On the Influence of Resonant Scattering on Cosmic Microwave Background Polarisation Anisotropies

Abstract

We implement the theory of resonant scattering in the context of Cosmic Microwave Background (CMB) polarisation anisotropies. We compute the changes in the E-mode polarisation (EE) and Temperature E-mode (TE) CMB power spectra introduced by the scattering on a resonant transition with a given optical depth $τ_X$ and polarisation coefficient $E_1$. The latter parameter, accounting for how anisotropic the scattering is, depends on the exchange of angular momentum in the transition and enables, {\it a priori}, to distinguish among different possible resonances. We use this formalism in two different scenarios: cosmological recombination and cosmological reionisation. In the context of cosmological recombination, we compute predictions in frequency and multipole space for the change in the TE and EE power spectra introduced by scattering on the $H_α$ and $P_α$ lines of Hydrogen. This constitutes a fundamental test for the standard model of recombination, and the sensitivity it requires is comparable to that needed in measuring the primordial CMB B-mode polarisation component. In the context of reionisation, we study the scattering off metals and ions produced by the first stars, and find that polarisation anisotropies, apart from providing a consistency test for intensity measurements, give some insight on how reionisation evolved: since they keep memory of how anisotropic the line scattering is, they should be able to discern the OI 63.2$μ$m transition from other possible transitions associated to OIII, NII, NIII, etc. The amplitude of these signals are, however, between 10 to 100 times below the (already challenging) level of CMB B-mode polarisation anisotropies.