A hidden reionization prior biases cosmological inference

Precision cosmology assumes that cosmic reionization was a single smooth transition. We show that this assumption is in tension with current observations: the Planck optical depth, patchy kinetic Sunyaev--Zel'dovich (PkSZ)limits from SPT and ACT, and the Ly alpha forest endpoint cannot be simultaneously reproduced by any viable monotonic ionization history. Non-parametric reconstructions and Planck EE polarization provide independent support for an additional ionization component at z 12. Incorporating this early phase relaxes the upper bound on the summed neutrino mass to sum m nu < 0.39 eV (95% CL) and shifts sigma 8 toward values preferred by weak-lensing surveys, both consequences of the standard A s -- tau e degeneracy. These shifts arise from relaxing a hidden prior on reionization shape rather than from new physics, and identify reionization shape as an implicit prior in cosmological inference. Next generation of CMB and 21 cm experiments will be able to test this directly,which will convert what has been an unflagged systematic on sigma 8 and sum m nu into a quantified statistical uncertainty.