Multimessenger observations bound the gravitational-wave speed to be luminal, imposing a strong filter on modified gravity. We develop a symmetry-selected Palatini framework with torsion in which exact luminality at quadratic order is guaranteed by construction rather than by parameter tuning. Two structural ingredients govern the observable sector: (i) a scalar PT projector that keeps scalar densities real and parity-even; and (ii) projective invariance enforced via a nondynamical Stueckelberg compensator that enters only through its gradient. Under explicit assumptions (A1–A6) we establish: (C1) algebraic uniqueness of torsion to a pure-trace form aligned with the compensator gradient; (C2) bulk equivalence, up to improvements, among a rank-one determinant route, a closed-metric deformation, and a PT-even CS/Nieh–Yan route; and (C3) a coefficient-locking identity that enforces K = G for tensor modes on admissible domains, hence cT = 1 with two propagating polarizations. Beyond leading order, the framework predicts a distinctive, falsifiable next-to-leading correction δcT² (k) = b k²/Λ² (for k ≪ Λ), implying slope ≈ 2 in log–log fits across frequency bands (PTA/LISA/LVK). The analysis is fully reproducible, with a public repository providing figure generators, coefficients, and tests that directly validate (C1) – (C3).
Chien-Chih Chen (2025) studied this question.