Reduced bispectrum seeded by helical primordial magnetic fields

TL;DR

This paper explores how helical primordial magnetic fields influence the cosmic microwave background's reduced bispectrum, revealing the significance of IR cut-off and helicity in constraining magnetic field properties.

Contribution

It derives the full three-point statistics of helical magnetic fields and computes their impact on the CMB bispectrum, highlighting the role of IR cut-off and helicity.

Findings

Negative bispectrum signal due to helical terms

Largest contribution from non-zero IR cut-off for causal fields

Passive modes enhance the bispectrum for negative spectral indices

Abstract

In this paper, we investigate the effects of helical primordial magnetic fields (PMFs) on the cosmic microwave background (CMB) reduced bispectrum. We derive the full three-point statistics of helical magnetic fields and numerically calculate the even contribution in the collinear configuration. We then numerically compute the CMB reduced bispectrum induced by passive and compensated PMF modes on large angular scales. There is a negative signal on the bispectrum due to the helical terms of the fields and we also observe that the biggest contribution to the bispectrum comes from the non-zero IR cut-off for causal fields, unlike the two-point correlation case. For negative spectral indices, the reduced bispectrum is enhanced by the passive modes. This gives a lower value of the upper limit for the mean amplitude of the magnetic field on a given characteristic scale. However, high values of IR cut-off in the bispectrum, and the helical terms of the magnetic field relaxes this bound. This demonstrates the importance of the IR cut-off and helicity in the study of the nature of PMFs from CMB observations.