A very extended reionization epoch ?
  The recent observations of cross temperature-polarization power spectra of the Cosmic Microwave Background (CMB) made by the WMAP satellite are in better agreement with a high value of the Thomson scattering optical depth \tau \approx 0.17. This value is close to \tau = 0.3, which is taken as the upper limit in the parameter extraction analysis made by the WMAP team. However, models with \tau \sim 0.3 provide a good fit to current CMB data and are not significantly excluded when combined with Large Scale Structure data. By making use of a self-consistent reionization model, we verify the astrophysical feasibility of models with \tau \sim 0.3. It turns out that current data on various observations related to the thermal and ionization history of the intergalactic medium are not able to rule out \tau \sim 0.3. The possibility of a very extended reionization epoch can significantly undermine the WMAP constraints on crucial cosmological parameters such as the Hubble constant, the spectral index of primordial fluctuations and the amplitude of dark matter clustering. 
A. Melchiorri, T. Roy Choudhury, P. Serra, A. Ferrara