The CMB optical depth constrains the duration of reionization

TL;DR

This paper explores how the CMB optical depth, when combined with Ly$eta$ forest constraints, can effectively measure the duration of reionization, revealing tension with kSZ effect limits at higher $ au_{ m CMB}$ values.

Contribution

It demonstrates that fixing the reionization endpoint allows $ au_{ m CMB}$ to serve as a probe of reionization duration, highlighting tension with pkSZ constraints at higher optical depths.

Findings

$ au_{ m CMB} = 0.09$ is in $ oughly 2σ$ tension with pkSZ limits.

Ly$eta$ forest data constrains reionization's endpoint, refining $ au_{ m CMB}$ interpretation.

Higher $ au_{ m CMB}$ values conflict with existing CMB-based reionization duration limits.

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.