Understand
A primordial magnetic field in the early universe will cause Faraday rotation of the linear polarization of the cosmic microwave background generated via Compton scattering at the surface of last scattering.
- This rotation induces a non-zero parity-odd (B-mode) polarization component.
- The Wilkinson Microwave Anisotropy Probe (WMAP) 5-year data puts an upper limit on the magnitude of the B-polarization power spectrum; assuming that the B-polarization signal is totally due to the Faraday rotation effect, the upper limits on the comoving amplitude of a primordial stochastic magnetic field range from $6 \times 10^{-8}$ to $2 \times 10^{-6}$ G on a comoving length scale of 1 Mpc, depending on the power spectrum of the magnetic field.
Reading the bibliography…