Cosmic microwave background constraints in light of priors over reionization histories
Institut Lagrange de Paris (ILP), Sorbonne Universités, 98bis boulevard Arago, 75014
2 Institut d’Astrophysique de Paris (IAP), UMR7095, CNRS & Sorbonne Universités, 98bis boulevard Arago, 75014 Paris, France
Accepted: 24 May 2018
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, τ, or correlated parameters such as the neutrino mass sum, Σmν, at the level of 0.3–0.4 σ, meaning 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.
Key words: cosmic background radiation / dark ages / reionization / first stars / cosmological parameters
© ESO 2018
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.