Discriminating Planck Reionisation Histories with the kSZ Effect

TL;DR

This paper explores how the kinetic Sunyaev--Zel'dovich effect can differentiate between various reionisation histories derived from Planck data, highlighting its potential to improve understanding of cosmic reionisation.

Contribution

It introduces an analytical framework to compute kSZ signatures for different reionisation histories, demonstrating their distinguishability despite uncertainties.

Findings

Current kSZ measurements are not yet precise enough to distinguish reionisation scenarios.

Two broad classes of reionisation histories ('short' and 'long') produce distinct kSZ signatures.

A future measurement with ~0.4 μK² sensitivity at ℓ ~2000 could discriminate between these scenarios.

Abstract

The epoch of reionisation is a key phase in cosmic history, but its detailed evolution remains poorly constrained by current cosmic microwave background (CMB) observations. We investigate whether the kinetic Sunyaev--Zel'dovich (kSZ) effect can discriminate among reionisation histories consistent with current large-scale CMB constraints. Using histories derived from Planck data, we compute the corresponding kSZ angular power spectra within an analytical framework, separating late-time and patchy contributions and accounting for uncertainties in both the ionisation history, $x_e(z)$, and astrophysical parameters constrained by the LORELI II simulations. The allowed histories fall into two broad classes, `short' and `long' duration reionisation, yielding distinct kSZ signatures. Uncertainties from $x_e(z)$ and astrophysical parameters produce comparable amounts of dispersion, yet the two classes remain clearly separable, with variations within each class at the $\sim$10\% level. Current kSZ measurements ($\sim$0--3 $\mu$K$^2$) are not yet precise enough to distinguish between these scenarios, although they tend to favor a `short' reionisation. The kSZ effect thus provides a promising probe of reionisation beyond optical depth constraints. In particular, a measurement of the kSZ power spectrum at $\ell \sim 2000$ with $\sim$0.4 $\mu$K$^2$ sensitivity would discriminate between `short' and `long' reionisation scenarios.