A team of physicists from the University of California at Berkeley has developed an unprecedentedly precise instrument designed to hunt down dark energy, the elusive force accelerating the expansion of the universe.
The results of their experiment were published today in a prestigious journal Nature – targeting a hypothesized particle known as the chameleon, which could hold the key to understanding this mysterious cosmic force.
First identified in 1998, dark energy accounts for about 70 percent of the total mass and energy of the universe. Despite numerous theories, its true nature remains a mystery.
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Among the main hypotheses is the existence of a fifth force, different from the four known fundamental forces of nature – gravity, electromagnetism and the strong and weak nuclear forces.
This power would be mediated by a particle called a chameleon due to its ability to hide in plain sight.
The UC Berkeley experiment, led by Professor Holger Müller, uses an advanced atom interferometer combined with an optical grating.
If this sounds technical, that’s because it is. Essentially, this setup allows for precise measurements of gravity by holding free-falling atoms in place for a certain amount of time.
The longer you can keep the atoms hanging, the better your chance of finding (or not finding) the chameleon’s tracks.
“Atomic interferometry is the art and science of using the quantum properties of a particle, that is, the fact that it is both a particle and a wave. We split the wave so that the particle takes two paths at the same time and then interfere with them at the end,” said Müller
“Waves can either be in phase and add up, or they can be out of phase and cancel each other out.” The trick is that whether they’re in or out of phase depends very sensitively on some quantity you might want to measure, like acceleration, gravity, rotation, or fundamental constants.”
While previous experiments were only able to immobilize atoms for milliseconds at a time, the new device can hold them for much longer – seconds to tens of seconds. This represents a significant improvement that increases the most accurate measurement by a factor of five.
In one recent article published in the journal Natural physicsMüller and his colleagues extended the dwell time to an incredible 70 seconds.
To discover whether the chameleon particle is indeed the mastermind of dark energy, scientists need to find holes in the results predicted by the accepted theory of gravity – something no one has done since Isaac Newton established his theories 400 years ago.
In their recent tests, Müller and his team found no deviations from Newtonian gravity, suggesting that if chameleons exist, their effects are extremely subtle.
Still, researchers remain optimistic. Improving the accuracy of their instrument means that future experiments could provide the evidence needed to confirm or disprove the presence of chameleons or other hypothetical particles contributing to dark energy.
About the expert
Holger Müller At the age of 14, he successfully applied for his first patent. He later did his undergraduate work with Jürgen Mlynk at the University of Constance in Germany. He graduated from the Humboldt-University in Berlin with Achim Peters as his advisor. Müller received a fellowship from the Alexander von Humboldt Foundation and joined Steven Chu’s group at Stanford as a postdoc. In July 2008, he joined the physics faculty at UC Berkeley, where he now chairs as a professor of physics. He is now the principal investigator of his research group, Müller’s group.
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