Chameleon particle
| Composition | Unknown |
|---|---|
| Interactions | Gravity, electroweak |
| Status | Hypothetical |
| Mass | Variable, depending on ambient energy density |
| Electric charge | 0 |
| Spin | 0 |
The chameleon is a hypothetical scalar particle that couples to matter more weakly than gravity, postulated as a dark energy candidate. Due to a non-linear self-interaction, it has a variable effective mass which is an increasing function of the ambient energy density—as a result, the range of the force mediated by the particle is predicted to be very small in regions of high density (for example on Earth, where it is less than 1mm) but much larger in low-density intergalactic regions: out in the cosmos chameleon models permit a range of up to several thousand parsecs. As a result of this variable mass, the hypothetical fifth force mediated by the chameleon is able to evade current constraints on equivalence principle violation derived from terrestrial experiments even if it couples to matter with a strength equal or greater than that of gravity. Although this property would allow the chameleon to drive the currently observed acceleration of the universe's expansion, it also makes it very difficult to test for experimentally.
Chameleon particles were proposed in 2003 by Khoury and Weltman.
In most theories, chameleons have a mass that scales as some power of the local energy density: , where .
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