A hidden reionization prior biases cosmological inference

Abstract

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 $z \gtrsim 12$. 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.