Even the strongest The planet-passing gravitational waves, created by the distant collisions of black holes, only stretch and compress every mile of Earth’s surface by one-thousandth the diameter of an atom. It’s hard to imagine how small these ripples in the fabric of spacetime are, let alone detect them. But in 2016, after physicists spent decades building and refining an instrument called the Laser Interferometer Gravitational-Wave Observatory (LIGO), they have a.
With nearly 100 gravitational waves currently recorded, the landscape of invisible black holes is opening up. But that’s only part of the story.
Gravitational wave detectors are picking up some side gigs.
“People are starting to ask: ‘Could there be more to these machines than just gravitational waves?'” Rana Adhikaria physicist at the California Institute of Technology.
Inspired by the extreme sensitivity of these detectors, researchers are figuring out how to use them to search for other elusive phenomena: dark matter, above all, non-luminous matter. hold the galaxies together.
In December, a group led by Hartmut Grote of Cardiff University report in Nature that they used a gravitational wave detector to search for scalar dark matter, a little-known candidate for the missing mass in and around galaxies. The team found no signal, ruling out a large class of scalar-field dark matter models. Now, the thing that can only exist if it affects normal matter is very weak – at least a million times weaker than was previously thought possible.
“It was a very nice result,” said Keith Rilesa gravitational wave astronomer at the University of Michigan who was not involved in the study.
Until a few years ago, the leading candidate for dark matter was a slow-moving, weakly interacting particle similar to other elementary particles – a type of heavy neutrino. But test searches for these so-called WIMPs continue to come with empty handsgive sit to countless alternatives.
“We have reached the stage of searching for dark matter, where we are looking everywhere,” said Kathryn Zureka theoretical physicist at Caltech.
In 1999, three physicists propose that dark matter can be made up of particles so light and so numerous that they are considered the best as a whole, as an energy field throughout the universe. This “scalar field” has a value at each point in space and this value oscillates with a characteristic frequency.
Scalar-field dark matter will subtly alter the properties of other fundamental particles and forces. For example, the mass of the electron and the strength of the electromagnetic force would fluctuate with the amplitude of the scalar field.
For several years, Physicists wondered whether a gravitational wave detector could detect such oscillations. These detectors sense slight disturbances using a method called interferometry. The laser light first enters a “beam splitter,” which splits the light, sending the beams in two directions at right angles to each other, much like the arm of an L. The beams are reflected off the mirror at the ends of both arms, then return to the hinge of the L and combine. If the returning laser beams are synchronously ejected — for example, by a passing gravitational wave, which will lengthen one arm of the interferometer during the shortening of the other — a shape separate interference patterns of dark and light fringes will form.
Can scalar-field dark matter push beams out of sync and cause interference patterns? “The general thinking,” says Grote, is that any deformity will affect both arms equally, and will be suppressed. But then in 2019, Grote realized. “I woke up one morning and an idea came to me: The beam splitter was exactly what we needed.”
