Equivalence principle in chameleon models

KRAISELBURD, LUCILA; LANDAU, SUSANA J.; SALGADO, MARCELO; SUDARSKY, DANIEL; VUCETICH, HÉCTOR

Physical Review D

American Physical Society

Año: 2018 vol. 97 p. 104044 - 104044

2470-0010

Most theories that predict time and/or space variation of fundamentalconstants also predict violations of the weak equivalence principle(WEP). In 2004 Khoury and Weltman [1] proposed the so called chameleonfield arguing that it could help avoiding experimental bounds on the WEPwhile having a nontrivial cosmological impact. In this paper we revisitthe extent to which these expectations continue to hold as we enter theregime of high precision tests. The basis of the study is thedevelopment of a new method for computing the force between two massivebodies induced by the chameleon field which takes into account theinfluence on the field by both, the large and the test bodies. Weconfirm that in the thin shell regime the force does depend nontriviallyon the test body´s composition, even when the chameleon couplingconstants βi=β are universal. We also propose asimple criterion based on energy minimization, that we use to determinewhich of the approximations used in computing the scalar field in a twobody problem is better in each specific regime. As an application of ouranalysis we then compare the resulting differential acceleration of twotest bodies with the corresponding bounds obtained from Eötvöstype experiments. We consider two setups: (1) an Earth based experimentwhere the test bodies are made of Be and Al; (2) the Lunar Laser Rangingexperiment. We find that for some choices of the free parameters of thechameleon model the predictions of the Eötvös parameter arelarger than some of the previous estimates. As a consequence, we put newconstrains on these free parameters. Our conclusions strongly suggestthat the properties of immunity from experimental tests of the WEP,usually attributed to the chameleon and related models, should becarefully reconsidered. An important result of our analysis is that ourapproach leads to new constraints on the parameter space of thechameleon models.