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The chameleon is a hypothetical
scalar particle A scalar boson is a boson whose spin equals zero. ''Boson'' means that the particle's wave function is symmetric under particle exchange and therefore follows Bose–Einstein statistics. The spin-statistics theorem implies that all bosons have an ...
that couples to matter more weakly than gravity, postulated as a
dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univer ...
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 The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...
. As a result of this variable mass, the hypothetical
fifth force In physics, there are four observed fundamental interactions (also known as fundamental forces) that form the basis of all known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative the ...
mediated by the chameleon is able to evade current constraints on
equivalence principle In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (suc ...
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 The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not exp ...
, it also makes it very difficult to test for experimentally. In 2021, physicists suggested that an excess reported at the dark matter detector experiment
XENON1T Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
rather that being a dark matter candidate could be a dark energy candidate particularly chameleon particles yet in July 2022 a new analysis by XENONnT discarded the excess.


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: m_\text \sim \rho^\alpha, where \alpha \simeq 1. Chameleons also couple to photons, allowing photons and chameleons to oscillate between each other in the presence of an external
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. 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.


Experimental searches

A number of experiments have attempted to detect chameleons along with
axions An axion () is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interes ...
. 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 5 T magnetic field. The ends of the chamber are glass windows, allowing light from a laser to enter and afterglow to exit. GammeV set the limited coupling to photons in 2009. CHASE (CHameleon Afterglow SEarch) results published in November 2010, 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 \beta > 5.8 \times 10^8, where the effective potential of the chameleon quanta is written as V_=V(\Phi)+e^ \rho, \rho being the mass density of the environment, V(\Phi) the chameleon potential and M'_\text the reduced Planck mass. The
CERN Axion Solar Telescope The CERN Axion Solar Telescope (CAST) is an experiment in astroparticle physics to search for axions originating from the Sun. The experiment, sited at CERN in Switzerland, was commissioned in 1999 and came online in 2002 with the first da ...
has been suggested as a tool for detecting chameleons.


References


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External links

{{DEFAULTSORT:Chameleon Particle Dark energy Bosons Physical cosmology Astroparticle physics Hypothetical elementary particles Subatomic particles with spin 0 Force carriers