A team of physicists from the University of California, Berkeley, has developed an unprecedentedly precise instrument designed to hunt for dark energy, the elusive force accelerating the expansion of the universe.
Their experiment – the results of which were published today in the prestigious journal Nature – focuses on a hypothesized particle known as the chameleon that could hold the key to understanding this mysterious cosmic force.
Dark energy, first identified in 1998, represents about 70 percent of the total matter and energy in the universe. Despite numerous theories, its true nature remains a mystery.
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One of the leading hypotheses is the existence of a fifth force, distinct from the four known fundamental forces of nature: gravity, electromagnetism, and the strong and weak nuclear forces.
This force is said to be mediated by a particle called the chameleon for its ability to hide in plain sight.
The UC Berkeley experiment, led by Prof. Holger Müller, uses an advanced atomic interferometer combined with an optical grating.
If that sounds technical, that’s because it is. Essentially, this setup allows for precise gravity measurements by holding freely falling atoms in place for a specified period of time.
The longer you can leave the atoms hanging there, the more likely you are to find (or not find) traces of a chameleon.
‘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 finally interferes with it,” Müller said
“The waves can either be in phase and add up, or the waves can be out of phase and cancel each other out. The trick is that whether they are in or out of phase depends very sensitively on some quantities you might want to measure, such as acceleration, gravity, rotation or fundamental constants.”
While previous experiments could immobilize atoms for only milliseconds at a time, the new device can hold them for much longer: seconds to tens of seconds. This represents a significant improvement that improves the most accurate measurements by a factor of five.
In a recent article published in the journal PhysicsMüller and his colleagues extended the waiting time to as much as 70 seconds.
To discover whether the chameleon particle is indeed the mastermind behind dark energy, scientists must find holes in the results predicted by the accepted theory of gravity – something no one has been able to do since Isaac Newton first formulated 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, the researchers remain optimistic. The improvements in the accuracy of their instrument mean that future experiments could provide the evidence needed to confirm or refute the presence of chameleons or other hypothetical particles that contribute to dark energy.
About the expert
Holger Muller He successfully applied for his first patent when he was 14. Later he did his bachelor’s thesis with Jürgen Mlynek at the University of Konstanz, Germany. He graduated from Humboldt University, Berlin, with Achim Peters as his advisor. Müller received a grant 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 is now a professor of physics. He is now the principal investigator of his research group, the Müller group.
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