even the strongest Gravitational waves traveling through Earth are produced by distant collisions of black holes, stretching and compressing every mile of Earth’s surface by only one thousandth of the diameter of an atom. It’s hard to imagine how small the ripples in these space-time fabrics 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 unfolding. But that’s only part of the story.
Gravitational wave detectors are taking over some side jobs.
“People are starting to ask: ‘Maybe we’re getting more than gravitational waves from these machines?'” says Caltech physicist Rana Adhikari.
Inspired by the extreme sensitivity of these detectors, researchers are devising ways to use them to find other elusive phenomena: most importantly, dark matter, the non-luminous matter that holds galaxies together.
In December, a team led by Hartmut Grote of Cardiff University nature They used gravitational-wave detectors to look for scalar-field dark matter, a little-known candidate for missing mass in and around galaxies. The team found no signal, ruling out a large class of scalar-field dark matter models. Now, the substance only exists if it has a very weak effect on normal matter—at least a million times weaker than previously thought possible.
“This is a very good 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 leading candidate for dark matter was a slow-moving, weakly interacting particle similar to other elementary particles—a type of heavy neutrino. But experimental searches for these so-called WIMPs have come up empty-handed, making room for countless alternatives.
“We’re at the point where we’re looking everywhere for dark matter,” said Kathryn Zurek, a theoretical physicist at Caltech.
In 1999, three physicists proposed that dark matter could be composed of particles so light and so numerous that they are best described collectively as an energy field that permeates the universe. This “scalar field” has a value at every point in space, and that value oscillates at a characteristic frequency.
Scalar-field dark matter subtly changes the properties of other particles and fundamental forces. For example, the mass of the electron and the strength of the electromagnetic force oscillate with the amplitude of the oscillation of the scalar field.
For years, physicists have wondered whether gravitational-wave detectors could detect this wobble. These detectors use a method called interferometry to detect slight disturbances. First, the laser enters a “beam splitter,” which splits the beam into two mutually perpendicular directions, like the arms of an L. The beam reflects off mirrors at the ends of both arms, then returns to the L’s hinge and recombines. If the returning laser beam is out of sync—for example, by gravitational waves, which briefly lengthen one arm of the interferometer while contracting the other—a distinct interference pattern of dark and bright fringes forms.
Can scalar field dark matter desynchronize beams and cause interference patterns? “The general idea,” Grote said, is that any twist affects both arms equally, counteracting. But then in 2019, Grote realized it. “I woke up one morning and thought: A beamsplitter is just what we need.”