|Mass||Variable, depending on ambient energy density|
The chameleon is a hypothetical scalar particle which couples to matter, 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. Whilst 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: , where .
Chameleons also couple to photons, allowing photons and chameleons to oscillate between each other in the presence of an external magnetic field.
Chameleons can be confined in hollow containers because their mass increases rapidly as they penetrate the container wall, causing them to reflect. One strategy to search experimentally for chameleons is to direct photons into a cavity, confining the chameleons produced, and then to switch off the light source. Chameleons would be indicated by the presence of an afterglow as they decay back into photons.
The GammeV experiment is a search for axions, but has been used to look for chameleons too. It consists of a cylindrical chamber inserted in a 5T magnetic field. The ends of the chamber are glass windows, allowing light from a laser to enter and afterglow to exit.
The latest results were published in November 2010 (nothing found in the range of photon and matter coupling they tested)
- J. Khoury and A. Weltman, Phys. Rev. D 69, 044026 (2004)
- Erickcek, A. L.; Barnaby, N; Burrage, C; Huang, Z (2013). "Catastrophic consequences of kicking the chameleon". Physical review letters 110 (17): 171101. PMID 23679701.
- J.H. Steffen et al., "Constraints on chameleons and axion-like particles from the GammeV experiment" 
- Rybka, G; Hotz, M; Rosenberg, L. J.; Asztalos, S. J.; Carosi, G; Hagmann, C; Kinion, D; Van Bibber, K; Hoskins, J; Martin, C; Sikivie, P; Tanner, D. B.; Bradley, R; Clarke, J (2010). "Search for chameleon scalar fields with the axion dark matter experiment". Physical review letters 105 (5): 051801. PMID 20867906.
- GammeV experiment at Fermilab
- The CHASE laboratory search for chameleon dark energy
- Khoury, J.; Weltman, A. (2004). "Chameleon fields: awaiting surprises for tests of gravity in space". Physical Review Letters 93 (17): 171104. arXiv:astro-ph/0309300. Bibcode:2004PhRvL..93q1104K. doi:10.1103/PhysRevLett.93.171104. PMID 15525066.
- Khoury, J.; Weltman, A. (2004). "Chameleon cosmology". Physical Review D 69 (4): 044026. arXiv:astro-ph/0309411. Bibcode:2004PhRvD..69d4026K. doi:10.1103/PhysRevD.69.044026.
- Brax, P.; van de Bruck, C.; Davis, A.-C.; Khoury, J.; Weltman, A. (2004). "Detecting dark energy in orbit: The cosmological chameleon". Physical Review D 70 (12): 123518. arXiv:astro-ph/0408415. Bibcode:2004PhRvD..70l3518B. doi:10.1103/PhysRevD.70.123518.
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