Primordial Magnetism in the CMB: Exact Treatment of Faraday Rotation and WMAP7 Bounds

Understand

Faraday rotation induced B-modes can provide a distinctive signature of primordial magnetic fields because of their characteristic frequency dependence and because they are only weakly damped on small scales, allowing them to dominate B-modes from other sources.

• By numerically solving the full CMB radiative transport equations, we study the B-mode power spectrum induced by stochastic magnetic fields that have significant power on scales smaller than the thickness of the last scattering surface.
• Constraints on the magnetic field energy density and inertial scale are derived from WMAP 7-year data, and are stronger than the big bang nucleosynthesis (BBN) bound for a range of parameters.
• Observations of the CMB polarization at smaller angular scales are crucial to provide tighter constraints or a detection.