Event

Cosmic microwave background (CMB) photons are partially polarized, both by free electrons around recombination, and by free electrons from reionization. The free electron fraction as a function of redshift directly affects the observed large-scale polarized CMB, by changing both the amplitude and the shape of the E-mode power spectrum. Planck and WMAP have constrained the reionization optical depth, but this integrated quantity can only constrain the redshift of reionization assuming the intergalactic medium (IGM) instantaneously transitions from neutral to ionized. In this talk, I will demonstrate that future CMB measurements with a near cosmic variance-limited E-mode measurement will be able to distinguish between different models of reionization that have not yet been ruled out by direct measurements of the ionization state of the IGM. Specifically, I will consider a simple model with a period of an early ionization fraction of 5% at z~28 in addition to the standard reionization scenario. I will also demonstrate that a cosmic variance measurement requires a moderate sensitivity of 10 μK arcmin, and will be achieved by the CLASS experiment.