Shining Light on Modifications of Gravity

TL;DR

This paper demonstrates that high-precision photon experiments can constrain disformal scalar field theories, which are difficult to test with traditional laboratory methods, thus providing new avenues to explore modifications of gravity.

Contribution

It reveals that photon experiments, typically used for axion-like particles, can also effectively constrain disformal scalar field theories in gravity modifications.

Findings

Photon experiments constrain disformal scalar couplings.

Disformal terms are undetectable in static non-relativistic sources.

Laboratory experiments are crucial for testing disformal modifications.

Abstract

Many modifications of gravity introduce new scalar degrees of freedom, and in such theories matter fields typically couple to an effective metric that depends on both the true metric of spacetime and on the scalar field and its derivatives. Scalar field contributions to the effective metric can be classified as conformal and disformal. Disformal terms introduce gradient couplings between scalar fields and the energy momentum tensor of other matter fields, and cannot be constrained by fifth force experiments because the effects of these terms are trivial around static non-relativistic sources. The use of high-precision, low-energy photon experiments to search for conformally coupled scalar fields, called axion-like particles, is well known. In this article we show that these experiments are also constraining for disformal scalar field theories, and are particularly important because of the difficulty of constraining these couplings with other laboratory experiments.