Space isotropy and weak equivalence principle in a scalar theory of gravity

TL;DR

This paper develops a scalar gravity theory with a preferred frame, modifies the metric to be locally isotropic, and shows that the weak equivalence principle holds in this new version, correcting previous anisotropic assumptions.

Contribution

It introduces a locally-isotropic scalar gravity model (v2) that preserves the weak equivalence principle, unlike earlier anisotropic versions, and adapts the post-Newtonian approximation accordingly.

Findings

Weak equivalence principle holds in v2

Isotropic dilation modifies the scalar field equation

Previous violations due to anisotropy are corrected in v2

Abstract

We consider a preferred-frame bimetric theory in which the scalar gravitational field both influences the metric and has direct dynamical effects. A modified version ("v2") is built, by assuming now a locally-isotropic dilation of physically measured distances, as compared with distances evaluated with the Euclidean space metric. The dynamical equations stay unchanged: they are based on a consistent formulation of Newton's second law in a curved space-time. To obtain a local conservation equation for energy with the new metric, the equation for the scalar field is modified: now its l.h.s. is the flat wave operator. Fluid dynamics is formulated and the asymptotic scheme of post-Newtonian approximation is adapted to v2. The latter also explains the gravitational effects on light rays, as did the former version (v1). The violation of the weak equivalence principle found for gravitationally-active bodies at the point-particle limit, which discarded v1, is proved to not exist in v2. Thus that violation was indeed due to the anisotropy of the space metric assumed in v1.