Although the motions of galaxies provide evidence that dark matter exists, scientists have yet to directly detect the invisible stuff, or figure out what it could be made of.
The prevailing theory over the past several decades has been that dark matter is made up of particles that act like teeny, tiny billiard balls bouncing around in space. Considering that all the matter we can see is made up of particles that behave that way, the idea seems logical. But in recent years, a growing number of physicists have been exploring the hypothesis that dark matter primarily exists in a different form: invisible waves.
“Instead of particles bouncing around you, you’re living in waves.”
To be wavelike suggests that dark matter is ultralight—a millionth or even billionth the mass of an electron. From cosmological observations, scientists have an estimate of the total mass of all dark matter in the universe; the lighter dark matter particles are, the more profuse they would need to be to add up to that amount. This cascade of almost massless dark matter would act like smooth waves in water.
Imagine waves in the ocean flowing toward shore and pushing a swimmer around, says Lindley Winslow, an experimental nuclear and particle physicist and associate professor at MIT. “You don't have those interference patterns, but rather coherent waves. That’s what wavelike dark matter is doing. Instead of particles bouncing around you, you’re living in waves.”
What are the waves made of?
Multiple theories describe different versions of wavelike dark matter. Physicists’ current favorite candidate is the quantum chromodynamics axion, or QCD axion.
In the late 1970s, particle physicists Roberto Peccei and Helen Quinn were looking to solve a longstanding quandary in nuclear physics known as the CP problem. The problem arose in experimental results when the symmetry between matter and antimatter particles seemed broken in a way that violated physicists’ current understanding of the universe.
When Peccei and Quinn proposed a mechanism that could reconcile results with theory, they realized that this mechanism would produce a particle: the QCD axion. QCD axions would be ultralight particles that would not interact with much, but their gravitational pull could explain the motion of galaxies attributed to dark matter.
Alternative wavelike dark matter candidates fit into different theories that solve different problems. “What's cool about axion-like particles is that they may be relics from higher order theories,” Winslow says. “For example, they would be the first proof that something like string theory exists and is the right way to understand the universe.”
Another wavelike dark matter candidate is the dark photon, which “you can think of as being like a cousin of the photon,” says Tien-Tien Yu, a particle phenomenologist and associate professor at the University of Oregon.
The dark photon would be similar to the photon, except that it would have a very small electromagnetic charge and could also have a mass, she says.
A needle in the haystack
The way scientists search for wavelike dark matter is quite different from the way they search for dark matter particles, Yu says. “In the particle case, you’re looking for one particle that either scatters or absorbs. Whereas with wavelike dark matter, you’re looking for a large number of particles that are working together.”
So instead of looking for particles bouncing off things like billiard balls, scientists are searching for something more akin to a radio signal, says Gray Rybka, an associate professor at the University of Washington and co-spokesperson for the Axion Dark Matter Experiment, ADMX. “Most of our experiment to detect wavelike dark matter is essentially a very fancy AM radio with a magnetic apparatus to convert axions into microwaves.”
The ADMX experiment, based at the University of Washington and sponsored by the US Department of Energy, consists of a large magnet, a microwave cavity and ultra-sensitive low-noise quantum electronics. “In essence, we’re looking for a constant stream of power that seems to be coming from nowhere, but it's actually dark matter,” he says.
The search for wavelike dark matter, like the search for particles of dark matter, involves trying to detect a faint signal that would be difficult to tell apart from a flurry of other things that can mimic that signal. “So understanding everything that could possibly look like what you're trying to look for is a big, big challenge,” Yu says.
To increase the chances of detection, a group of physicists from the University of California, Irvine, the Kavli Institute for the Physics and Mathematics of the Universe in Japan, and the University of Delaware recently proposed an experiment that would send atomic clocks to the inner reaches of our solar system, between Mercury and the sun, where some models predict a higher density of dark matter and presumably a greater chance of detecting it. The researchers forecast that sensitive and precise atomic clocks could detect the slight perturbations in the electromagnetic field caused by wavelike dark matter.
“This is really a time for maximum originality and creativity,” says Yu-Dai Tsai, a postdoctoral scholar at UCI and the lead author proposing the atomic clock experiment. “We need to break the boundaries between disciplines and support cross-frontier efforts.”
For Yu, the interdisciplinary nature of dark matter research is precisely what keeps the search so compelling. “A lot of efforts to look for wavelike dark matter bring together fairly different communities to create new ways of thinking about the problem,” she says. “That’s exciting to me, because as scientists we’re all after the same goal, which is simply to understand how our world and the universe works.”
Editor’s note: Madison Langweil contributed to the reporting of this article.