Primordial Magnetic Field Limits from CMB Trispectrum - Scalar Modes and Planck Constraints

Understand

Cosmic magnetic fields are observed to be coherent on large scales and could have a primordial origin.

• Non-Gaussian signals in the cosmic microwave background (CMB) are generated by primordial magnetic fields as the magnetic stresses and temperature anisotropy they induce depend quadratically on the magnetic field.
• We compute the CMB scalar trispectrum on large angular scales, for nearly scale-invariant magnetic fields, sourced via the Sachs-Wolfe effect.
• The trispectra induced by magnetic energy density and by magnetic scalar anisotropic stress are found to have typical magnitudes of approximately $10^{-29}$ and $10^{-19}$, respectively.