CMB polarization can constrain cosmology better than CMB temperature

We demonstrate that for a cosmic variance limited experiment, cosmic microwave background (CMB) E polarization alone places stronger constraints on cosmological parameters than CMB temperature. For example, we show that C_^EE can constrain parameters better than C_^TT by up to a factor 2. 8 when a multipole range of =30--2500 is considered. We expose the physical effects at play behind this remarkable result and study how it depends on the multipole range included in the analysis. In most relevant cases, C_^TE or C_^EE surpass the C_^TT-based cosmological constraints. This result is important as the small-scale astrophysical foregrounds are expected to have a much reduced impact on polarization, thus opening the possibility of building cleaner and more stringent constraints of the model. This is relevant especially for proposed future CMB satellite missions, such as CORE or PRISM, that are designed to be cosmic variance limited in polarization till very large multipoles. We perform the same analysis for a Planck-like experiment, and conclude that even in this case C_^TE alone should determine the constraint on ₂h^2 better than C_^TT by 15%, while determining ₁h^2, nₒ and with comparable accuracy. Finally, we explore a few classical extensions of the model and show again that CMB polarization alone provides more stringent constraints than CMB temperature in the case of a cosmic variance limited experiment.