2025ApJ...987L..29C

Recently, it was pointed out that invoking a large value of the cosmic microwave background (CMB) optical depth, τ_CMB = 0.09, could help resolve tensions between Dark Energy Survey Instrument DR2 baryon acoustic oscillation data and the CMB. This is larger than the value of τ_CMB = 0.058 measured from the Planck low-ℓ polarization data. Traditionally, τ_CMB is thought of as a constraint on reionization's midpoint. However, recent observations and modeling of the Lyα 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 Lyα forest constraints, renders τ_CMB a sensitive probe of the duration of reionization, as well as its midpoint. We compare low and high values of τ_CMB to upper limits on the patchy kinematic Sunyaev-Zel'dovich (pkSZ) effect, another CMB observable that constrains reionization's duration, and find that a value of τ_CMB = 0.09 is in ≈2σ tension with existing limits on the pkSZ from the South Pole Telescope (SPT). The strength of this tension is sensitive to the choices involved in modeling the other CMB foregrounds in the SPT measurement, and in the modeling of the pkSZ signal itself.