The Inevitable Decrease of Light Frequency During Propagation: The Illusion of Cosmic Expansion

This paper establishes a structural theorem demonstrating that the frequency of light cannot remain constant during long‑distance propagation in any physically realistic universe. The result follows from three minimal and theory‑independent premises: (i) light carries energy, (ii) no physical universe provides an infinitely flat propagation environment, and (iii) every finite light source possesses a monotonic radial geometry. From these premises alone, it is shown that exact frequency constancy is logically impossible, cumulative interaction effects cannot cancel, and the inherent directional asymmetry imposed by source monotonicity forces the net frequency evolution to be downward.This conclusion does not rely on cosmological expansion, gravitational field equations, spacetime metrics, or any dynamical model. It identifies redshift as a structural consequence of wave propagation in a non‑flat universe rather than as evidence of cosmic expansion. The traditional assumption of frequency constancy is thereby recognized as an artifact of an idealized Minkowski model with no physical analogue at cosmological scales. The theorem provides a foundational, model‑independent explanation for redshift phenomena and reframes frequency evolution as a geometric inevitability of energy‑carrying radiation in a structured universe.