A scalar-tensor theory uses a metric tensor and an additional scalar field to mediate gravity. A scalar multiplier of the Ricci scalar makes the curvature coupling depend on that field. The Brans-Dicke theory is a standard example; the choice of scalar potential and kinetic coupling distinguishes different members of the class.
Scalar–tensor theory is a class of theories in theoretical physics that combines both scalar fields and tensor fields, typically used in the context of gravity. The most well-known example of a scalar-tensor theory is Brans-Dicke theory, which was proposed to extend general relativity by incorporating a scalar field alongside the standard metric tensor field of gravity.
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