Circular polarization of the cosmic microwave background from vector and tensor perturbations

Circular polarization of the cosmic microwave background (CMB) can be induced by Faraday conversion of the primordial linearly polarized radiation as it propagates through a birefringent medium. Recent work has shown that the dominant source of birefringence from primordial density perturbations is the anisotropic background CMB. Here we extend prior work to allow for the additional birefringence that may arise from primordial vector and tensor perturbations. We derive the formulas for the power spectrum of the induced circular polarization and apply those to the standard cosmology. We find the root-variance of the induced circular polarization to be $\sqrt{<V^2>}\sim 3\times 10^{-14}$ for scalar perturbations and $\sqrt{<V^2>}\sim 7\times 10^{-18} (r/0.06)$ for tensor perturbations with a tensor-to-scalar ratio $r$.