Scientists have been trying for decades to understand “dark matter,” the unknown essence that represents an estimated 85% of all matter in the universe. Its effects can be observed, but it has not yet been detected directly.
This proposal, published Monday, Dec. 5 in Nature Astronomy, would send two atomic clocks into the inner reaches of the solar system to search for ultralight dark matter, which has wavelike properties that could affect the operation of the clocks. The team behind the study include University of Delaware physicist Marianna Safronova, Yu-Dai Tsai of the University of California, Irvine and Joshua Eby of the University of Tokyo and the Kavli Institute for the Physics and the Mathematics of the Universe.
Atomic clocks, which tell time by measuring the rapid oscillations of atoms, already are at work in space, enabling the Global Positioning System (GPS). Future space clocks could help spacecraft navigate and provide links to Earth-based clocks. Safronova has been part of other proposals, including one published in July that would link Earth-bound clocks to atomic clocks in orbit and test gravity. Putting atomic clocks into the variable gravity environment of space could produce gravity tests that are far more precise — by four orders of magnitude or 30,000 times more precise — than what is possible on Earth.
This proposal would send experiments that have been performed on Earth closer to the sun than Mercury, where there could be more dark matter to detect.
The work would be done by atomic, nuclear and molecular clocks that are still under development. They are frequently referred to as “quantum sensors.”
“This was inspired by the Parker Solar Probe,” Safronova said, referring to the ongoing NASA mission that sent a spacecraft closer to the sun than any other spacecraft has gone before. The probe flew across the sun’s corona for the first time in 2021 and continues to circle closer and closer.
“It has nothing to do with quantum sensors or clocks,” she said, “but it showed that you could send a satellite very close to the sun, sensing new conditions and making discoveries. That is much closer to the sun than what we are proposing here.”
NASA’s 2019 Deep Space Atomic Clock mission demonstrated the best atomic clock in space to date, Safronova said, but different types of clocks — based on much higher frequencies — have been developed in the past 15 years. Such “optical” clocks are orders of magnitude more precise and will not lose even a second of time in billions of years.
With that kind of technology now available on Earth, Safronova and her collaborators started talking about what sort of questions would be possible to study in space that cannot be done on Earth.
“It is a beautiful synergy between a quantum expert and particle theorists,” said Tsai, lead author of the Nature Astronomy article, “and we are working on new ideas at the intersection of these two fields.”
They settled on this study of ultralight dark matter, which scientists say could make a huge halo-like region, bound to the sun.
“It has very specific properties and is a very specific dark matter that is detectable by clocks,” Safronova said.
Such ultralight dark matter would cause the energies of atoms to oscillate, Safronova said, and that will change how the clock ticks. This effect depends on the atoms the clock uses. The scientists track the differences seen in the clocks to look for dark matter.
“What is observable is the ratio of those two clock frequencies,” she said. “That ratio should oscillate if such dark matter exists.”
All clocks mark time using some kind of repetitive process — a swinging pendulum, for example, Safronova said. The atomic clock uses laser technology to manipulate and measure the oscillations of atoms. These oscillations are very fast. A clock based on strontium atoms, for example, “ticks” 430 trillion times per second, she said, and atomic clocks are far more accurate and stable than any mechanical devices.
In a lab, these atomic clocks cover a table or several tables, Safronova said, but portable atomic clocks have been developed that can fit into a van. NASA’s Deep Space Atomic Clock is even smaller — about the size of a toaster.
Nuclear clocks, which are based on nuclear energy levels rather than atomic energy levels, would be the best clock for this research, Safronova said, and she is involved in the project to build a prototype, funded by the European Research Council.
“We now have portable clocks and it’s fun to think about how you would go about sending such high-precision clocks to space and establish what great things we can do,” Safronova said.
As technology advances, more proposals and opportunities will emerge. NASA’s Artemis program will pioneer new lunar-based research, for example.
“There are a lot of things we can do on the moon, such as building telescopes and even gravitational wave detectors, enabling new science,” she said. “We want to learn many more things about the moon first, for example its seismic activity.”
Studies using quantum sensors are part of the University’s new Quantum Science and Engineering Program, Safronova said, an interdisciplinary graduate program that was established earlier this year. Studies focus on understanding and exploiting the unusual behavior of particles and excitations governed by the laws of quantum mechanics.
Atomic clocks are important in the study of geodesy, for example, the study of Earth’s geometric shape, gravity and orientation in space.
“These now can sense a one-centimeter difference in height,” Safronova said. “So they’re getting better and better.”
And as the technology improves, new questions emerge.
“There is a whole range of great things we can do in space,” Safronova said. “We are at the very, very beginning of that.”