Dark matter is thought to make up some five sixths of all matter in the universe. Yet incredibly sophisticated projects ranging from the most powerful atom smasher ever built to vats of chilly liquid xenon have failed to find a trace of it. But now some scientists are hoping atomic clocks, the most precise timekeepers ever made, could be used to help explain this elusive phenomenon.
Many physicists believe dark matter is an invisible substance whose predicted gravitational effects on known matter would help explain a variety of cosmic mysteries, such as why galaxies can spin as fast as they do without flying apart. Despite its apparently colossal importance to the very structure of the universe, however, no one knows anything for certain about what it might be composed of or where it came from. On Monday a group of physicists in Poland published a study in Nature Astronomy that suggests fluctuations in the rate that an atomic clock “ticks” might help reveal how apparent dark matter can influence known matter.
Scientists have largely ruled out all known particles as possible explanations for dark matter. This suggests it may comprise a kind of particle that falls outside the Standard Model of physics, which is currently our best working description of the subatomic world. Another possibility is that dark matter is not made of particles at all; rather it is a field that permeates space much like gravity does. Previous research indicates structures could arise in such a dark matter field—stable “topological defects” shaped like single points, strings or sheets. These structures might have formed during the hot chaos after the big bang, and essentially frozen into stable forms when the early universe cooled down.
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If Earth passes through such a topological defect—which researchers say could in principle reach at least planet-size—it could trigger changes detectable by some of the most accurate scientific instruments ever built: atomic clocks. These machines keep time by monitoring the wobbles of atoms, much as grandfather clocks rely on swinging pendulums. Modern atomic clocks are so accurate that they would lose no more than one second every 15 billion years, longer than the 13.8 billion years the universe is thought to have existed.
Passing through a topological defect might make an atomic clock’s atoms temporarily wobble faster or slower, according to prior work from theoretical physicists Andrei Derevianko at the University of Nevada, Reno, and Maxim Pospelov at the Perimeter Institute for Theoretical Physics in Ontario. By looking at a network of synchronized atomic clocks that are spread far enough apart for a topological defect to affect some clocks but not others, Derevianko and Pospelov say, scientists could detect a topological defect’s existence and measure some of its properties.
But in Monday’s Nature Astronomy report, physicist Piotr Wcisło at Nicolaus Copernicus University in Poland and his colleagues suggest a single atomic clock might be sensitive enough to shed light on the nature of dark matter. The researchers analyzed how topological defects might influence an individual optical atomic clock, which uses visible laser beams to measure the dances of atoms when they are slowed down by cooling them to temperatures near absolute zero. The researchers’ model demonstrates that passing through a defect in the proposed dark matter field could increase or decrease the overall strength of the electromagnetic force, which in turn would alter how atoms would respond to illumination.
Derevianko and Pospelov had shown that at least two atomic clocks would be needed to detect the effects of a topological defect. According to this model, "if we want to see that one clock is ticking faster or slower, we need to have some other clock serving as a reference," Wcisło says. But he and his colleagues suggest it might instead work to look at two elements within a single optical atomic clock: its supercooled vibrating atoms and the chamber in which laser light pinpoints the frequency at which those atoms vibrate. Passing through a topological defect might cause measurable changes in the frequencies both of vibrating atoms and the laser light within the chamber.
Wcisło and his colleagues tested their idea with experiments using optical atomic clocks. Their findings refined previous limits for the strength of any interaction between atoms and topological defects by more than a thousandfold over prior work. If optical atomic clocks were to spot any signature of topological defects, “it could give answers to some of the most fundamental questions of the modern physics, like the nature of dark matter, the relation between the Standard Model and gravity, or if the fundamental constants are really constant," Wcisło says.
Although these findings suggest a single clock may be able to help detect topological defects, Derevianko notes networks of atomic clocks can still shed light on other aspects of this potential explanation of dark matter—such as the possible sizes of these hypothetical defects. He calculates that as the solar system revolves around the center of the Milky Way at about 300 kilometers per second, passing through an Earth-size topological defect would influence an intercontinental network of atomic clocks on Earth for about 30 seconds.
Several networks of atomic clocks already exist, including the rubidium and cesium ones onboard satellites of the Global Positioning System (GPS). “We are mining a decade of archival GPS clock data to search for dark matter signatures,” Derevianko says. “Essentially we are using the GPS as the largest human-built dark matter detector.”
Still, these satellite clocks lag behind state-of-the-art laboratory optical atomic clocks. Wcisło notes his team's findings show the best atomic clocks can unite to form a giant dark matter observatory without needing fiber-optic links thousands of kilometers long to connect them as previously thought. "Our approach makes the idea of building a global network of such detectors possible without any further developments in experimental apparatus," he adds.