UC Berkeley team searches for hidden dark energy ‘chameleon’ particles

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Sierra Brown/Staff

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A team of UC Berkeley physicists is trying to determine whether dark energy — the mysterious force responsible for the acceleration of the expansion of the universe — may be hiding right under our noses in the form of hypothetical “chameleon” particles.

The team, headed by assistant physics professor Holger Muller, conducted an experiment published last week that attempted to detect the presence of these chameleon particles, so called because they are camouflaged around normal-density objects. This camouflaging could explain why previous experiments concerning dark energy have failed to identify chameleon particles, or the “chameleon field” as it is alternatively viewed.

Ever since dark energy was first discovered in 1998, when astronomers observed that the expansion of the universe was in fact speeding up, determining its nature has remained one of the great mysteries of modern science. Despite composing roughly 70 percent of the universe, dark energy has yet to be directly detected, and previous attempts to characterize it have largely been discredited by empirical data.

“The traditional explanation for dark energy is that space itself has some energy density, which is called the ‘cosmological constant,’ ” said Paul Hamilton, an assistant professor of physics at UCLA who is on Muller’s team. “Physicists calculate that this energy should be enormous — about 120 orders of magnitude greater than what we see. So there’s this huge mystery of why, if it is the cosmological constant, its energy density is so much smaller than we would expect.”

Another explanation for dark energy is that it is actually an entirely new type of field exerting what is called a “fifth force,” which would be unique from the four forces that are already known. According to Muller, though, “all the evidence right now suggests that fifth forces don’t exist.”

It is possible, however, that chameleon particles exist and have gone undetected by previous experiments because of their hidden effect around large masses. Unlike normal forces, the chameleon field may be present only on the very surface of objects so that under ordinary circumstances, it would be imperceptible.

“Gravity is the weakest known force, but all of the Earth exerts gravity,” Muller said. “Because chameleon particles would only couple to the outer shell of an object — maybe just the outermost nanometer — the net effect would be much weaker than gravity.”

With its experiment, Muller’s group attempted to amplify the effect of the chameleon force by employing single-cesium atoms, whose surface represents their entire mass as opposed to just a tiny fraction of it.

The team then dropped the atoms above a 1-inch-diameter aluminum sphere in a vacuum and used highly sensitive lasers to measure the forces acting on the atoms during the 10 to 20 milliseconds that they were in free fall.

By moving the sphere in and out, the team was able to account for the effect of gravity and look only for an additional force acting on the atoms. After analyzing its data, Muller said, his team did not find any such force but concluded that this result in itself provides “useful insights” about the nature of dark energy.

By not finding any mysterious forces, the experiment narrowed the range of possible energies exerted by chameleon particles by a factor of 1,000, as compared with previous research, but did not rule out the chameleon particles’ existence completely.

According to Hamilton, the team is already capable of making the necessary modifications to its experiment to either prove or disprove the chameleon mechanism once and for all, and it hopes to do so over the next few years.

Contact Logan Goldberg at [email protected].