Since the 1950s, ground-based atomic clocks have been the gold standard for timekeeping. These clocks use very stable and precise frequencies of light emitted by specific atoms to regulate the time kept by more traditional mechanical, quartz crystal clocks. As a result, the clock system can be ultra-stable for decades.
A new study published in Nature Astronomy suggests that studying an atomic clock on board a spacecraft inside Mercury’s orbit and very close to the Sun could be the key to understanding the nature of dark matter.
Dark matter makes up more than 80 percent of mass in the universe, but it has so far evaded detection on Earth, despite decades of experimental efforts. A key component of these searches is an assumption about the local density of dark matter, 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 can be much higher than is usually assumed, and dark matter can become more concentrated in some regions compared to others.
Atomic or nucleic searches are an important class of experimental searches because they have achieved incredible sensitivity to dark matter signals. This is possible in part because dark matter particles with very small masses cause oscillations in the fundamental constants of nature. These oscillations, such as those in electron mass or electromagnetic force interaction strength, modify the transition energies of atoms and nuclei in predictable ways.
The more dark matter there is around the experiment, the larger these oscillations are, so the local density of dark matter matters a lot when analyzing the signal.
Joshua Eby
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 University of Delaware Professor Marianna S. Safronova, saw 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 may be exceedingly large, which would mean exceptional sensitivity to the oscillating signals.
These signals could be picked up by atomic clocks, which operate by carefully measuring the frequency of photons emitted in transitions of different states in atoms. Ultralight dark matter in the vicinity of the clock experiment could modify those frequencies, as the oscillations of the dark matter slightly increase and decrease the photon energy.
“The more dark matter there is around the experiment, the larger these oscillations are, so the local density of dark matter matters a lot when analyzing the signal,” Eby explained.
While the precise density of dark matter near the Sun is unknown, the researchers argue that even a relatively low-sensitivity search could yield valuable information.
The density of dark matter in the Solar System is only limited by information about planet orbits. In the region between the Sun and Mercury, the planet nearest to the Sun, there is almost no constraint. As a result, a measurement onboard a spacecraft could quickly uncover world-leading limits on dark matter in these models.
The technology to put their theory to the test already exists. Eby says the NASA Parker Solar Probe, which has been operating since 2018 with the help of shielding, has traveled closer to the Sun than any human-made craft in history, and is currently operating inside the orbit of Mercury, with plans to move even closer to the Sun within a year.
Other than the search for dark matter, atomic clocks in space are already highly motivated.
“Long-distance space missions, including possible future missions to Mars, will necessitate exceptional timekeeping, which atomic clocks in space would provide. A future mission with shielding and a trajectory similar to the Parker Solar Probe, but carrying an atomic clock apparatus, could be enough to carry out the search” Eby stated.
