Is the CMB reionization optical depth underestimated?
This text mentions work from 2024 on. Those parts were partly written without a live literature search and have not been checked by an editor yet.
In plain words
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 statement
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.
What would settle it
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.
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