Physical frame selection in scalar–tensor gravity

Determine in which conformal frame—the Jordan frame or the Einstein frame—the field equations of scalar–tensor theories provide the most appropriate description of gravitational phenomena.

Background

Scalar–tensor theories commonly admit both a Jordan-frame formulation, in which the scalar field is non-minimally coupled to gravity, and an Einstein-frame formulation, in which the scalar field is minimally coupled after a conformal transformation of the metric. Because a conformal transformation acting only on the metric can alter the compatibility relation between the metric and affine connection, the two frames may correspond to different underlying geometric structures and may lead to different interpretations of gravitational measurements.

The paper identifies the absence of a generally accepted criterion for selecting the physically appropriate frame as an unresolved issue. Although the proposed Weyl-integrable formulation is presented as a framework for addressing the frame problem, the quoted passage leaves the question of which frame gives the most appropriate description unresolved.