Theory of Cosmic Microwave Background Polarization

Abstract

These lectures introduce some of the basic theory of cosmic microwave background (CMB) polarization with the primary aim of developing the theory of CMB polarization from inflationary gravitational waves, as well as some of the related theory of weak gravitational lensing (cosmic shear) of CMB polarization. We begin with production of polarization by Thomson scattering. We then discuss tensor-harmonic analysis (the ``grad-curl'' or ``E-B'' decomposition) on a flat and full sky in some detail. The Boltzmann/Einstein equations required to predict the CMB temperature/polarization pattern due to primordial gravitational waves are derived. We show that gravitational waves produce a curl component of the CMB polarization while density perturbations (at linear order) do not. We then show how cosmic shear induces a curl component from a curl-free surface of last scattering. We describe, though in less detail, how higher-order correlations can be used to subtract the cosmic-shear--induced curl. Several exercises are provided.