Luminal Propagation of Gravitational Waves in the Scalar Temporal Field Ontology

The coincident observation of gravitational waves and gamma rays from GW170817 constrains the propagation speed of tensor modes to agree with the speed of light to one part in 10^15. This result eliminates large classes of modified gravity theories whose tensor sector deviates from luminal propagation. In this paper we prove that the Scalar Temporal Field Ontology (STFO), formulated as a covariant scalar–temporal congruence theory with elastic self-energy and generalized kinetic sector, predicts exactly luminal gravitational wave propagation on all cosmological and astrophysical backgrounds compatible with observational constraints. Starting from the covariant STFO reference action with generalized kinetic term F(X), temporal elasticity (Ω^2, C_{μν}C^{μν}), and no curvature–derivative couplings, we derive the linearized tensor perturbation equations around a general FLRW background. We show explicitly that all non-Einstein–Hilbert contributions vanish identically in the transverse–traceless sector. As a result, the quadratic tensor action is exactly that of general relativity and the gravitational wave speed satisfies c_T = c exactly, independent of background evolution, elastic couplings, or temporal kinetic nonlinearities. This establishes full structural compatibility of STFO with GW170817 and all present and future multimessenger gravitational wave constraints, without the need for parameter tuning.