Chameleon Cosmology and Modified Gravity Theories
Summary
Chameleon cosmology and related modified gravity theories introduce light scalar degrees of freedom that couple to matter in a density-dependent way, thereby evading stringent local tests of general relativity while still playing a significant role on cosmological or galactic scales. In chameleon models the effective mass of the scalar field increases in high-density environments, suppressing any long-range “fifth force” in the solar system yet allowing detectable deviations from Einstein’s theory in the low-density cosmic web. Symmetron and K-mouflage variants implement alternative screening mechanisms based on symmetry restoration or nonlinear kinetic terms, respectively. A closely related implementation appears in f(R) gravity, where modifications to the Ricci scalar reproduce chameleon-type behaviour. Collectively, these theories offer unified frameworks for dark energy and dark matter phenomena, potentially addressing the Hubble tension, structure formation, and scalar interactions with photons. Progress in analytical and numerical methods has clarified the structure of screening surfaces, while astrophysical and laboratory probes continue to constrain viable parameter space. As a result, chameleon and other screened scalar scenarios remain a vibrant frontier linking fundamental theory, cosmological observation and precision tests of gravity.
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Chameleon Cosmology and Modified Gravity Theories publication trend
The graph below shows the total number of articles in chameleon cosmology and modified gravity theories across all publications each year (not limited to Nature Index journals).
Technical terms
Chameleon mechanism: A screening process in which the scalar field’s effective mass increases with ambient matter density, suppressing fifth-force effects in high-density regions.
Screening mechanism: A generic term for any dynamical process—such as chameleon, symmetron or K-mouflage—that hides additional scalar interactions from local tests of gravity.
Fifth force: A hypothetical non-gravitational interaction mediated by a light scalar field, whose effects must be screened to comply with laboratory and solar-system observations.
f(R) gravity: A class of modified gravity theories in which the Ricci scalar in the Einstein–Hilbert action is replaced by a nonlinear function f(R), often leading to chameleon-like screening behaviour.
References
- Accurate computation of the screening of scalar fifth forces in galaxies. Journal of Cosmology and Astroparticle Physics (2024).
- Testing screened modified gravity with SDSS-IV-MaNGA. Monthly Notices of the Royal Astronomical Society (2024).
- Testing Screened Modified Gravity. Universe (2021).
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