Discriminating Planck Reionisation Histories with the kSZ Effect

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 $\mathit{\text{Planck}}$ data, we compute the corresponding kSZ angular power spectra within an analytical framework. The allowed histories fall into two broad classes, ‘late’ and ‘early’ start, yielding distinct kSZ signatures, which remain clearly separable even when accounting for modelling uncertainties in both the reionisation scenario, $x_e(z)$, and the properties of early galaxies. Current kSZ measurements ($\sim$0-3 $\mu$K${}^2$) tend to favour `late’ reionisation models but are not yet sensitive enough to definitely distinguish between the scenarios - a measurement of the kSZ power spectrum at $\mathrm{\ell }\sim 2000$ with $\sim$0.4 $\mu$K${}^2$ sensitivity, achievable in the coming years, will be sufficient to do so. This work demonstrates that CMB data alone can constrain the reionisation midpoint $z_{\mathrm{r}\mathrm{e}}$ with extremely narrow error bars ($7.94<z_{\mathrm{r}\mathrm{e}}<8.17$), even when effectively marginalising over modelling uncertainties.