Primordial Magnetic Field Limits from Cosmic Microwave Background Bispectrum of Magnetic Passive Scalar Modes

Understand

Primordial magnetic fields lead to non-Gaussian signals in the cosmic microwave background (CMB) even at the lowest order, as magnetic stresses and the temperature anisotropy they induce depend quadratically on the magnetic field.

• In contrast, CMB non-Gaussianity due to inflationary scalar perturbations arises only as a higher order effect.
• Apart from a compensated scalar mode, stochastic primordial magnetic fields also produce scalar anisotropic stress that remains uncompensated till neutrino decoupling.
• This gives rise to an adiabatic-like scalar perturbation mode that evolves passively thereafter (called the passive mode).