In a feat of scientific repurposing, a team of physicists has used a detector designed to sense gravitational waves—fleeting ripples in space and time set off by, for example, the collisions of two black holes—to search for something even more elusive, dark matter. Although researchers found no sign of the particular hypothetical particles they sought, the experiment highlights the emergence of a new frontier—and a new tool—in the quest to identify dark matter, the mysterious stuff thought make up 80% of matter in the universe.
“It’s a great example of how you can use an existing detector for a secondary purpose,” says Daniel Carney, a quantum information scientist at Lawrence Berkeley National Laboratory, who was not involved in the work. Nancy Aggarwal, a gravitational wave physicist at Northwestern University, says, “The idea has been out there for a while and it’s nice to see it implemented.”
Astrophysicists have many reasons to believe galaxies are filled with dark matter. For example, the stars in spiral galaxies whirl too fast to be held in by their own gravity, implying that gravity from invisible dark matter reins them in. But scientists don’t know what dark matter actually is. For decades, many theorists have thought it could consist of weakly interacting massive particles (WIMPs), which would weigh about 100 times as much as a proton and would occasionally interact with atomic nuclei through the so-called weak force. But with ever-more-sensitive detectors failing to see WIMPs bouncing off of atomic nuclei, experimenters are beginning to target other hypothetical particles in higher and lower mass ranges.
Now, physicists have searched for one kind of ultralight particle with GEO600, a gravitational wave detector near Hanover, Germany. “We only use data that’s already been recorded,” says Hartmut Grote, an experimental physicist at Cardiff University and a member of the GEO600 collaboration. “We didn’t have to touch the experiment.” A test bed for bigger gravitational wave detectors, GEO600 is a large, L-shaped optical instrument called an interferometer. Within it, a semireflective mirror, or beam splitter, sends half of a laser beam down each of the 600-meter-long arms of the L. After bouncing back and forth between mirrors at the ends of the arms, the light waves return to the beam splitter where they interfere. Whether the light flows out of the interferometer depends on the arms’ relative lengths. A passing gravitational wave will typically stretch the arms by different amounts, causing a signal to warble out of the detector.
Dark matter that consists of ultralight particles called scalars could also produce a signal in an interferometer like GEO600, Grote says. The particles would have a mass less than one-quintillionth of that of a proton, too little to create a signal by bouncing off a nucleus. To generate dark matter’s gravitational effects, however, huge numbers of the particles would have to be jammed into every cubic centimeter of space. According to quantum mechanics, such light-weight particles act less like particles and more like waves, with wavelengths kilometers long. Hordes of them permeating Earth should form one big overlapping wave.
When that wave of dark matter passes through the interferometer, Grote says, it could make all the material objects in the interferometer expand and contract very slightly. Were the device entirely symmetrical, that throbbing would have no effect. However, the light in one arm of the interferometer reflects off the surface of the beam splitter while the light in the other passes through it. Because of that key difference, as the beam splitter itself expands and contracts, the light detector would pick up a signal. Whereas a gravitational wave generates a short-lived chirp, Grote says, the ever-present dark matter wave should produce a steady hum at a frequency set by dark matter particles’ mass.
In 7 daylong tranches of data collected over 3 years, the researchers saw more than 1000 relatively steady signals. But they concluded that all of them were due to vibrations and electronic noise in the detector—because, for example, their frequency and amplitude varied from run to run. Because the dark matter particle’s mass would be constant, its signal would be rock-steady, Grote says. The absence of a viable signal allowed the team to put limits on how strongly ultralight dark matter particles can interact with ordinary matter, they report today in Nature.
So, is this a big advance in pinning down the nature of dark matter? Not necessarily, Carney says. Compared with WIMPs, the theory doesn’t offer compelling reasons why ultralight scalar dark matter should exist, he says. “This [idea] is a bit, ‘Well, if it’s allowed to be anything, it could be this.’”
Nevertheless, as WIMP searches come up empty, physicists are exploring more options, Aggarwal says. “This is sort of a new paradigm for casting a wider net for dark matter,” she says. Grote notes that, without much fuss, physicists could tweak the designs of future gravitational wave detectors to ensure they’d be sensitive to ultralight dark matter, too.