Tests of Chameleon Gravity (1709.09071v2)

Abstract: Theories of modified gravity where light scalars with non-trivial self-interactions and non-minimal couplings to matter-chameleon and symmetron theories-dynamically suppress deviations from general relativity in the solar system. On other scales, the environmental nature of the screening means that such scalars may be relevant. The highly-nonlinear nature of screening mechanisms means that they evade classical fifth-force searches, and there has been an intense effort towards designing new and novel tests to probe them, both in the laboratory and using astrophysical objects, and by reinterpreting existing datasets. The results of these searches are often presented using different parametrizations, which can make it difficult to compare constraints coming from different probes. The purpose of this review is to summarize the present state-of-the-art searches for screened scalars coupled to matter, and to translate the current bounds into a single parametrization to survey the state of the models. Presently, commonly studied chameleon models are well-constrained but less commonly studied models have large regions of parameter space that are still viable. Symmetron models are constrained well by astrophysical and laboratory tests, but there is a desert separating the two scales where the model is unconstrained. The coupling of chameleons to photons is tightly constrained but the symmetron coupling has yet to be explored. We also summarize the current bounds on $f(R)$ models that exhibit the chameleon mechanism (Hu & Sawicki models). The simplest of these are well constrained by astrophysical probes, but there are currently few reported bounds for theories with higher powers of $R$. The review ends by discussing the future prospects for constraining screened modified gravity models further using upcoming and planned experiments.

• The paper presents a comprehensive review of experimental tests of chameleon gravity, detailing how scalar fields can screen modifications to General Relativity.
• It analyzes both chameleon and symmetron models, emphasizing environmental screening mechanisms that reconcile laboratory and astrophysical observations.
• Key findings suggest that while many chameleon models remain viable within constrained parameter spaces, future experiments could further unveil new gravitational insights.

An Analytical Review of "Tests of Chameleon Gravity"

The research paper titled "Tests of Chameleon Gravity" by Clare Burrage and Jeremy Sakstein offers a comprehensive examination of modified gravity theories, particularly focusing on chameleon and symmetron theories. These theories introduce light scalar fields with non-trivial interactions, which are hypothesized to suppress observable deviations from General Relativity (GR) in the solar system while remaining significant on larger scales. This review paper systematically consolidates current experimental and observational constraints on these theories and assesses their viability in light of recent advances in astrophysical and laboratory tests.

Chameleon and Symmetron Theories

The paper introduces chameleon and symmetron models, both of which encapsulate screening mechanisms that allow scalar fields to mimic GR on solar system scales but diverge at astronomical distances. The chameleon mechanism achieves this by making the effective mass of the scalar field an increasing function of the ambient density, ensuring dense environments like the solar system are screened. The symmetron model, on the other hand, leverages symmetry-breaking dynamics dependent on environmental density to nullify deviations from GR under certain conditions. This distinction underscores the potential flexibility of these models in addressing current cosmological puzzles, such as the nature of dark energy.

Experimental and Observational Constraints

The paper surveys various detection techniques across different scales—from controlled laboratory experiments to cosmic-scale observations—providing a critical evaluation of each method's efficacy in probing these elusive scalar fields. For instance, precision tests such as torsion balance experiments and atom interferometry are emphasized for their potential to detect fifth forces and deviations from the inverse-square law in gravitational physics. Additionally, astrophysical tests using the dynamics of dwarf galaxies and gravitational lensing in galaxy clusters are reviewed for their relevance in constraining these screening mechanisms on astronomical scales.

Numerous tests reveal that common chameleon models, especially those with negative power potentials, are highly constrained but not entirely ruled out, suggesting wide swathes of parameter space that remain viable. Symmetron models, similarly, face substantial constraints, although certain parameter regimes between laboratory and astrophysical scales remain underexplored, highlighting a key area for future research.

Implications and Future Directions

The review highlights significant implications for both theoretical and experimental physics. The convergence of bounds from disparate methods allows a sweeping assessment of viability for chameleon and symmetron theories in explaining dark energy phenomena without conflicting with GR on verified scales. The need to explore less studied regions of parameter space is stressed, particularly in relation to more exotic predictions resulting from dense, unscreened environments.

Moreover, the research prompts further inquiry into potential couplings of such scalar fields with other fundamental forces or particles, such as photons, which remain less explored or tested. Upcoming and proposed experiments, especially those harnessing novel astrophysical phenomena or improved laboratory sensitivity, are expected to refine or redefine the permissible landscape for these modified gravity theories.

Conclusion

Burrage and Sakstein's paper is pivotal in outlining the current frontiers in testing chameleon and symmetron gravity theories. It effectively synthesizes experimental data across multiple disciplines to present a nuanced view of current constraints, emphasizing the potential for meaningful breakthroughs with forthcoming technological advancements and experimental designs. This discussion not only provides a clear roadmap for future research efforts but also underscores the critical nature of developing innovative methodologies to either validate or exclude these compelling theories of modified gravity.