Abstract

The inter-galactic medium at redshifts less than about 7 is highly ionized. But when was it ionized? What can we learn about the first sources of UV photons and the emergence of structure from the dark ages? Observations of the cosmic microwave background can offer clues, particularly from measurements of large-angle polarization anisotropies. In this talk, I will review the constraints on the optical depth to reionization, tau, from WMAP , the Planck LFI (low frequency instrument) and HFI (high frequency instrument). Each of these provides a near-independent measurement of tau. Control of systematic errors in these experiments is critical, but highly technical. I will therefore try to give a non-technical account suitable for non-specialists. By the end of the talk, I hope that people will have a feel for the issues involved and whether the recent Planck results indicating tau ~ 0.05 are reliable.