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, 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.
Highlights
The exact details of the epoch of reionization, during which the universe transitioned from a mostly neutral hydrogen gas into the highly ionized state we see today, are still of considerable uncertainty. This transition left several imprints on the cosmic microwave background (CMB) which can be used to constrain the physics of reionization, and serve as a “nuisance” uncertainty which must be marginalized over when considering constraints on other parameters
One of the most widely used methods for reconstructing or marginalizing over the reionization history from CMB data has been based on decomposing the history into a set of principal component amplitudes (PCAs), described initially by Hu & Holder (2003; hereafter HH03)
We focus on the sum of neutrino masses, Σmν, as it is the cosmological parameter expected to be most impacted by the details of reionization
Summary
The exact details of the epoch of reionization, during which the universe transitioned from a mostly neutral hydrogen gas into the highly ionized state we see today, are still of considerable uncertainty. The FlexKnot model follows almost exactly the same procedure used previously to generically reconstruct the primordial power spectrum from Planck data (Vázquez et al 2012; Planck Collaboration XX 2016), and is well suited to the problem here as well Using these reionization models, we calculate new constraints on the history of reionization coming from Planck intermediate data, in particular using the simlow likelihood (Planck Collaboration Int. XLVI 2016). We comment on the ability of combining with other probes, such as DESI-BAO, or a cosmic-variancelimited CMB large-scale polarization measurement, to reduce this uncertainty Given these biases, we conclude that generic reconstructions of the reionization history are likely not enough to achieve the most accurate bounds on Σmν. These sections are a fairly technical description of the methodology used, and those wishing to see the results should skip to Sects. 4–6 where we discuss constraints from Planck 2015, intermediate, and future data, respectively
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