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

• Published in: Physical review. D 2021-03, Vol.103 (6), Article 063518
• Main Authors: Alvarez, Marcelo A., Ferraro, Simone, Hill, J. Colin, Hložek, Renée, Ikape, Margaret
• Format: Article
• Language: English
• Subjects: ASTRONOMY AND ASTROPHYSICS; Big Bang theory; Contaminants; Cosmic microwave background; Cosmology; Ionization; Kinematics; Large scale structure of the universe; Neutrinos; Optical thickness; Parameters; Perturbation; Phase transitions; Sensitivity
• ISSN: 2470-0010; 2470-0029
• DOI: 10.1103/PhysRevD.103.063518

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, τ, and the amplitude of scalar perturbations, As. 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 τ and the duration of reionization, Δz. We show that by combining the kSZ two-point function and the reconstructed kSZ four-point function, degeneracies between τ and Δz can be strongly broken, yielding tight constraints on both parameters. We forecast σ(τ)=0.003 and σ(Δz)=0.25 for a combination of CMB-S4 and Planck data, including detailed treatment of foregrounds and atmospheric noise. The constraint on τ 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.