The Optical Phase-Delay Signature in Massive Elliptical Lenses: A Parameter-Free Phenomenological Ansatz Applied to the SLACS Catalog

The modeling of strong gravitational lensing systems is fundamentally constrained by the mass-sheet degeneracy, a mathematical ambiguity that typically requires the introduction of highly parameterized non-baryonic dark matter halos to reconcile visible baryonic mass distributions with observed Einstein radii. In this work we present an alternative phenomenological model that describes the macroscopic optical deflection of light via a parameter-free optical-hydrodynamic ansatz. Treating the cosmic vacuum as a viscous fluid governed by a universal phase-impedance constant β = 0.028 (calibrated from independent galactic dynamics, not from lensing data), we formulate the Cortez Refractive Index (ηC) and perform a blind recovery of source redshifts (zS) for the Sloan Lens ACS Survey (SLACS). Applying a zero-parameter state equation (N = 0) that uses an integrated thick-vacuum drag and a linear causal metric to a complete sample of 59 systems (all with non-missing data) yields a Mean Absolute Error (MAE) of ∆z = 0.2946. For comparison, a naive geometric baseline zS = 2zL gives ∆z = 0.2324, but it has no physical foundation, while the standard ΛCDM approach employs several free parameters per galaxy to force a perfect fit. Using the exact bias parameters reported by Sonnenfeld (2025) for the SDSS fibre aperture (a systematic upward shift of 5% in σv, an intrinsic log-normal scatter of 8.4%, and a 5% uncertainty in θE), a Monte Carlo simulation shows that the expected error floor for any perfect model is 0.309±0.073– fully consistent with the observed MAE of 0.2946. Our rigid N = 0 equation thus reveals the instrumental noise floor rather than absorbing it. These results provide evidence that the mass-sheet degeneracy can be parsimoniously interpreted as a macroscopic optical propagation effect in a viscous spatial fabric, offering a deterministic geometric constraint without invoking non-baryonic dark matter.