If We Live in a Sea of Dark Matter, This Tiny Mirror Might Be Able to Detect It

If We Live in a Sea of Dark Matter, This Tiny Mirror Might Be Able to Detect It
A membrane mechanical resonator that acted as the test mass for a proposed dark photon accelerometer. (Image: Aman Agrawal)

Despite the vast and expensive effort to figure out the identity of the invisible stuff that seems to make up much of the universe, no proposed dark matter candidate has been detected by any scientific experiment. Now, a team of researchers has suggested a design for a new dark matter experiment, one that relies on a super-thin mirror and would aim to detect something called dark photons.

The research, published last week in the journal Physical Review Letters, describes a coin-sized accelerometer that would, in theory, be able to measure the presence of particles too small to be seen by older experiments. The work follows up on an earlier paper by the same team published last summer.

The theoretical particles they are hoping to find are called dark photons. They are not be confused with photons of light. “Just like photons, they are represented by an equivalent of the ‘electromagnetic field,” said Jack Manley, the new paper’s lead author and a quantum optics research in the lab of Swati Singh at the University of Delaware. “However, unlike photons, dark photons have mass. This property makes them a candidate for dark matter.”

The two papers explored different sorts of dark matter signatures; basically, different avenues that physicists think could offer a sniff at the real stuff. “Both papers consider the possibility that our galaxy is swimming in a sea of particles that are trillions of times lighter than an electron, and these yet-to-be-detected particles make up all of dark matter,” said co-author Swati Singh, a quantum optics theorist at the University of Delaware. “However, there are key differences between what such particles are and how they interact with normal matter.”

Physicists are looking for something they’ve named “dark matter” because when they look out into the cosmos, they see gravitational effects that suggest there’s a lot more mass present than the regular matter we can detect. Hence, they believe there must be some “dark” — invisible to our current technologies — stuff to account for all that extra gravity. One theory of dark matter is that things called axions are responsible for the observable gravitational effects of the invisible matter. (A recent suggestion held that such axions may pop in and out of existence at the mysterious cores of neutron stars). Some longstanding contenders are known as weakly interacting massive particles, or WIMPs. Yet another idea is that dark matter could be explained by petite black holes from the primordial universe. All these various ideas for how dark matter manifests and where it resides typically come with ideas about how we might detect it.

Singh explained that there’s vast uncertainty about how densely packed dark matter is. There’s approximately a squirrel’s worth of dark matter for every mass the size of Earth, she said; the question is not how much dark matter there is, but whether that dark matter is actually concentrated in a squirrel-size mass evenly distributed throughout the Earth-sized mass in an ultra-fine mist of particles.

The team’s proposition is a coin-sized, 100-nanometre-thick card made of a silicon nitride membrane and a beryllium mirror. The materials are extremely sensitive, and when light bounces between the surfaces, the detector would be able to measure if the distance between the mirror and the membrane changed at all; that would indicate that something pushed them apart — a hint of new physics. Like a tuning fork, the team’s device can be set to “listen” for dark matter at a given frequency. If lots of these detecters were built, each one could be set to different channels (frequencies) to watch out for dark matter; if no results come in a certain amount of time, they could just keep channel surfing.

The device is scalable and affordable, a far cry from the more traditional dark matter detector layouts that require things like a ton of xenon buried under a mountain. In fact, the team’s design would be tabletop, though not in the way you may immediately understand.

“We mean an optical table, which usually sits on pneumatic legs that isolate it from the Earth, like the suspension system of a luxury car,” said co-author Dalziel Wilson, a quantum optics experimentalist at the University of Arizona. “If that’s not good enough — which it won’t be — then we’ll have to build another suspension system on top of the table. And then another, etc.” The better suspended the detector is (the closer to free-fall it can be), the better its results can cut through the noise of terrestrial activity. Wilson noted that the papers are also not necessarily about the specific experimental design the team came up with, but rather a call-to-arms to similar groups working on precision measurement devices for such difficult tasks.

Singh estimated that experimental runs with the detector could be operational in about five years. Will these pint-sized sensors find any dark matter? Well, no one has yet. But even a null result tells physicists what dark matter isn’t, which will help refine future searches.