Polarization bispectrum for measuring primordial magnetic fields

TL;DR

This paper explores how polarization bispectra of the CMB can improve constraints on primordial magnetic fields, especially through B-mode polarization, potentially detecting fields as weak as 1 nG.

Contribution

It introduces a comprehensive analysis of polarization bispectra, including B-modes, for constraining primordial magnetic fields with improved sensitivity.

Findings

E-mode bispectra improve measurement precision by 80%.

PMFs of 2.2-2.6 nG detectable with Planck or PRISM.

B-mode bispectra can detect PMFs below 1 nG.

Abstract

We examine the potential of polarization bispectra of the cosmic microwave background (CMB) to constrain primordial magnetic fields (PMFs). We compute all possible bispectra between temperature and polarization anisotropies sourced by PMFs and show that they are weakly correlated with well-known local-type and secondary ISW-lensing bispectra. From a Fisher analysis it is found that, owing to E-mode bispectra, in a cosmic-variance-limited experiment the expected uncertainty in the amplitude of magnetized bispectra is 80% improved in comparison with an analysis in terms of temperature auto-bispectrum alone. In the $\mathit{Planck}$ or the proposed PRISM experiment cases, we will be able to measure PMFs with strength 2.6 or 2.2 nG. PMFs also generate bispectra involving B-mode polarization, due to tensor-mode dependence. We also find that the B-mode bispectrum can reduce the uncertainty more drastically and hence PMFs comparable to or less than 1 nG may be measured in a PRISM-like experiment.