Was there an early reionization component in our universe?

TL;DR

Current Cosmic Microwave Background data do not necessitate an early reionization phase, favoring the simplest model with a single optical depth, though some marginal models suggest possible early contributions.

Contribution

This study assesses the necessity of an early reionization component using present CMB data and finds strong evidence supporting the simplest reionization scenario.

Findings

Data do not require early reionization.

A single optical depth model is strongly favored.

Some marginal models hint at early reionization possibilities.

Abstract

A deep understanding of the Epoch of Reionization is still missing in our knowledge of the universe. While future probes will allow us to test the precise evolution of the free electron fraction from redshifts between $z\simeq 6$ and $z\simeq 20$, at present one could ask what kind of reionization processes are allowed by present Cosmic Microwave Background temperature and polarization measurements. An early contribution to reionization could imply a departure from the standard picture where star formation determines the reionization onset. BBy considering a broad class of possible reionization parameterizations, we find that current data do not require an early reionization component in our universe and that only one marginal class of models, based on a particular realization of reionization, may point to that. In addition, the frequentist Akaike Information Criterion (AIC) provides strong evidence against alternative reionization histories, favoring the most simple reionization scenario, which describes reionization by means of only one (constant) reionization optical depth $\tau$.