Even the strongest gravitational waves that pass through the planet, created by the distant collisions of black holes, only stretch and compress each mile of Earth’s surface by one-thousandth the diameter of an atom. It’s hard to conceive of how small these ripples in the fabric of spacetime are, let alone detect them. But in 2016, after physicists spent decades building and fine-tuning an instrument called the Laser Interferometer Gravitational-Wave Observatory (LIGO), they got one.
With nearly 100 gravitational waves now recorded, the landscape of invisible black holes is unfurling. But that’s only part of the story.
Gravitational wave detectors are picking up some side gigs.
“People have started to ask: ‘Maybe there’s more to what we get out of these machines than just gravitational waves?’” said Rana Adhikari, a physicist at the California Institute of Technology.
Inspired by the extreme sensitivity of these detectors, researchers are devising ways to use them to search for other elusive phenomena: above all, dark matter, the nonluminous stuff that holds galaxies together.
In December, a team led by Hartmut Grote of Cardiff University reported in Nature that they had used a gravitational wave detector to look for scalar-field dark matter, a lesser-known candidate for the missing mass in and around galaxies. The team didn’t find a signal, ruling out a large class of scalar-field dark matter models. Now the stuff can only exist if it affects normal matter very weakly—at least a million times more weakly than was previously thought possible.
“It’s a very nice result,” said Keith Riles, a gravitational wave astronomer at the University of Michigan who wasn’t involved in the research.
Until a few years ago, the leading candidate for dark matter was a slow-moving, weakly interacting particle similar to other elementary particles—a sort of heavy neutrino. But experimental searches for these so-called WIMPs keep coming up empty-handed, making room for myriad alternatives.
“We’ve kind of reached the stage in dark matter searches where we’re looking everywhere,” said Kathryn Zurek, a theoretical physicist at Caltech.
In 1999, three physicists proposed that dark matter might be made of particles that are so light and numerous that they’re best thought of collectively, as a field of energy that permeates the universe. This “scalar field” has a value at each point in space, and the value oscillates with a characteristic frequency.
Scalar-field dark matter would subtly alter the properties of other particles and fundamental forces. The electron’s mass and the strength of the electromagnetic force, for example, would oscillate with the oscillating amplitude of the scalar field.
For years, physicists have wondered whether gravitational wave detectors could spot such a wobble. These detectors sense slight disturbances using an approach called interferometry. First, laser light enters a “beam splitter,” which divides the light, sending beams in two directions at right angles to each other, like arms of an L. The beams reflect off mirrors at the ends of both arms, then return to the hinge of the L and recombine. If the returning laser beams have been pushed out of sync—for instance, by a passing gravitational wave, which briefly lengthens one arm of the interferometer while contracting the other—a distinct interference pattern of dark and light fringes forms.
Could scalar-field dark matter push the beams out of sync and cause an interference pattern? “The common thinking,” said Grote, was that any distortions would affect both arms equally, canceling out. But then in 2019, Grote had a realization. “One morning I woke up and the idea came to me suddenly: The beam splitter is exactly what we need.”
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