The CMB optical depth constrains the duration of reionization

Abstract

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

8 pages, 4 figures, to be submitted to ApJL. Comments welcome