Atomic clocks on Earth could reveal secrets about dark matter across the universe

Scientists are using atomic clocks to investigate some of the universe's greatest mysteries, including the nature of dark matter, in a laboratory. In the process, they say they're bringing cosmology and astrophysics "down to Earth."

The project, which is a collaboration between the University of Sussex and the National Physical Laboratory (NPL) in the U.K., uses the ticks of these incredibly precise clocks to hunt for hitherto unknown ultra-light particles. 

These particles could be connected to dark matter, the mysterious substance that makes up an estimated 85% of all matter in the universe but remains effectively invisible to us because it does not interact with light or, more precisely, electromagnetic radiation. Scientists believe most galaxies are enveloped by a cloud of dark matter, but its presence can only be inferred by the effect it has on gravity.

"Our universe, as we know it, is governed by laws of physics, so gravity is governed by general relativity and particle physics by the Standard Model of particle physics," Xavier Calmet, project leader and a professor of physics at the University of Sussex, told Space.com. "We call deviations from these laws  'breakdown in physics' — basically, that is a synonym for new physics beyond our current understanding of the universe."

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This new physics could be used to explain the nature of dark matter, something that doesn't fit within the Standard Model.

"One of the biggest mysteries is the nature of dark matter. We know that it is out there, we see its impact in our universe, but we don't have a valid explanation within the Standard Model of particle physics," Calmet continued. "There must be new physics, but we do not know how to describe these new particles and how they couple to regular matter."

How can "new physics" be spotted with atomic clocks?

According to established laws of physics, clocks should tick at a constant rate, but physics beyond the Standard Model's scope would result in tiny charges in atomic energy levels. This should affect the rate at which clocks tick, but the variation would be so small it could only be spotted with an incredibly precise clock — and that's where atomic clocks come in. 

"Atomic clocks bring cosmology and astrophysics down to Earth, enabling searches for ultra-light particles that could explain dark matter in a laboratory," Calmet said.

Atomic clocks measure time using atoms with two potential energy states. When atoms absorb energy, they go to a higher energy state. Then, they eventually release this energy and drop back down to their lower ground state. 

In atomic clocks, groups of atoms are prepared by placing them in a higher energy state using microwave energy, and the characteristic and consistent rates at which they vibrate between states — their resonance frequencies — are used to precisely measure time.

So, for example, all atoms of cesium resonate at the same frequency, meaning the standard measure of a second can be defined as 9,192,631,770 cycles of cesium. Because this cycling per second occurs with far less variation than, say, the swinging of a pendulum, this makes atomic clocks incredibly precise.

"It has been recently realized that dark matter could be made of ultra-light particles that interact extremely weakly with regular matter," Calmet explained. "If that is the case, dark matter would essentially behave as a classical wave that interacts with electrons and protons. This dark matter wave would give some small kicks to these particles."

Calmet added that these ultra-light dark matter particle kicks to the building blocks of the atom would lead to a time variation in fundamental constants of the universe, such as the fine-structure constant or "alpha" — a measure of how strong particles couple via the electromagnetic force — and the mass of the proton. 

"Because atomic clocks are amazingly precise devices, they would be able to detect these kicks and thus discover ultra-light dark matter," he continued. "By comparing two clocks, one sensitive to changes in alpha and the other one less sensitive to changes in alpha, we can obtain a limit on the time variation of this fundamental constant and thus set constraints on ultra-light particles."

Calmet thinks the technique could potentially also be used to investigate another problematic aspect of the universe for physicists: Dark energy, the unknown force that is driving the accelerating expansion of space.

While Calmet acknowledges that dark energy is more likely explained by the cosmological constant, a form of energy that acts almost in opposition to gravity to stretch the fabric of space and push apart galaxies, there is a small chance it could be connected to an ultra-light particle. In this vein, future clocks could also be sensitive to that particle and its associated wave.

"While the clocks have not discovered new physics at this stage, we were able to develop a new theoretical framework to probe generic new physics with clocks and were able to derive the first model-independent limits on physics beyond the standard model within this approach," Calmet concluded. "We are creating a new field at the interface of atomic, molecular, and optical physics and traditional particle physics. 

"These are exciting results!"

These results are set to be published in a future edition of the New Journal of Physics.