CMB Faraday rotation as seen through the Milky Way

TL;DR

This paper investigates the impact of the Milky Way's magnetic field on CMB Faraday Rotation measurements and assesses the potential to detect primordial magnetic fields with future experiments, accounting for galactic contributions.

Contribution

It provides forecasts for detecting galactic and primordial magnetic fields via CMB Faraday Rotation, considering realistic experimental and galactic subtraction scenarios.

Findings

Galactic RM will not be detected by Planck but affects future measurements.

Future sub-orbital experiments can detect primordial fields of 0.1 nG.

Space-based experiments can detect even weaker primordial magnetic fields.

Abstract

Faraday Rotation (FR) of CMB polarization, as measured through mode-coupling correlations of E and B modes, can be a promising probe of a stochastic primordial magnetic field (PMF). While the existence of a PMF is still hypothetical, there will certainly be a contribution to CMB FR from the magnetic field of the Milky Way. We use existing estimates of the Milky Way rotation measure (RM) to forecast its detectability with upcoming and future CMB experiments. We find that the galactic RM will not be seen in polarization measurements by Planck, but that it will need to be accounted for by CMB experiments capable of detecting the weak lensing contribution to the B-mode. We then discuss prospects for constraining the PMF in the presence of FR due to the galaxy under various assumptions that include partial de-lensing and partial subtraction of the galactic FR. We find that a realistic future sub-orbital experiment, covering a patch of the sky near the galactic poles, can detect a scale-invariant PMF of 0.1 nano-Gauss at better than 95% confidence level, while a dedicated space-based experiment can detect even smaller fields.