A circular polarimeter for the Cosmic Microwave Background

TL;DR

This paper investigates the potential for primordial circular polarization in the Cosmic Microwave Background (CMB) to reveal early magnetic fields, using observational data to constrain their properties and effects.

Contribution

It introduces a method to use CMB polarization data to set limits on primordial magnetic fields and circular polarization, providing new constraints on early universe magnetism.

Findings

Constraints on the amplitude and spectral slope of primordial V-mode polarization.

Limits on magnetic field strength prior to photon decoupling.

Potential to rule out certain magnetic field ranges with future measurements.

Abstract

A primordial degree of circular polarization of the Cosmic Microwave Background is not observationally excluded. The hypothesis of primordial dichroism can be quantitatively falsified if the plasma is magnetized prior to photon decoupling since the initial V-mode polarization affects the evolution of the temperature fluctuations as well as the equations for the linear polarization. The observed values of the temperature and polarization angular power spectra are used to infer constraints on the amplitude and on the spectral slope of the primordial V-mode. Prior to photon decoupling magnetic fields play the role of polarimeters insofar as they unveil the circular dichroism by coupling the V-mode power spectrum to the remaining brightness perturbations. Conversely, for angular scales ranging between 4 deg and 10 deg the joined bounds on the magnitude of circular polarization and on the magnetic field intensity suggest that direct limits on the V-mode power spectrum in the range of 0.01 mK could directly rule out pre-decoupling magnetic fields in the range of 10-100 nG. The frequency dependence of the signal is located, for the present purposes, in the GHz range.