The Cosmic Microwave Background Optical Depth Constrains the Duration of Reionization

Abstract

Recently, it was pointed out that invoking a large value of the cosmic microwave background (CMB) optical depth, τCMB = 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 τCMB = 0.058 measured from the Planck low-ℓ polarization data. Traditionally, τCMB 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 τCMB a sensitive probe of the duration of reionization, as well as its midpoint. We compare low and high values of τCMB 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 τCMB = 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.