Infrared Structure of Disformal Scalar–Tensor Theories in the Interstellar Medium

We present the infrared (IR) structure of a scalar–tensor theory in which matter and radiation propagate on distinct effective metrics. A scalar field with a normalized gradient defines a dynamical preferred direction, leading to a restricted disformal matter metric and a conformal optical metric for radiation. In low‑acceleration environments such as the interstellar medium, the nonlinear IR regime becomes active and modifies the geometry of matter‑coupled wave propagation. We derive the IR field equations, the effective matter metric, and the resulting wave operator, and identify three geometric, falsifiable predictions: anisotropic plasma dispersion, metric slip across IR boundaries, and rotation of the fastest‑propagation axis determined by the scalar gradient rather than magnetic geometry. These effects arise naturally from the IR kinetic structure and are not expected from standard magnetohydrodynamic processes. The framework provides a basis for interpreting transition‑scale features observed near the heliopause, which will be analyzed in a companion paper.