Tau has marched from 0.17 to 0.089 to 0.066 to 0.054, with PR4-era reprocessings spread between 0.051 and 0.063: each revision exceeded its predecessor's stated uncertainty, and the current spread across defensible analyses is wider than any quoted error. The number anchors reionization, calibrates sigma_8, and limits the neutrino bound, so its instability propagates through every claim of precision closure.
The model compresses an epoch into one number by assuming reionization was simple: a single roughly monotonic transition with one midpoint. Every estimator is then measuring the same tau by definition, and persistent scatter can only be systematics, excavated indefinitely, because the parameterization has no room for a structured history.
SCT's reionization is structured by construction: collision-seeded sources ionize early and locally, recombination reclaims regions in the natively clumpy deposited medium, and later heating episodes re-ionize, the staggered multi-phase history of P47 and P36, spatially patterned by the deposition geometry (P34). An epoch like that does not possess a single tau: estimators weighting different scales and redshift kernels legitimately recover different effective values, and the twenty-year scatter, each analysis precise, the ensemble unstable, is the signature of one number being asked to summarize a history with phases. The same structure is what absorbs the JWST photon-budget surplus (powerful early sources, late forest-anchored completion), one mechanism serving both reionization tensions.
The reading converts the scatter from embarrassment to data: the spread between scale-weighted estimators becomes a measurement of the history's structure, with the registered expectation that low-ell polarization will show reionization-bump structure beyond the single-midpoint shape, correlated with the patchiness large-scale structure already hints at. The multi-phase framework is developed in Paper 4, From Chaos To Collisothermal Cosmogenesis, with the reheating premises from Paper 8.
Two premises structure the epoch, the same two already paying for the photon-budget crisis and the 21 cm contingencies: the framework's reionization sector runs three tensions on one mechanism.
The settlement test: LiteBIRD-class low-ell polarization measuring a clean single-phase reionization bump, tau converging across all estimators to one stable value with the historical scatter fully traced to instrument systematics, would remove the structured-epoch reading and restore the simple history. SCT requires residual structure: estimator-dependent effective tau and a bump shape beyond the one-midpoint parameterization.