Distinguishing Primordial Magnetic Fields from Inflationary Tensor Perturbations in the Cosmic Microwave Background

TL;DR

This paper proposes a method to differentiate primordial magnetic fields from inflationary tensor perturbations in the CMB by detecting small-scale Faraday rotation, enabling clearer identification of the B-mode polarization sources.

Contribution

It introduces a novel approach using four-point statistics to identify magnetic field-induced B-modes, improving the accuracy of cosmological tensor perturbation detection.

Findings

Faraday rotation can distinguish magnetic fields from tensor perturbations

Detectable in upcoming polarization experiments for tensor-scalar ratios > 0.001

Magnetic fields induce small-scale signals not mimicked by tensor perturbations

Abstract

A claimed detection of cosmological tensor perturbations from inflation via B-mode polarization of the cosmic microwave background requires distinguishing other possible B-mode sources. One such potential source of confusion is primordial magnetic fields. For sufficiently low-amplitude B-mode signals, the microwave background temperature and polarization power spectra from power-law tensor perturbations and from a power-law primordial magnetic field are indistinguishable. However, we show that such a magnetic field will induce a small-scale Faraday rotation which is detectable using four-point statistics analogous to gravitational lensing of the microwave background. The Faraday rotation signal will distinguish a magnetic-field induced B-mode polarization signal from tensor perturbations for effective tensor-scalar ratios larger than 0.001, detectable in upcoming polarization experiments.