Cosmic Microwave Background Constraints in Light of Priors Over Reionization Histories

TL;DR

This paper demonstrates that the constraints on reionization history derived from CMB data are sensitive to the choice of priors, affecting interpretations of early reionization signals and emphasizing the need for improved data or models.

Contribution

It highlights the prior dependence in non-parametric reionization reconstructions and introduces the first application of such methods to newer Planck data, showing the disappearance of early reionization hints.

Findings

Prior choices can shift constraints on reionization optical depth by 0.3-0.4σ.

Hints of early reionization in Planck 2015 data are overestimated due to prior dependence.

In newer Planck data, early reionization signals are not supported.

Abstract

Non-parametric reconstruction or marginalization over the history of reionization using cosmic microwave background data necessarily assumes a prior over possible histories. We show that different but reasonable choices of priors can shift current and future constraints on the reionization optical depth, $\tau$, or correlated parameters such as the neutrino mass sum, $\Sigma m_\nu$, at the level of 0.3-0.4$\sigma$, i.e., that this analysis is somewhat prior dependent. We point out some prior-related problems with the commonly used principal component reconstruction, concluding that the significance of some recent hints of early reionization in Planck 2015 data has been overestimated. We also present the first non-parametric reconstruction applied to newer Planck intermediate (2016) data and find that the hints of early reionization disappear entirely in this more precise dataset. These results limit possible explanations of the EDGES 21cm signal which would have also significantly reionized the universe at $z\,{>}\,15$. Our findings about the dependence on priors motivate the pursuit of improved data or searches for physical reionization models which can reduce the prior volume. The discussion here of priors is of general applicability to other non-parametric reconstructions, for example of the primordial power spectrum, of the recombination history, or of the expansion rate.