Reionization's clocks disagree: Planck's optical depth wants a compact event near z of 7.7, while quasar spectra show neutral islands surviving to z of 5.3 to 5.5 with large sightline scatter and hints of much earlier activity (Planck 2020; Bosman 2022). The smooth sigmoid history cannot be simultaneously late, compact, patchy, and source-driven.
The model's reionization is one epoch with one driver: the gradually assembling galaxy population ionizes the IGM in a single smooth sweep. Every measured deviation, the late stragglers, the opacity scatter, the early hints, must be absorbed by tuning escape fractions and faint-source budgets, because the single-sweep shape itself is not negotiable.
SCT's ionization history is multi-phase by construction. The cascade's thermalization was not uniform, multi-phase heating across the overlap volume left baked-in inhomogeneity (P44), and the deposited structure ignited its sources on the web's geography rather than on a single assembly clock (P25, P34): dense deposit regions ionized their surroundings early, sparse regions late, and post-collision reheating events staggered additional ionizing episodes through the early epochs (P47). The result is exactly the timeline the two clocks jointly describe: activity beginning earlier than the assembly clock permits, neutral islands persisting later than the sigmoid allows, and sightline scatter as the native texture of a patchwork rather than a noise term to be excused.
The integrated optical depth then measures the patchwork's sum, naturally compact in its central weight while the edges straggle in both directions, dissolving the duration mismatch without per-dataset tuning. The early-source half is the same accelerated ignition carrying the star formation history (recid 111) and the early chemical maturity (recid 113); the multi-phase structure is registered through the canonical reionization mechanism (P35, P36, P40 family) laid out in Paper 1, From Chaos to Convergent Foundations, with the thermalization texture in Paper 3, From Chaos To Concordance Spectra.
Keystone economy: P44 supplies the patchwork, P47 staggers the episodes, P25 starts the clock early. The two clocks were both right; the model's single sweep was the error.
21-cm tomography carries the kill: SKA mapping of the neutral-hydrogen history will reconstruct reionization's actual shape, and a clean single-phase sigmoid, smooth in time and space with scatter fully attributable to known source statistics, would refute the multi-phase cascade texture. SCT separately requires the patchiness to correlate with the deposited large-scale structure; ionization topology uncorrelated with the web's geography would break the deposit-driven reading.