Signatures of Pseudoscalar Photon Mixing in CMB Radiation

TL;DR

This paper models how photon and ultra-light pseudoscalar mixing affects CMB polarization, deriving analytical expressions and setting observational limits on the effect's magnitude.

Contribution

It provides an analytic framework for pseudoscalar-photon mixing effects on CMB polarization and constrains model parameters using observational data.

Findings

CMB circular polarization can reach up to 10^{-7} within current parameter limits.

Analytic expressions for Stokes parameters increase linearly with the number of magnetic domains.

Numerical simulations confirm the linear relationship in polarization effects.

Abstract

We model the effect of photon and ultra-light pseudoscalar mixing on the propagation of electromagnetic radiation through the extragalactic medium. The medium is modelled as a large number of magnetic domains, uncorrelated with one another. We obtain an analytic expression for the different Stokes parameters in the limit of small mixing angle. The different Stokes parameters are found to increase linearly with the number of domains. We also verify this result by direct numerical simulations. We use this formalism to estimate the effect of pseudoscalar-photon mixing on the Cosmic Microwave Background (CMB) polarization. We impose limits on the model parameters by the CMB observations. We find that the currently allowed parameter range admits a CMB circular polarization up to order $10^{-7}$.