Ambient Scalar–Tensor Gravity: How Local Speed, Density, and Temperature Tune the Gravitational Force and Eliminate the Dark Sector

We present a scalar–tensor theory of gravity in which the effective gravitational coupling depends on the local ambient parameters—relative velocity of test masses, mass density of the environment, and temperature. The theory is derived from a well-defined action principle, preserving general covariance, and introduces a dynamical scalar field that settles into a local equilibrium determined by the matter distribution. As a result, gravity is enhanced in low-density, high-velocity environments. The core of the theory is the ambient function Φ(v, ρ, T) = 1 + α (v/c)² e⁻ᵝρ Γ(T), which acts as a volume knob for the gravitational constant. We show that the theory satisfies all Solar System constraints, flattens galactic rotation curves without dark matter, and yields cosmic acceleration without a cosmological constant. Testable predictions are given for laboratory measurements of G with forced oscillations, atomic clock spectroscopy, and the cosmic microwave background. A step-by-step derivation of the coupling function from the scalar field equation is provided in the Appendix. Keywords: modified gravity, scalar–tensor theory, dark matter, dark energy, galactic rotation curves, cosmological constant problem, varying gravitational constant, ambient environment coupling