Emergent Spacetime Optics from a Finite-Response Cosmological Medium: Weak Lensing Suppression, Non-Metric Propagation, and CMB Phase Signatures

We present a cosmological framework in which spacetime geometry is not assumed as fundamental, but emerges as an effective description of light propagation in a finite-response medium. In this picture, the vacuum is modeled as a condensate-like system—the Emergent Condensate Superfluid Medium (ECSM)—whose local response properties govern the transport of radiation. Light is treated as an inductive excitation propagating through this medium, rather than as a test particle following null geodesics of a predefined metric. A central parameter is the induction coherence length, which controls the spatial scale over which the medium responds coherently. In the limit of infinite coherence, standard metric behaviour is recovered. At finite coherence, propagation becomes explicitly non-metric, local, and scale-dependent. We derive the corresponding ray equations from first principles and perform numerical ray tracing through ECSM density fields. The results show that lensing is naturally localized to condensed structures and suppressed along extended lines of sight. This leads to a breakdown of standard lensing consistency relations: convergence and shear fields become decorrelated in a scale- and redshift-dependent manner that cannot be absorbed into a single amplitude rescaling. The framework also provides a reinterpretation of the cosmic microwave background (CMB) as radiation emitted across a finite-thickness phase-transition boundary. Finite emission depth and coherence during propagation produce small, correlated phase shifts in the TT, TE, and EE power spectra while preserving the overall acoustic structure. These shifts are peak-dependent and cannot be mimicked by simple angular-diameter distance rescaling or standard late-time parameter adjustments. Taken together, the model predicts a unified set of observational signatures: (i) suppression of weak lensing relative to structure growth, (ii) scale-dependent decorrelation between lensing and large-scale structure, and (iii) correlated, peak-dependent phase shifts across CMB temperature and polarization spectra. These signatures provide a direct and falsifiable test of whether cosmological light propagation is fundamentally metric or instead governed by an emergent, finite-response medium. All derivations and numerical results are provided to enable independent verification and further testing with current and upcoming observational data. How to cite: Sheldrick, A. (2026). Emergent Spacetime Optics from a Finite-Response Cosmological Medium: Weak Lensing Suppression, Non-Metric Propagation, and CMB Phase Signatures. Zenodo. https://doi.org/10.5281/zenodo.19518946