The Thomson scattering optical depth τ measures how much the cosmic microwave background was scattered by free electrons after reionization. It's one of six ΛCDM parameters and one of the hardest to pin down — constrained almost entirely by the large-scale E-mode polarization pattern.
The authors (arXiv:2603.22454) build a multi-frequency likelihood combining all available satellite polarization data: Planck LFI 70 GHz, Planck HFI 100 and 143 GHz (SRoll2 processing), and WMAP Ka, Q, and V bands. The key methodological insight: retaining all cross-spectra and the WMAP-LFI auto-spectrum eliminates a significant bias present when all auto-spectra are included, while preserving comparable precision.
Result: τ = 0.0575 (+0.0048/-0.0058). Combined with high-ℓ CMB, CMB lensing, and DESI BAO data, the neutrino mass upper bound tightens to Σm_ν < 0.069 eV.
The through-claim: τ is robustly low, and this has consequences beyond reionization history. A low optical depth constrains late-time cosmological models — it rules out using a high τ to explain or reduce the tension between DESI-BAO and CMB observations. The measurement that was hardest to make now closes a loophole.