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Will gravitational waves solve one of physics’ greatest mysteries? (Weekend feature)

gravitational waves

“Gravitational waves will bring us extremely accurate maps of black holes – maps of their space-time. These maps will clearly indicate whether or not we are dealing with black holes as described by general relativity,” said the winner of the Nobel Prize, Caltech. Kip Thorne. Scientists at Cardiff University’s Gravity Exploration Institute are using the technologies behind one of the greatest scientific breakthroughs of the century – the detection of gravitational waves led by Thorne – in the long-running search for dark matter .

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

“First Second After” – Gravitational waves reveal the secrets of the Big Bang

Laser interferometers

In a study published in Nature, the Cardiff team took the first step towards this goal by using the instruments, known as laser interferometers, to search for a new type of dark matter for the first time ever. Until recently, dark matter was widely believed to be composed of heavy elementary particles. These have not been 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 dark matter is actually something called a scalar field, which behaves like 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 (perhaps not at all) with normal matter except by gravity. “We realized that our instruments could be used to hunt this new type of dark matter, although they were originally designed to detect gravitational waves,” said the professor. Hartmut Largefrom the Gravity Exploration Institute at Cardiff University, which carried out the investigation.

“We excluded dark matter from the scalar field in certain mass ranges and for a coupling constant six orders of magnitude lower than that which had been excluded by previous experiments,” Grote said. The Everyday Galaxy. “To explain: we have been looking for a particular type of dark matter, called scalar-field dark matter, which has now been a bit more constrained than previous searches. It’s a piece of the big dark matter puzzle.

In a laser interferometer, two beams of light bounce between mirrors before meeting on a detector. From there, scientists can gauge with great precision how out of sync the light beams are, which itself is an indicator of any disturbance the beams are experiencing.

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

Where we are and where we are going in gravitational wave physics. Kip Thorne’s Nobel Prize speech below:

The British/German GEO 600 detector in Germany, where Grote was the principal scientist from 2009 to 2017, is another highly sensitive interferometer and has been 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 specifically search for dark matter. “The scalar-field dark matter waves would pass through the Earth and our instruments, but in doing so would vibrate objects such as mirrors ever so slightly,” said lead researcher Sander Vermeulen, also from the University of Gravity Exploration Institute. Cardiff.

“The vibrations of the mirrors would disturb the light beams in instruments like GEO600 or LIGO detectors in a particular way characteristic of dark matter, which we should be able to detect, based on the exact properties of this dark matter”, explains Vermeulen. .

Although 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 invisible matter can explain why galaxies spin the way they do and how they could have formed in the first place.

Is dark matter just the tip of an invisible universe of unknown forces?

Important first strides

Although the team failed to make any kind of detection in this new study, they say they are taking important first steps in terms of introducing this technology to dark matter searches and have already made progress. in terms of reducing some parameters for future studies. “I was surprised at how sensitive an instrument can be to hunt dark matter when it was originally built for an entirely different purpose,” Grote continued.

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

New experiments yet to be built to detect dark matter must therefore be designed to be sensitive to dark matter with a different mass or weaker coupling strength if they are to have any hope of making a detection.

The last word

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

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

Top image credit: Shutterstock License

Maxwell Moeastrophysicist, NASA Einstein Fellow, University of Arizona via Hartmut Large, Zander Vermeulen, Cardiff University and Nature

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