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The CMB optical depth constrains the duration of reionization
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abstract
Recently, it was pointed out that invoking a large value of the CMB optical depth, $\tau_{\rm CMB} = 0.09$, could help resolve tensions between DESI DR2 BAO data and the CMB. This is larger than the value of $\tau_{\rm CMB} = 0.058$ measured from the Planck low-$\ell$ polarization data. Traditionally, $\tau_{\rm CMB}$ is thought of as a constraint on reionization's midpoint. However, recent observations and modeling of the Ly$\alpha$ forest of high-$z$ quasars at $5 < z < 6$ have tightly constrained the timing of the last $10-20\%$ of reionization, adding nuance to this interpretation. Here, we point out that fixing reionization's endpoint, in accordance with the latest Ly$\alpha$ forest constraints, renders $\tau_{\rm CMB}$ a sensitive probe of the duration of reionization, as well as its midpoint. We compare low and high values of $\tau_{\rm CMB}$ to upper limits on the patchy kinematic Sunyaev-Zeldovich (pkSZ) effect, another CMB observable that constrains reionization's duration, and find that a value of $\tau_{\rm CMB} = 0.09$ is in $\approx 2\sigma$ tension with existing limits on the pkSZ from the South Pole Telescope.
Forward citations
Cited by 7 Pith papers
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Characterising the epoch of reionisation using the cross-correlation of the kSZ effect and CMB lensing
The kSZ-squared field cross-correlated with CMB lensing is a new observable that, in simulations, is sensitive to the duration and midpoint of reionisation and could be detected by a CMB-HD-like experiment.
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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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Rapid late-time reionization: constraints and cosmological implications
Reionization is inferred to be rapid and late (midpoint z≈7, duration Δz50≈1.1), yielding an optical depth τ=0.0492 from Lyman-alpha + BAO + BBN, independent of CMB data.
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Probing the ionizing photon output of galaxies near cosmic dawn with the patchy kSZ effect
In calibrated reionization simulations, the patchy kSZ power at ell~3000 correlates strongly with the start of reionization once the end is fixed by Lyman-alpha forest data, yielding model-dependent limits on early io...
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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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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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