  We study how uncertainty in the reionization history of the universe affects
estimates of other cosmological parameters from the Cosmic Microwave
Background. We analyze WMAP7 data and synthetic Planck-quality data generated
using a realistic scenario for the reionization history of the universe
obtained from high-resolution numerical simulation. We perform parameter
estimation using a simple sudden reionization approximation, and using the
Principal Component Analysis (PCA) technique proposed by Mortonson and Hu. We
reach two main conclusions: (1) Adopting a simple sudden reionization model
does not introduce measurable bias into values for other parameters, indicating
that detailed modeling of reionization is not necessary for the purpose of
parameter estimation from future CMB data sets such as Planck. (2) PCA analysis
does not allow accurate reconstruction of the actual reionization history of
the universe in a realistic case.
