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The chameleon is a hypothetical scalar particle that couples to matter more weakly than gravity,〔( Tiny fountain of atoms sparks big insights into dark energy ) by Adrian Cho on August 20, 2015. Published at the "American Association for the Advancement of Science"〕 postulated as a dark energy candidate.〔J Khoury and A. Weltman, Phys. Rev. D 69, 044026 (2004)〕 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. ==Hypothetical properties== 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 . 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.〔J.H. Steffen et al., "Constraints on chameleons and axion-like particles from the GammeV experiment" ()〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chameleon particle」の詳細全文を読む スポンサード リンク
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