Is dark energy, the unseen force believed to explain the acceleration of the universe’s expansion, hiding in a group of hypothetical particles found all around us? If so, the new experiment led by University of Berkeley professor Holger Müller should be able to find them.
As Müller and his colleagues explained in a statement, dark energy may be hiding in this type of particles, which are known as “chameleons,” and the new technique narrows down the search for these particles one-thousand times in comparison to previous tests. Müller, an assistant professor of physics, believes that his next experiment will confirm or deny that the particles are dark energy.
First discovered in 1998, dark energy is believed to make up more than two-thirds of all energy in the cosmos, and several theories have been proposed to explain this mysterious force. One of those theories, proposed in 2004 by Justin Khoury of the University of Pennsylvania, explained that dark energy particles may not have been found yet because they’re hiding from us.
These so-called chameleon particles, Khoury said, vary in mass depending upon the density of the matter surrounding them. In space, they would have a small mass and exert force over a long distance, while in a laboratory, they would have a shorter reach and a larger mass. If proven true, this hypothesis would explain why dark energy has been so hard to find in the lab.
Searching for the elusive chameleon field
Müller, a faculty scientist at Lawrence Berkeley National Laboratory, and post-doctoral fellow Paul Hamilton, explained in a paper to be published in the journal Science, that they used an atom interferometer to test a theory indicating how this particle could be detected. They measured the attraction the chameleon field caused between an atom and a larger mass rather than that shared by two lager masses, which may have made the field undetectable.
“A chameleon is a postulated new particle,” Müller told redOrbit via email, “and as such, it would be expected to cause a new kind of force, just like photons cause electric forces. For chameleons, this force is exceedingly weak, because it would couple to a think outer shell of objects only (perhaps the outermost nanometer). Thus, the force between two large spheres would be very weak. Our idea was to replace one of the large spheres by an atom.”
They did this by dropping cesium atoms above an inch-diameter aluminum sphere, then using a sensitive laser to measure the forces on those atoms during free fall for a period of 10 to 20 milliseconds. The only force they were able to detect was Earth’s gravity, which eliminated a large range of possible energies for these particles.
“The outermost nanometer of an atom is the entire atom, so the chameleon would have an un-suppressed effect,” said Müller. “The data shows that the force, if it exists, must be about a million times weaker than the force caused by Earth’s gravity (the gravity between the sphere and the atom is weaker still). This rules out a large range of chameleon theories. At our current sensitivity, the chameleon theory can still be ‘tuned’ to evade our bounds. We are planning an improved version of the experiment that would rule out all chameleons – or detect them.”
Experiments could also find other dark energy fields
In addition, their experiments could narrow down the search for other hypothetical dark energy fields, such as symmetrons and forms of modified gravity, including so-called f(R) gravity, the study authors explained in a statement.
“These other fields are similar in that they can explain the observed acceleration of the universe, and yet would not have been detected in current experiments,” Müller explained to redOrbit. “By being sensitive to a lot of them, the experiment has a higher chance of actually detecting the dark energy. If nothing else, it will tell us more and more what dark energy is not, thus helping us to focus on models that are still viable.”
(Image credit: Holger Müller)
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