scientist.

They found extra dimensions could have two effects on gravitational waves—firstly, they would have what they call a "breathing mode." This provides another way for gravitational waves to deform, stretch and shrink space.

"The breathing mode deforms the space in a specific manner, giving a distinct signature," they wrote. To observe this change, they would need three detectors like LIGO all working to observe the same thing at the same time—something that "should be available in a near future," they wrote.

The second effect is a "massive tower" of extra gravitational waves. These waves could be detected at high frequencies, something our current technologies prevent. To detect changes at the frequencies they propose, LIGO would need to be thousands of times more sensitive.

The scientists are clear that such apparatus does not exist, but note: "If such a detector were available, however, one could hope for a very clean signal, since there is no known astrophysical process emitting gravitational waves with frequencies much greater than 1,000Hz. Such high frequencies may thus be clear symptoms of new physics."

However, Bobby Acharya, professor of Theoretical Physics at King's College London, U.K., who was not involved in the study, is not convinced by the findings. In an interview with Newsweek , he says that while he firmly believes in the existence of extra dimensions, models suggest these dimensions would be extremely small: "That means that in order to excite them and create waves in those extra dimensions you require a lot of energy," he says.

"And if you did produce the gravitational wave that propagated in the extra dimensions, the fact that extra dimensions are so small it means the frequency of this gravitational wave will be very high—much higher than the LIGO gravitational wave detectors can detect."

He said you would need a "very optimistic point of view" to try to detect gravitational waves propagating in extra dimensions: "[The extra dimensions] would have to be rather large and then it would be difficult to make the model consistent with other observations. I'm not so positive about the result."

This story has been updated with corrections to the frequency of the gravitational waves and how the waves would move in space.​