Wavelength-Constrained Propagation as a Substrate Mechanism for Evolving Dark Energy: Mapping the Entropic-Lagrangian Framework to DESI DR2 Observations

Recent results from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 show evidence for a time-varying dark energy equation of state, with w_0 > -1 and w_a < 0 at 2.5–4.2σ significance, disfavoring the cosmological constant ΛCDM at increasing confidence. I propose that this evolution is not a property of dark energy itself but a direct consequence of a substrate mechanism I term the ρ/c exponent exchange: as the universe expands and mean energy density decreases, the coupling balance between the density field ρ and the propagation field c shifts toward the c-dominated regime, reducing the effective vacuum energy density over cosmic time. Within this Entropic-Lagrangian Framework (ELF), electromagnetism and gravity are not independent forces but opposite spectral limits of a single wavelength-constrained propagation spectrum. Short-wavelength modes propagate freely as electromagnetism; long-wavelength modes accumulate amplitude torsion and seal into closed temporal bubbles identified as gravitational fields. The apparent hierarchy of 10³⁶ between electromagnetic and gravitational coupling strengths emerges as a wavelength ratio without fine-tuning. This framework makes three testable predictions: (1) the DESI dark energy equation of state evolution should follow a specific functional form tied to the cosmic mean energy density history; (2) gravitational wave signals from compact binary mergers should exhibit a distance-dependent phase lag relative to electromagnetic counterparts; and (3) co-located optical atomic clocks of different species should exhibit a persistent differential phase-rate offset δR ≈ −7 × 10⁻¹⁸ that does not vanish under improved environmental isolation. Taken together, these predictions provide laboratory-scale, solar-system-scale, and cosmological-scale tests of the same underlying substrate mechanism — precisely the class of observable that Koch et al. identified as needed to distinguish quantum gravity candidates.