Mitigating the optical depth degeneracy using the kinematic Sunyaev-Zel'dovich effect with CMB-S4 data

The epoch of reionization is one of the major phase transitions in the history of the universe, and is a focus of ongoing and upcoming cosmic microwave background (CMB) experiments with improved sensitivity to small-scale fluctuations. Reionization also represents a significant contaminant to CMB-derived cosmological parameter constraints, due to the degeneracy between the Thomson-scattering optical depth, tau, and the amplitude of scalar perturbations, A(s). This degeneracy subsequently hinders the ability of large-scale structure data to constrain the sum of the neutrino masses, a major target for cosmology in the 2020s. In this work, we explore the kinematic Sunyaev-Zel'dovich (kSZ) effect as a probe of reionization, and show that it can be used to mitigate the optical depth degeneracy with high-sensitivity, high-resolution data from the upcoming CMB-S4 experiment. We discuss the dependence of the kSZ power spectrum on physical reionization model parameters, as well as on empirical reionization parameters, namely tau and the duration of reionization, Delta z. We show that by combining the kSZ two-point function and the reconstructed kSZ four-point function, degeneracies between tau and Delta z can be strongly broken, yielding tight constraints on both parameters. We forecast sigma(tau) = 0.003 and sigma(Delta z) = 0.25 for a combination of CMB-S4 and Planck data, including detailed treatment of foregrounds and atmospheric noise. The constraint on tau is nearly identical to the cosmic-variance limit that can be achieved from large-angle CMB polarization data. The kSZ effect thus promises to yield not only detailed information about the reionization epoch, but also to enable high-precision cosmological constraints on the neutrino mass.

Automated summary

This study investigates the kinematic Sunyaev-Zel'dovich (kSZ) effect as a probe of reionization, demonstrating its potential to provide detailed information about the reionization epoch and enable high-precision cosmological constraints on the neutrino mass using high-sensitivity, high-resolution data from the upcoming CMB-S4 experiment. By combining the kSZ two-point function and the reconstructed kSZ four-point function, degeneracies between key parameters can be strongly broken, resulting in tight constraints on both parameters. The forecasted constraints on tau and Delta z show promising results for future cosmological studies.