Nonlinear electrodynamics and CMB polarization
  Recently WMAP and BOOMERanG experiments have set stringent constraints on the
polarization angle of photons propagating in an expanding universe: $\Delta
\alpha = (-2.4 \pm 1.9)^\circ$. The polarization of the Cosmic Microwave
Background radiation (CMB) is reviewed in the context of nonlinear
electrodynamics (NLED). We compute the polarization angle of photons
propagating in a cosmological background with planar symmetry. For this
purpose, we use the Pagels-Tomboulis (PT) Lagrangian density describing NLED,
which has the form $L\sim (X/\Lambda^4)^{\delta - 1}\; X $, where $X=1/4
F_{\alpha\beta} F^{\alpha \beta}$, and $\delta$ the parameter featuring the
non-Maxwellian character of the PT nonlinear description of the electromagnetic
interaction. After looking at the polarization components in the plane
orthogonal to the ($x$)-direction of propagation of the CMB photons, the
polarization angle is defined in terms of the eccentricity of the universe, a
geometrical property whose evolution on cosmic time (from the last scattering
surface to the present) is constrained by the strength of magnetic fields over
extragalactic distances.
