Researchers say space atomic clocks could help uncover the nature of dark matter

Studying an atomic clock aboard a spacecraft inside Mercury’s orbit and very close to the sun could be the trick to uncovering the nature of dark matter, suggests a new study published in natural astronomy.

Dark matter makes up more than 80% of the mass of the universe, but it has so far escaped detection on Earth, despite decades of experimental effort. A key part of this research is an assumption about the local dark matter density, which determines the number of dark matter particles passing through the detector at any given time, and therefore the experimental sensitivity.

In some models, this density may be much higher than usually assumed, and dark matter may become more concentrated in some regions than in others.

An important class of experimental research is that which uses atoms or nuclei, as these have achieved incredible sensitivity to signals from dark matter. This is possible, in part, because when dark matter particles have very small masses, they induce oscillations in the very constants of nature. These oscillations, for example in the mass of the electron or the interaction strength of the electromagnetic force, modify the transition energies of atoms and nuclei in predictable ways.

An international team of researchers, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) project researcher Joshua Eby, University of California, Irvine, postdoctoral fellow Yu-Dai Tsai and Professor Marianna S. Safronova from the University of Delaware, saw the potential in these oscillating signals. They claimed that in a particular region of the solar system, between the orbit of Mercury and the sun, the density of dark matter can be extremely large, which would mean exceptional sensitivity to oscillating signals.

These signals could be picked up by atomic clocks, which work by carefully measuring the frequency of photons emitted during transitions of different states in atoms. Ultralight dark matter in the vicinity of the clock experiment could alter these frequencies, as dark matter oscillations slightly increase and decrease photon energy.

“The more dark matter there is around the experiment, the stronger these oscillations are, so local dark matter density matters a lot when analyzing the signal,” Eby said.

While the precise density of dark matter near the sun isn’t well known, the researchers say even a relatively low-sensitivity search could yield important insights.

The density of dark matter is limited in the solar system only by information about the orbits of the planets. In the region between the sun and Mercury, the planet closest to the sun, there are almost no constraints. So a measurement aboard a spacecraft could quickly reveal the global limits of dark matter in these models.

The technology to test their theory already exists. Eby says NASA’s Parker Solar Probe, which has been operating since 2018 with the help of shielding, has approached the sun closer than any man-made craft in history, and is working currently inside Mercury’s orbit, with plans to move even closer to the sun within a year.

Atomic clocks in space are already well motivated for many reasons other than the search for dark matter.

“Long-range space missions, including possible future missions to Mars, will require exceptional timekeeping, as would atomic clocks in space. A possible future mission, with shielding and trajectory very similar to the solar probe Parker, but wearing an atomic clock device, might be enough to perform the search,” Eby said.

Details of their study were published in natural astronomy.