We still don’t know what dark matter is. The most widely accepted possibility is Weakly Interacting Massive Particles, or WIMPs, and most dark matter searches are looking for those. But other possibilities remain, and these alternatives to WIMPs, the "monstrous creatures at the edges of the dark matter map," are still generally particles, theoretical, exotic, or otherwise. These particles could comprise the mysterious matter that holds the galaxies together and makes up 26.8 percent of the mass-energy of the Universe.

Yet there’s another possibility, a different sort of monstrous creature, one that doesn't involve particles. Some physicists have been exploring the idea that dark matter might be ‘topological defects’ in a quantum field. Rather than solid particles, these would be perturbations, or oscillations.

This week, two physicists proposed a way to look for such defects using only atomic clocks. Atomic clocks are “arguably the most accurate scientific instruments ever built,” the researchers write in their paper. And, crucially, the clocks necessary already exist in the form of our GPS system.

Here’s how it works.

Principle

The topological defects could have formed in the early Universe as quantum fields underwent phase transitions. If these phase transitions were incomplete, bits of the field would be trapped in a high-energy state, a defect in the Universe itself. Due to the equivalence of mass and energy, these defects would provide a gravitational pull. The defects can take a variety of forms, including objects familiar to string theory proponents: zero-dimensional monopoles, one-dimensional strings, and two-dimensional domain walls. One or more of these could potentially exist, but they're entirely theoretical—it's not clear at this point whether any of them do.

(A zero-dimensional object is essentially like a point. It has no length, width, or height. One-dimensional objects, like the strings of string theory, have only length, while two dimensional objects have length and width—making a flat surface, like a sheet of paper. In our everyday world, of course, objects are three-dimensional.)

The defects could exist in large, gas-like clouds and travel straight through solid objects like the Earth. If that’s the case, relativity tells us that they should slightly alter the ticking of a clock as they pass through it. So if multiple atomic clocks were set up a considerable distance away from one another—say, on opposite sides of the Earth—then the passing of the cloud should become apparent, as it would take time for a cloud of defects to get from one clock to another.

When the cloud reaches the first clock, the two clocks become out of sync. For the time it takes the cloud to reach the second clock, the two clocks will remain out of sync. Then, the cloud will hit the second clock, altering that one as well, and the two clocks will be in sync again. Here, the researchers make a reasonable assumption—that the material will be traveling at the well-known speed that galactic objects, passing through the Solar System, tend to move: roughly 300 kilometers per second.

If so, that leaves plenty of time for the discrepancy between the clocks to be detected before the cloud reaches the second clock and brings them back into sync: about 30 seconds.

Constellation of clocks

To make a detector, a network of many atomic clocks could be set up, spaced all over the Earth’s surface. The authors recommend that each node of the network should have multiple clocks, to confirm the effect. If there really is a huge cloud of topological defects passing through and altering a clock, then close-by clocks should feel the same thing at the same time. That way, if only one clock falls out of sync but the others nearby do not, it’s more likely to be an effect specific to that clock rather than a defect cloud passing over the whole node. “Moreover, a large number of clocks in a network will help in determining the direction of arrival of the [topological defect cloud], its velocity and spatial extent,” the authors write in their paper.

This kind of a network would also be able to distinguish between different kinds of topological defects; for example, it would be able to differentiate domain walls from monopoles. We'd get even better measurements if there are clocks of different types at each node in the network.

As previously noted, the equipment for such a test already exists. Most notably, GPS satellites have on-board atomic clocks. “We envisage using the GPS constellation as a 50,000-km-aperture dark-matter detector,” the authors write. Such a wide aperture makes for an accurate detector, especially if ground-based atomic clocks are included in the network as well.

While this gives a high degree of certainty about measurements by the prospective network, it is also susceptible to a lot of noise. As with many other experiments designed to look for elusive objects from space, it’s very important to rule out as much noise as possible in such an experiment. While GPS satellite clocks are affected by things like solar flares and temperature, none of these will propagate through the network at 300 kilometers per second like the topological defects will. For this reason, the scientists are confident that the defect cloud signal should stand out from the noise and be detectable.

Topological Dark Matter is a minority candidate for understanding the Universe, with WIMPs still being the best possibility. Still, that could change very quickly if data from the GPS network hints at the presence of defect clouds.

Nature Physics, 2014. DOI: doi:10.1038/NPHYS3137 (About DOIs).