‘Chameleon’ forces remain elusive in a new dark energy experiment

A chameleon-like force that shifts its nature based on its environment could explain a major physics quandary: how the mysterious substance called dark energy is compelling the cosmos to expand faster and faster. But a new experiment casts doubt on some chameleon theories, researchers report August 25 in Nature Physics.

The chameleon force would be a fifth type of force beyond the basic four: gravitational, strong, weak and electromagnetic. And like a chameleon changing its colors, the hypothetical fifth force would morph depending on the density of its surroundings. In dense environments like Earth, this fifth force would be feeble, camouflaging its effects. In the sparseness of space, the force would be stronger and long-ranged.

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This force would result from a chameleon field — an addition to the known fields in physics, such as electric, magnetic and gravitational fields. A chameleon field with these morphing properties could drive the accelerating expansion of the universe without disagreeing with measurements on Earth.

But it’s a challenge to suss out such a changeling force. On Earth, says astrophysicist Jianhua He of Nanjing University in China, “it’s very, very tiny. That’s the most difficult part.”

So He and colleagues designed a detector to search for a subtle fifth force. A wheel with plastic films attached spins past another film sitting on a magnetically levitated piece of graphite. If a chameleon force really exists, the films spinning by would cause a periodic force on the levitating plastic, pulling it up and down. (Gravity also acts this way, but thanks to the device’s design, it should be much weaker than a chameleon force.)

The team was able to rule out a category of chameleon theories. In the future, the researchers hope to improve their results by chilling their device to allow for more sensitive measurements.