The \gamma\ parameter in Brans-Dicke-like (light-)Scalar-Tensor theory with a universal scalar/matter coupling

TL;DR

This paper investigates the post-Newtonian parameter  in Brans-Dicke-like Scalar-Tensor theories with a universal scalar/matter coupling, revealing that  can be positive, zero, or negative depending on the coupling function, contrasting with previous results.

Contribution

It provides a new explicit scalar-field equation using a perfect fluid formalism and explores the conditions under which  varies in scalar-tensor theories with matter coupling.

Findings

 can be positive, zero, or negative depending on the coupling function

The metric can be expressed in standard post-Newtonian form

Scalar-tensor theories without coupling predict <1 for finite _0

Abstract

The post-Newtonian parameter \gamma\ resulting from a universal scalar/matter coupling is investigated in Brans-Dicke-like Scalar-Tensor theories where the scalar potential is assumed to be negligible. Conversely to previous studies, we use a perfect fluid formalism in order to get the explicit scalar-field equation. It is shown that the metric can be put in its standard post-Newtonian form. However, it is pointed out that 1-\gamma\ could be either positive, null or negative for finite value of \omega_0, depending on the coupling function; while Scalar-Tensor theories without coupling always predict \gamma<1 for finite value of \omega_0.