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arXiv · 2505.15899

The CMB optical depth constrains the duration of reionization

Christopher Cain·Alexander Van Engelen·Kevin S. Croker·Darby Kramer·Anson D'Aloisio·Garett Lopez
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Abstract

Recently, it was pointed out that invoking a large value of the CMB optical depth, $τ_{\rm CMB} = 0.09$, could help resolve tensions between DESI DR2 BAO data and the CMB. This is larger than the value of $τ_{\rm CMB} = 0.058$ measured from the Planck low-$\ell$ polarization data. Traditionally, $τ_{\rm CMB}$ is thought of as a constraint on reionization's midpoint. However, recent observations and modeling of the Ly$α$ 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$α$ forest constraints, renders $τ_{\rm CMB}$ a sensitive probe of the duration of reionization, as well as its midpoint. We compare low and high values of $τ_{\rm CMB}$ 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 $τ_{\rm CMB} = 0.09$ is in $\approx 2σ$ tension with existing limits on the pkSZ from the South Pole Telescope.

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Christopher Cain, Alexander Van Engelen, Kevin S. Croker, Darby Kramer, Anson D'Aloisio, Garett Lopez. 2025-05-21. The CMB optical depth constrains the duration of reionization. https://arxiv.org/abs/2505.15899

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