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Title: Is the CMB reionization optical depth underestimated?
Plain: Free electrons released when the first galaxies ionized hydrogen scatter microwave background light, an effect measured by a number called the optical depth. If it is larger than the Planck value, several current tensions, including the neutrino-mass one, ease.
Precise: Planck low-multipole EE polarization gives $\tau = 0.054 \pm 0.007$. Sailer, Farren, Ferraro and White (2025, arXiv:2504.16932) show that a larger $\tau$ removes the DESI BAO versus CMB tension in extended models, including the preference for very small or negative sum $m_\nu$, but conflicts with Planck large-scale polarization. Determine $\tau$ with $\sigma(\tau)$ below 0.003 from an independent large-angle E-mode measurement or from 21-cm and kinetic Sunyaev-Zeldovich reionization histories, and state whether it agrees with the Planck value.
Settled by: Large-angle E-mode polarization from LiteBIRD or a ground or balloon experiment such as CLASS, cross-checked with reionization histories from 21-cm and kSZ data.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: (empty)