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 .