Hunting dark matter with GPS data

WASHINGTON, D.C.—A team of physicists has used data from GPS satellites to hunt for dark matter, the mysterious stuff whose gravity appears to hold galaxies together. They found no signs of a hypothetical type of dark matter, which consists of flaws in the fabric of space called topological defects, the researchers reported here on Saturday at a meeting of the American Physical Society. But the physicists say they have vastly narrowed the characteristics for how the defects—if they exist—would interact with ordinary matter. Their findings show how surprisingly innovative—and, in this case, cheap—methods might be used to test new ideas of what dark matter might be.

“It is so interesting and refreshing and exciting, and the cost is basically zero,” says Dmitry Budker, an experimental physicist at the Johannes Gutenberg University of Mainz in Germany, who was not involved in the work. “It’s basically the cost of the students analyzing the data.”

Astrophysicists think that dark matter makes up 85% of all the matter in the universe. Yet so far they have inferred dark matter’s existence only from its gravitational pull. For decades, many physicists have tried to directly detect a promising candidate for particles of dark matter, so-called weakly interacting massive particles, or WIMPs. But enthusiasm is waning as ever-more-sensitive detectors have failed to find the particles floating through our galaxy and passing through Earth. So many physicists are thinking more broadly about what dark matter might be.

For example, instead of a new subatomic particle, dark matter could be something far bigger and weirder: macroscopic faults in the vacuum of space called topological defects. Topological defects are best explained with an analogy to magnetic materials such as nickel. Nickel atoms act like little magnets themselves, and below a certain temperature, neighboring atoms tend to point in the same direction, so that their magnetic fields reinforce one another. But that orderly alignment can suffer defects if, for example, atoms in different regions point in different directions. When this happens, the regions meet along a craggy surface called a “domain wall,” which is one type of topological defect. There can be pointlike and linelike defects, too.

A similar thing might happen in space itself. Some theories predict that empty space is filled with a quantum field. If that field interacts with itself, then, as the infant universe expanded and cooled, the field may have taken on a value or “phase,” which would be a bit like the direction in which nickel atoms point. Regions of space with different phases would then meet at domain walls. These domain walls would have energy and, through Einstein’s famous equivalence, E=mc², mass. So they would generate gravity and could be dark matter.

Now, Benjamin Roberts and Andrei Derevianko, two physicists at the University of Nevada in Reno, and their colleagues say they have performed the most stringent search yet for topological dark matter, using archival data from the constellation of 31 orbiting GPS satellites. Each satellite  carries an atomic clock and broadcasts timing signals. Receivers on Earth use the timing information from multiple satellites to determine how far it is from each of them and, hence, its location.

To use those data to search for dark matter, the researchers had to invoke another bit of speculative physics. Theory suggests that within a topological defect, the constants of nature will change. In particular, the passing of a topological defect should fiddle with the so-called fine structure constant, which determines the strength of the electromagnetic force and the precise frequency of radiation that an atom will absorb or emit as an electron in it jumps from one quantized energy level to another. But an atomic clock works by measuring just such a frequency. So were a GPS satellite to pass through a topological defect, the defect should cause the satellite’s atomic clock to skip a beat.

One jump in one atomic clock wouldn’t be proof enough for topological defects. So the researchers looked for a stronger signal, the wave of time shifts that would sweep across the whole 50,000-kilometer-wide GPS network if Earth passed through a large domain wall as the galaxy rotates in its cloud of dark matter. Combing 16 years of GPS data, they found no evidence of a shift greater than half a nanosecond, Roberts told the meeting. They placed limits on the number of such topological defects and how strongly they interact with matter—limits that are up to six orders of magnitude more stringent than ones set by previous studies of supernova explosions. The researchers haven’t yet reached the limitations set by the clocks’ noise, Roberts reported, “so there’s a lot of room to improve.”

“It seems like a worthwhile study to pursue,” says Glennys Farrar, a theorist at New York University in New York City. “The idea of how you’d extract a signal was fun to think about.” Still, she says, the particular model of dark matter that Roberts, Derevianko, and colleagues test seems “rather narrow.” For example, she notes, they have to arbitrarily assume that the domain wall isn’t much thicker than Earth is wide. Budker agrees with that point. But he also notes that the work is just one example of a raft of new ideas physicists are hatching to look for different types of dark matter.