A chameleon-like force that changes its nature based on its surroundings could explain a great dilemma in physics: how the mysterious substance called dark energy forces the cosmos to expand ever faster. But a new experiment doubts on some chameleon theories, researchers report August 25 in physics of nature.
The chameleon force would be a fifth type of force beyond the four basic ones: gravitational, strong, weak and electromagnetic. And like a color-changing chameleon, the hypothetical fifth force would transform depending on the density of its surroundings. In dense environments like Earth, this fifth force would be weak, camouflaging its effects. In the scarcity of space, the force would be stronger and more far-reaching.
This force would result from a chameleon field, an addition to the fields known in physics, such as the electric, magnetic, and gravitational fields. A chameleon field with these changing properties could drive the acceleration of the expansion of the universe without disagreeing with the measurements on Earth.
But it is a challenge to discover such a changeable force. On Earth, says astrophysicist Jianhua He of Nanjing University in China, “it’s very, very small. That’s the hardest part.”
So He and his colleagues designed a detector to look for a subtle fifth force. A wheel with plastic films attached to it rotates next to another film on top of a magnetically levitated piece of graphite. If a chameleon force really exists, the spinning films would cause a periodic force on the levitating plastic, pulling it up and down. (Gravity also works this way, but thanks to the design of the device, 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 cooling their device to allow for more sensitive measurements.