Sensitive test to detect a possible fifth fundamental force known as the chameleon force provides the best evidence yet that it isn’t a valid candidate for dark energy
By Alex Wilkins
The chameleon force may have been an important player in the very early universe A. Schaller (STScI)/ESA
A test to measure a leading candidate for dark energy, a fifth fundamental force called the chameleon force, has found no evidence that it is real.
The researchers behind the test say this rules out the basic chameleon model as an explanation for dark energy, but some people think that it could still play a role.
Dark energy has been suggested as an explanation for the observation that galaxies appear to be accelerating away from each other. One possible candidate for how it works is the chameleon force, an additional fundamental force to the four we know are at work in the universe: gravity, the strong force, the weak force and electromagnetism.
The strength of the chameleon force would depend on the density of matter around it. In less dense regions of space over cosmic distances, it would have a significant effect, forcing matter apart. But on Earth, a dense region by cosmological standards, it is so small it is dwarfed by the other forces, making it difficult to measure.
People have tried to measure the chameleon force before by measuring any displacement between a test mass and groups of atoms in a lab, or between a test mass and a pendulum, but limitations in the size or scope of those experiments mean that the force might still have been lurking at energy scales we couldn’t probe.
Now, He Jian-hua at Nanjing University in China and his colleagues have devised an experiment that uses a magnetically levitated strip of graphite and a test mass to look for evidence for this mysterious effect at previously unmeasured scales that fill in the gap between the past pendulum and atom experiments.
He and his team designed the experiment to minimise every other possible force, by having masses so thin that the effect of gravity on them is negligible, and placing the test mass inside a Faraday cage, which blocks out electric fields. They then used an optical microscope and a laser system to measure any chameleon-driven displacement between the two masses.
“If dark energy really exists, then in our experiment we should be able to detect [the chameleon force],” says He. “We have a very sensitive force sensor, but we found nothing.”
The result is important for narrowing down the scales and energies at which a chameleon force might act, says Jeremy Sakstein at the University of Hawai’i at Manoa. However, it doesn’t decisively rule out possibilities for its existence if dark energy works at different energy scales, he says.
He and his colleagues chose particular values for dark energy and the chameleon force, says Adrienne Erickcek at the University of North Carolina, and while their work has made a possible link between the chameleon force and dark energy more unlikely, a chameleon force unrelated to dark energy could still exist.
The researchers hope to improve their apparatus by cooling it to very low temperatures so they can further probe the chameleon idea and test other dark energy candidates at even smaller energy scales, such as symmetron theory, another potential fifth force that would work like the Higgs boson.
Journal reference: Nature Physics, DOI: 10.1038/s41567-022-01706-9
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