Measuring the reionization optical depth without large-scale CMB polarization

Understand

We study the possibility of measuring the optical depth at reionization, $\tau$, without relying on large-scale Cosmic Microwave Background (CMB) polarization.

• Our analysis is driven by the need to obtain competitive measurements that can validate the state-of-the-art constraints on this parameter, widely based on E-mode polarization measurements at $\ell\le 30$.
• This need is partially motivated by the typical concerns regarding anomalies observed in the Planck large-scale CMB data as well as by the remarkable fact that, excluding these latter, $\tau$ consistently exhibits correlations with anomalous parameters, such as $A_{\rm lens}$ and $\Omega_k$, suggesting that slightly higher values of the optical depth at reionization could significantly alleviate or even eliminate anomalies.
• Within the $\Lambda$CDM model, our most constraining result is $\tau = 0.080 \pm 0.012$, obtained by combining Planck temperature and polarization data at $\ell > 30$, the Atacama Cosmology Telescope (ACT) and Planck measurements of the lensing potential, Baryon Acoustic Oscillations (BAO), and Type-Ia supernova data from the Pantheon+ catalogue.