A new study published in Nature Astronomy suggests that studying an atomic clock aboard a spacecraft in Mercury’s orbit and very close to the Sun could be the key to understanding the nature of dark matter.
Dark matter accounts for more than 80% of the universe’s mass, but it has so far eluded detection on Earth, despite decades of experimental efforts. The number of dark matter particles passing through the detector at any given time, and thus the experimental sensitivity, is determined by an assumption about the local density of dark matter. This density can be much higher in some models than is commonly assumed, and dark matter can become more concentrated in some regions than 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.
These oscillating signals piqued the interest of an international team of researchers, including 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. They claimed that in a specific region of the Solar System, between Mercury’s orbit and the Sun’s, the density of dark matter could be extremely high, implying exceptional sensitivity to oscillating signals.
These signals could be detected by atomic clocks, which operate by carefully measuring the frequency of photons emitted during atomic transitions. Because the oscillations of the dark matter slightly increase and decrease the photon energy, ultralight dark matter in the vicinity of the clock experiment could modify those frequencies.
“The more dark matter there is around the experiment, the larger these oscillations are, so when analyzing the signal, the local density of dark matter matters a lot,” Eby explained.
While the precise density of dark matter near the Sun is unknown, the researchers argue that even a low-sensitivity search could yield valuable information.
Only information about planet orbits constrains the density of dark matter in the Solar System. There is almost no constraint between the Sun and Mercury, the planet closest to the Sun. As a result, a measurement onboard a spacecraft could quickly reveal world-leading dark matter limits in these models.
The technology to test their theory already exists. According to Eby, the NASA Parker Solar Probe, which has been operating with shielding since 2018, has traveled closer to the sun than any human-made craft in history and is currently operating inside Mercury’s orbit, 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 require exceptional timekeeping, as would be provided by atomic clocks in space.” A future mission with shielding and a trajectory similar to the Parker Solar Probe, but carrying an atomic clock apparatus, could be sufficient to carry out the search, according to Eby.