We define a small phenomenological extension of the standard ΛCDM cosmological background by introducing an additional effective energy-density component modulated by a Hubble-threshold gate. The component reduces exactly to ΛCDM when its amplitude vanishes and preserves the present-day normalization. We specify the background formulation, the gate function, and two frozen benchmark configurations (Sₛafe, R2ₘain), together with candidate mechanisms and explicit falsifiable predictions. The model is implemented in the Boltzmann code CLASS, with background-level reproducibility confirmed by a local audit; spectra and P (k) outputs exist and differ between benchmarks. We document preliminary likelihood comparisons against Planck plik-lite, where an apparent improvement is high-ℓ–driven and does not survive an information-criterion penalty for the added parameters. This record is a priority and reproducibility snapshot of a testable candidate. It is NOT a detection, NOT a claim of statistical preference over ΛCDM, and NOT a confirmation of higher-dimensional physics. Full Planck likelihood analysis, production MCMC, posterior inference, a complete perturbation implementation with phantom-divide handling and the structure-growth (σ8, S8) evaluation that follows from it, and independent external reproduction remain as future work.
Taro SEKI (Sat,) studied this question.