The cosmic microwave background and the ionization history of the Universe

Details of how the primordial plasma recombined and how the Universe later reionized are currently somewhat uncertain. This uncertainty can restrict the accuracy of cosmological parameter measurements from the cosmic microwave background (CMB). More positively, future CMB data can be used to constrain the ionization history, using observations. We first discuss how current uncertainties in the recombination history impact parameter constraints, and show how suitable parametrizations can be used to obtain unbiased parameter estimates from future data. Some parameters can be constrained robustly; however, there is clear motivation to model recombination more accurately with quantified errors. We then discuss constraints on the ionization fraction binned in redshift during reionization. Perfect CMB polarization data could, in principle, distinguish different histories that have the same optical depth. We discuss how well the Planck satellite may be able to constrain the ionization history, and show the currently very weak constraints from Wilkinson Microwave Anisotropy Probe three-year data.