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Reionization and the Hubble Constant: Correlations in the Cosmic Microwave Background
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abstract
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, $\tau_{\rm reio}$, in some mild disagreement with that inferred from measurements of cosmic microwave background (CMB). Intriguingly, the CMB would prefer larger values of $\tau_{\rm reio}$, more consistent with the recent JWST hint, if the large-scale measurements (i.e. $\ell <30$) of E-mode polarization are removed. In addition, $\tau_{\rm reio}$ has an indirect correlation with today's Hubble constant $H_0$ in $\Lambda$CDM. Motivated by these interesting observations, we investigate and reveal the underlying mechanism for this correlation, using the CMB dataset without the low-$\ell$ polarization data as a proxy for a potential cosmology with a larger $\tau_{\rm reio}$. We further explore how this correlation may impact the Hubble tension between early and late universe measurements of $H_0$, in $\Lambda$CDM 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 $\tau_{\rm reio}$ 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.
Forward citations
Cited by 7 Pith papers
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No way ou$\tau$: Epoch of Reionization Observations Do not Support Large Values of the Optical Depth to Reionization
Epoch-of-reionization hydrogen probes, combined with CMB and BAO data but no CMB polarization, yield tau_reio = 0.067 +/- 0.011 and leave the dynamical-dark-energy preference at ~2 sigma.
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The $H_0$ world cup. II. A comprehensive competition between proposed Hubble tension solutions
Against a common 2025-26 dataset (Planck PR4, ACT DR6, SPT-3G, DESI DR2, Pantheon+), early dark energy and early modified gravity models win the H0 competition (~3σ residual tension), while radiation and late-time sol...
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Cosmological Concordance in an Especially Opaque Universe: A Tentative Cosmological Detection of Physical Neutrino Mass in $\Lambda$CDM
Imposing a high prior on τ = 0.11 ± 0.006 produces a 2σ positive neutrino mass sum of 0.10 eV and restores concordance between CMB and DESI data inside ΛCDM.
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Cosmic $\tau$ensions Indirectly Correlate with Reionization Optical Depth
The correlations between tau_reio and the parameters behind cosmic tensions are not intrinsic, but arise indirectly through networks of other cosmological parameters.
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Flash Ionization of the Early Universe by Pop III.1 Supermassive Stars
Pop III.1 supermassive stars flash-ionize much of the early universe at z~20-30, adding tau~0.04 to the CMB optical depth and potentially easing Hubble tension and DESI anomalies.
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Impact of CMB low-$\ell$ EE polarization data on dark energy parameterizations
Removing Planck's low-ℓ EE polarization data shifts A_s and τ_reio upward and makes three dark-energy parameterizations look more quintessence-like, with model-selection evidence depending on the dataset and prior.
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The Impact of Population III.1 Flash Reionization for CMB Polarization and Thomson Scattering Optical Depth
An early 'Pop III.1 Flash' reionization phase shifts CMB polarization power from l<8 to l>8, allowing a higher optical depth τ≈0.08–0.09 while staying closer to Planck's low-l EE data than a standard tanh reionization model.
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