Speaker
Description
Reionisation marks the epoch when the first luminous structures ionised the intergalactic medium after recombination, producing free electrons that Thomson-scatter CMB photons. This imprint is encoded in the large-scale E-mode polarisation of the CMB, which directly constrains the optical depth τ. Despite the high quality of Planck and WMAP data, a fully consistent framework that jointly exploits all low-ℓ polarisation information has been missing. To fill this gap, we present ELiCA (E-mode Likelihood for Cross-Analysis), an harmonic-based unified likelihood that consistently combines Planck (100 GHz and 143 GHz HFI channels and 70 GHz LFI channel) and WMAP (Ka, Q, and V bands) large-scale polarisation data. Validated on simulations, ELiCA delivers robust and precise constraints on the optical depth: τ = 0.0581^{+0.0048}_{-0.0059} (68% CL). This result strongly disfavors high-τ scenarios associated with early reionisation, reinforcing a low optical depth picture. The improved precision propagates to ΛCDM extensions, yielding a stringent upper bound on the summed neutrino mass: Σmν < 0.069 eV (95% CL). ELiCA sets a new benchmark for large-scale CMB polarisation analyses and provides a natural framework for next-generation missions such as LiteBIRD, where precision measurements of τ will be critical for reionisation physics and cosmological tensions. The ELiCA likelihood is publicly available.