The chameleon is a hypothetical scalar particle that couples to matter more weakly than gravity,[1] postulated as a dark energy candidate.
[2] 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 1 mm) 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.
In most theories, chameleons have a mass that scales as some power of the local energy density:
Chameleons would be indicated by the presence of an afterglow as they decay back into photons.
[10] A number of experiments have attempted to detect chameleons along with axions.
The ends of the chamber are glass windows, allowing light from a laser to enter and afterglow to exit.
[13] CHASE (CHameleon Afterglow SEarch) results published in November 2010,[14] improve the limits on mass by 2 orders of magnitude and 5 orders for photon coupling.
A 2014 neutron mirror measurement excluded chameleon field for values of the coupling constant
The CERN Axion Solar Telescope has been suggested as a tool for detecting chameleons.