Standard Model Prediction for Cosmological 21cm Circular Polarization

TL;DR

This paper derives a new expression for the circular polarization of 21-cm radiation caused by hydrogen spin polarization interactions with the CMB, providing a way to probe early universe conditions.

Contribution

It reformulates the derivation of 21-cm circular polarization using Cartesian tensors and computes its angular power spectrum within the standard cosmological model.

Findings

Derived a Cartesian tensor-based expression for 21-cm circular polarization.

Calculated the angular power spectrum of Stokes-V parameter in ΛCDM.

Showed the dependence of polarization on redshift and frequency.

Abstract

Before cosmic reionization, hydrogen atoms acquire a spin polarization quadrupole through interaction with the anisotropic 21-cm radiation field. The interaction of this quadrupole with anisotropies in the cosmic microwave background (CMB) radiation field gives a net spin orientation to the hydrogen atoms. The 21-cm radiation emitted by these spin-oriented hydrogen atoms is circularly polarized. Here, we reformulate succinctly the derivation of the expression for this circular polarization in terms of Cartesian (rather than spherical) tensors. We then compute the angular power spectrum of the observed Stokes-$V$ parameter in the standard $\Lambda$CDM cosmological model and show how it depends on redshift, or equivalently, the observed frequency.