Most scientists working on dark matter have become convinced that it consists of WIMPs, or weakly interacting massive particles. WIMPs provide the best match for the data we have on all dark matter effects, and they would have been produced early enough in the history of the Universe to explain the early influence of dark matter on the distribution of regular matter.
But WIMPs aren’t the only theoretical game in town. While the other proposals all have different problems — the wrong masses or energies, bad timing, and so on — our failure to actually detect a WIMP has kept them viable. One of the more bizarre ideas is that the universe itself is filled with what are called topological defects; areas where the quantum fields of the universe have not reached the same state as the field in which our normal physics operates.
Now a group of Polish researchers has calculated that searching for this form of dark matter requires no more than a few clocks. Very accurate atomic clocks, but still clocks.
The idea behind topological defects is based on our understanding of the quantum mechanical nature of the universe. In this view, the universe is filled with fields that control the behavior of particles, similar to how an electromagnetic field influences the behavior of charged particles within it. In this view, the empty space that makes up most of the universe is not actually empty. Instead, it’s a quantum vacuum, full of fields that describe all the physics we know.
The quantum vacuum of our universe is a relatively low energy state (whether it is the lowest energy ground state is a matter for another paper). But extremely early in the history of the universe, there are regions of space where several higher-energy quantum vacuums may have existed, with a distinct array of fields. As the energy density of the Universe decreased with its expansion, these regions would have undergone phase transitions, much like ice melting into water and decaying into the vacuum we are all familiar with.
Or at least most of them would have. Theorists have devised ways in which some of these high-energy fields could be shielded from phase transitions and persist in the current universe. The result is called a topological defect, a part of space that has different quantum fields than our known ones.
Topological defect questions are not just potential intellectual curiosities. If they have the right properties and are present in the right numbers, they could explain the observed effects of dark matter. Unlike particles, the existence of topological defects is purely hypothetical, so they are not as good a candidate as a WIMP. But if we could figure out how to test them, what harm could there be in looking?
That is exactly where the new work from Poland comes into play. It turns out that we may already have data that helps rule out topological defects up to a certain energy. And that data comes from things that are relatively easy to build and operate: atomic clocks.
Why would an atomic clock register a topological defect? The physics are generally simple. Atomic clocks work based on the energy transitions of electrons orbiting atoms. These transitions are based on fundamental physical constants, such as the mass of the electron and the fine structure constant that describes the interactions between charged particles.
But remember that a topological defect involves a different set of quantum fields than the ones we know. These defects would cause small but significant changes in the behavior of physics as they moved through the atomic clock. Which would mean that the physics on which atomic clocks are based would change. Relative to a clock that was not affected by the topological defect, the affected clock would tell the time differently. (Otherwise, but not necessarily inaccurate; it still obeys the physics it experiences.)
So if you have two clocks a good enough distance apart, they can show strange but similar glitches at a time determined by how fast the topological defect was sweeping across the Earth. And since most of the topological defects seen by Earth should move with the galaxy’s rotation, we have a good idea of what the time difference should be, though the authors note that we need to compensate for things like Earth’s rotation.
The authors calculate how accurate the measurements would be, and find that clocks can provide a 1,000-fold improvement in the limits we set for topological defects. And they come up with a system that is relatively easy to implement. (In their words, “Our method is not constrained by the need for a phase-noise-compensated fiber optic link with a length comparable to the size of the Earth.”) They also point out that existing atomic clock data could be reused. analyzed in the light of the new approach.
What they don’t do is do the reanalysis themselves. If they’re right and it really is that simple, we’d be surprised if no one did it. It probably won’t yield anything, but in doing so it will put energy limits on the possibility of dark matter topological defects. Which would certainly mean progress.
Nature Astronomy2016. DOI: 10.1038/s41550-016-0009 (About DOIs).