Will Gravitational Waves Solve One of the Biggest Mysteries in Physics?

 

Gravitational Waves

 

“Gravitational waves will bring us exquisitely accurate maps of black holes – maps of their space-time. Those maps will make it crystal clear whether or not what we’re dealing with are black holes as described by general relativity,” said Nobel Prize laureate, Caltech’s Kip Thorne. Now, scientists at Cardiff University’s Gravity Exploration Institute are using the technologies behind one of the biggest scientific breakthroughs of the century—the detection of gravitational waves led by Thorne— in the long-standing search for dark matter.

With extremely sensitive detectors now at their disposal, already proven through several outstanding discoveries, scientists believe that existing gravitational wave technology has the true potential to finally discover the elusive, exotic material. Dark matter is one of the biggest unsolved mysteries in modern physics, the gravitational wave observatories may finally find out what it is made of.

In a study published in Nature, the Cardiff team has taken the first step towards this goal by using the instruments, known as laser interferometers, to look for a new kind of dark matter for the very first time. Until recently, it was widely believed that dark matter was composed of heavy elementary particles. These were not discovered despite a multitude of efforts, and scientists are now turning to alternative theories to explain dark matter.

 

The Scalar Field

A recent theory says that dark matter is actually something called a scalar field, which would behave as invisible waves permeating all galaxies, including our own Milky Way. According to the scalar field hypothesis, ultralight dark matter behaves more like waves than particles, and it interacts extremely weakly (possibly not at all) with normal matter except through gravity. “We realized our instruments could be used to hunt for this new kind of dark matter, although they were initially designed for detecting gravitational waves,”‘ said professor Hartmut Grote, from Cardiff University’s Gravity Exploration Institute, who led the investigation.

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“We have ruled out scalar field dark matter in some mass ranges and for a coupling constant that is six orders of magnitude smaller than had been ruled out by previous experiments,” Grote told The Daily Galaxy. “To explain: we searched for a particular type of dark matter, named scalar field dark matter, which now has been constrained a bit more than by previous searches. It is a piece in the big puzzle of dark matter.”

Within a laser interferometer, two beams of light are bounced between mirrors before meeting up on a detector. From this, scientists can gauge with great accuracy how out of sync the beams of light are with each other, which is itself proxy for any disturbance the beams encounter.

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The Laser Interferometer Gravitational-Wave Observatory (LIGO) consists of two interferometers located in the US, each with two 4 km long arms arranged in the shape of an “L,” which were used to detect gravitational waves for the very first time in 2015, and many times since. 

Where we are and where we’re going in gravitational waves physics. Kip Thorne’s brilliant Nobel Prize talk below:

The UK/German GEO 600 detector in Germany, where Grote was lead scientist from 2009 to 2017, is another highly sensitive interferometer and was used to develop much of the technology needed to detect gravitational waves. The GE0600 detector was used, for the very first time, in this study to search specifically for dark matter. “Scalar field dark matter waves would pass right through the Earth and our instruments, but as they do so, would cause objects such as mirrors to vibrate ever so slightly,”‘ said lead investigator Sander Vermeulen, also from Cardiff University’s Gravity Exploration Institute.

“Vibrations of mirrors would disturb the beams of light in instruments like GEO600 or the LIGO detectors in a particular way characteristic of dark matter, which is something we should be able to detect, depending on the exact properties of that dark matter,” says Vermeulen.

Even though dark matter has never been directly detected, scientists suspect it exists due to its gravitational effect on objects across the universe. For example, a large amount of unseen matter may explain why galaxies rotate as they do, and how they could have formed in the first place.

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Important First Strides 

Though the team were unsuccessful in making any sort of detection in this new study, they say they are making important first strides in terms of introducing this technology to dark matter searches and have already made progress in terms of narrowing down certain parameters for future studies. “I was surprised by how sensitive an instrument can be for hunting dark matter when it was built for an entirely different purpose originally,” continued Grote.

“We have definitively ruled out some theories that say dark matter has certain properties, so future searches now have a better idea of what to look for,” said Vermeulen. “We believe these new techniques have the true potential to discover dark matter at some point in the future.”

“We have shown that – given a certain range of possible masses of the dark matter- the interactions between dark matter and normal matter are much weaker than the previously known upper limit,” wrote Vermeulen in reply to an email from The Daily Galaxy asking what dark-matter properties that were ruled out. “This result,” he notes, “thus rules out the existence of any dark matter with a mass in a certain range and with a certain coupling strength.  New yet-to-be-built experiments for detecting dark matter therefore need to be designed to be sensitive to dark matter with a different mass or a weaker coupling strength if they are to have any hope of making a detection.”

Source: Sander M. Vermeulen et al, Direct limits for scalar field dark matter from a gravitational-wave detector, Nature (2021). DOI: 10.1038/s41586-021-04031-y

Image credit top of page: Shutterstock License

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona  via Hartmut Grote, Sander VermeulenCardiff University and Nature

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