Reionization and the Hubble constant: correlations in the Cosmic Microwave Background

Recently, the James Webb Space Telescope (JWST) has found early galaxies producing photons from more efficient ionization than previously assumed. This may suggest a reionization process with a larger reionization optical depth, τ reio , in some mild disagreement with that inferred from measurements of cosmic microwave background (CMB). Intriguingly, the CMB would prefer larger values of τ reio , more consistent with the recent JWST hint, if the large-scale measurements (i.e. ℓ < 30) of E-mode polarization are removed. In addition, τ reio has an indirect correlation with today's Hubble constant H 0 in ΛCDM. Motivated by these interesting observations, we investigate and reveal the underlying mechanism for this correlation, using the CMB dataset without the low-ℓ polarization data as a proxy for a potential cosmology with a larger τ reio . We further explore how this correlation may impact the Hubble tension between early and late universe measurements of H 0 , in ΛCDM as well as two proposals to alleviate the Hubble tension: the dark radiation (DR) and early dark energy (EDE) models. We find that the Hubble tension gets further reduced mildly for almost all cases due to the larger τ reio and its positive correlation with H 0 , with either the Baryon Acoustic Oscillations (BAO) data before those from the Dark Energy Spectroscopic Instrument (DESI) or the DESI data.