Gravity Field Dynamics: Decoding mass from observed rotation curves

Galaxy rotation curves imply more mass than Newtonian gravity and visible matter predict. Two standard responses are dark matter halos (with parameters fitted per galaxy) and modified gravity such as MOND; neither offers a way to go from observed velocity to implied mass without assuming a mass model. We present a framework based on tetrad gravity and ghost-free bimetric gravity, with two field equations that are algebraically invertible: the same equations that give rotation velocity from mass also give enclosed mass from velocity. Applied to the Milky Way and M33 with no fitted gravitational parameters (only the constants G, a₀, and a topology-derived ratio 17/13), the inverse pipeline yields enclosed mass profiles that match independently inferred baryonic mass to within a factor of order unity, with no dark matter halo. We do not fit a mass model to the rotation curve; we transform observed velocity into enclosed mass using the same bidirectional covariant (the field equations that run both ways) that defines the theory.