Acoustic signatures in the Cosmic Microwave Background bispectrum from primordial magnetic fields

TL;DR

This paper numerically calculates the CMB bispectrum influenced by primordial magnetic fields, revealing two dominant magnetic shapes with unique oscillatory and divergence features, advancing understanding of magnetic effects on early universe signals.

Contribution

It introduces a detailed numerical analysis of the CMB bispectrum from primordial magnetic fields, identifying two distinct magnetic shapes and their unique signatures.

Findings

The bispectrum is dominated by two magnetic shapes.

The first shape resembles local-type primordial curvature perturbations.

The second shape's amplitude diverges at low multipoles.

Abstract

Using the full radiation transfer function, we numerically calculate the CMB angular bispectrum seeded by the compensated magnetic scalar density mode. We find that, for the string inspired primordial magnetic fields characterized by index $n_B=-2.9$ and mean-field amplitude $B_{\lam}=9{\rm nG}$, the angular bispectrum is dominated by two primordial magnetic shapes. The first magnetic shape looks similar to the one from local-type primordial curvature perturbations, so both the amplitude and profile of the Komatsu-Spergel estimator (reduced bispectrum) seeded by this shape are almost the same as those of the primary CMB anisotropies. However, for different parameter sets ($l_1,l_2$), this "local-type" reduced bispectrum oscillates around different asymptotic values in the high-$l_3$ regime because of the effect of the Lorentz force, which is exerted by the primordial magnetic fields on the charged baryons. This feature is different from the standard case where all modes approach to zero asymptotically in the high-$l$ limit. On the other hand, the second magnetic shape appears only in the primordial magnetic field model. The amplitude of the Komatsu-Spergel estimator sourced by the second shape diverges in the low-$l$ regime because of the negative slope of shape. In the high-$l$ regime, this amplitude is approximately equal to that of the first estimator, but with a reversal phase.