Quasar Lyman-α optical depth measurements indicate that cosmic reionization ended rapidly at z ≈ 5.3 to 5.5 (Bosman 2022; Choudhury 2021). This rapid termination is faster than ΛCDM gradual single-epoch reionization scenarios easily produce, creating tension between the need for a sudden ionization transition and the slow steady growth of the responsible stellar-population sources.
The standard model assumes a single-epoch smooth reionization history driven by the gradually-building galaxy population. Under that assumption, reionization should end gradually as the integrated UV photon budget from cumulative galaxy formation completes the ionization of the IGM. A sharp termination at z ≈ 5.3 to 5.5 is hard to produce without unphysically rapid changes in the galaxy population's ionizing-photon output.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field that became our visible universe. The cascade of multi-stage thermalization that produced our plasma was not perfectly uniform. It left density inhomogeneities baked into the post-recombination universe. Reionization in SCT proceeds through these inhomogeneities as a multi-phase process: different regions reionize at different epochs depending on the local cascade-deposit density variations (paper 4208, P40 alt).
High-density cascade-imprinted regions reionize early because cascade-seeded structures formed there first and produced the first ionizing-photon sources. Low-density cascade-imprinted regions reionize later because they had to wait for ionizing radiation to leak in from the early-reionizing high-density regions. The result is a multi-phase reionization history with extended early ionization in some regions and a rapid late synchronization phase as the last unionized regions complete reionization. The observed rapid termination at z ≈ 5.3 to 5.5 is the final synchronization phase: the last low-density regions completing reionization once enough ionizing flux has accumulated to overcome their slower-recombination resistance.
This is sharper than the gradual single-epoch picture ΛCDM expects, but it is exactly what multi-phase cascade reionization predicts. Combined with the observation that the integrated CMB optical depth τ ≈ 0.054 is moderate (Planck 2020), the multi-phase picture explains both the moderate global average and the sharp regional termination. The average is driven by extended early reionization in cascade-seeded high-density regions, and the sharp termination reflects the synchronization of the remaining low-density regions late in the process. Neither feature is anomalous in SCT; both are direct consequences of the multi-phase reionization that follows from the inhomogeneous cascade thermalization.
The toggle from hot-dense-center to superluminal-collision-and-thermalized-debris-field is what enables this. A perfectly uniform initial plasma (the ΛCDM assumption) cannot produce multi-phase reionization without invoking ad-hoc structure formation differences. The cascade-thermalized plasma has structure baked in from the cascade itself, and that structure naturally produces the spatial inhomogeneity in reionization timing that observations demand.
If next-generation Lyman-α forest measurements + 21-cm tomography (SKA, HERA) confirm a smooth single-epoch reionization history with no spatial inhomogeneity above survey precision, the multi-phase cascade reionization explanation is refuted.