A Scalar--Tensor Field Theory Based on a Cosmic Background Field

We propose that there exists a cosmic background field in four-dimensional spacetime, which takes scalar form within a selected class of coordinate charts. From this axiom we construct a scalar model, the model reproduces local special-relativistic kinematics and yields Newton's law of gravity from a single-body response : rest energy is the coupling energy of a particle to the background field, and the inertial scale is anchored to the cosmic background field. The speed of light is derived once electromagnetism is added, rather than entering as a postulate. We then upgrade the model to a scalar--tensor theory. The physical channel of the numerical scale of the speed of light is established; its mechanism layer (the denomination problem) is attributed to the cosmological initial-value problem.