The Cepheid period-luminosity relation depends on metallicity, but both the size and even the sign of this dependence are debated. Different studies find metal-rich Cepheids either brighter or fainter at fixed period (Sasselov 1997; Storm 2004; Efstathiou 2020). The metallicity sensitivity propagates directly into the SH0ES distance ladder and into local H₀, so plausible metallicity-correction choices can shift H₀ by several percent — comparable to the Hubble tension itself.
The standard model assumes a single global FLRW expansion with constant Λ, so the Cepheid distance ladder must deliver one universal H₀ value once metallicity systematics are controlled. The model has no framework that would explain why metallicity sensitivity is genuine but does not collapse the tension once corrected. Stellar-physics systematics get assigned the entire burden of resolving an apparent multi-probe disagreement.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, the Cepheid metallicity sensitivity is real but it does not collapse the tension because the underlying H₀ is genuinely environment-dependent. Λ_eff(x,t) = κ · U_local(x,t) / U_parent(x,t) (P17) varies between hosts depending on their embedding in the local KBC supervoid (P19) and on the integrated parent-frame mesh dissipation along the line of sight (P14, P15, P16, P18).
Pre-existing matter from prior cascade cycles (P25, P28) seeds host galaxies with heterogeneous chemical compositions: prior generations of stellar nucleosynthesis already enriched the matter that the cascade thermalized, so different host galaxies start from different metallicity baselines. The Cepheid metallicity dependence is therefore a real stellar-physics effect that tracks host-galaxy embedding history, not a pipeline systematic. The frame-tree corrections (P10) introduce additional small host-to-host distance-modulus shifts depending on the gravitational embedding hierarchy between Earth and the host.
Once the metallicity sensitivity is properly handled, SCT predicts H₀ ≈ 70.4 ± 0.4 km/s/Mpc (Paper 16, CAR-derived), intermediate between Planck (67.4) and SH0ES (73.0). The SH0ES value is biased high because it samples preferentially within our local KBC supervoid; the Planck value is biased low because it integrates the full cosmic-mean Λ_eff. The Cepheid metallicity scatter falls naturally into this framework as host-environment variability rather than as a calibration error.
Multi-host metallicity-stratified Cepheid surveys (precision JWST follow-up) finding zero correlation between host environment and inferred H₀ (after metallicity correction) at greater than 2σ would refute the M5 environmental explanation. The signature M5 prediction is that void-direction Cepheid hosts yield systematically higher H₀ than cluster-direction hosts, with the metallicity-correction residuals tracing the Λ_eff field rather than averaging to zero.