A new dark matter finding tool digs nothing

Even the strongest gravitational waves passing through the planet, created by distant black hole collisions, stretch and compress each mile of Earth’s surface by only one-thousandth of the diameter of an atom. It’s hard to imagine how tiny these space-time ripples 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 got one.

With nearly 100 gravitational waves registered, the landscape of invisible black holes is unfolding. But that’s only part of the story.

Gravitational wave detectors pick up some side effects.

“People have started asking, ‘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: especially dark matter, the non-luminous material 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 search for dark matter in a scalar field, a lesser-known candidate for the missing mass in and around galaxies. The team found no signal, ruling out a large class of dark matter models with scalar fields. Now the stuff can only exist if it affects normal matter very weakly — at least a million times weaker than previously thought possible.

“It’s a really nice result,” said Keith Riles, a gravitational wave astronomer at the University of Michigan who was not involved in the study.

Until a few years ago, the prime candidate for dark matter was a slow-moving, weakly interactive particle similar to other elementary particles — a type of heavy neutrino. But experimental searches for these so-called WIMPs continue to turn up empty-handed, leaving room for countless alternatives.

“We’ve kind of reached a stage in the dark matter search that we’re looking at everywhere,” said Kathryn Zurek, a theoretical physicist at Caltech.

In 1999, three physicists proposed that dark matter could be made of particles so light and numerous that they are best considered collective, like an energy field that permeates the universe. This “scalar field” has a value at any point in space, and the value oscillates at a characteristic frequency.

Dark matter in a scalar field would subtly alter the properties of other particles and fundamental forces. For example, the mass of the electron and the strength of the electromagnetic force would oscillate with the oscillating amplitude of the scalar field.

For years, physicists have wondered whether gravitational wave detectors could detect such wobble. These detectors detect light disturbances using an approach called interferometry. First, laser light enters a “beam splitter”, which splits the light and sends beams in two directions perpendicular to each other, like arms of an L. The beams bounce off mirrors at the ends of both arms, then return to the hinge of the L and recombine. If the returning laser beams are pushed out of sync — for example, by a passing gravitational wave, which briefly extends one arm of the interferometer while contracting the other — a distinct interference pattern of dark and light fringes forms.

Can dark matter with a scalar field throw the rays out of sync and create an interference pattern? “The general idea,” Grote said, was that any deformations would affect and lift both arms equally. But in 2019, Grote had a realization. “I woke up one morning and suddenly the idea came to me: the beamsplitter is exactly what we need.”

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