  Radiation propagating over cosmological distances can probe light weakly
interacting pseudoscalar (or scalar) particles. The existence of a spin-0 field
changes the dynamical symmetries of electrodynamics. It predicts spontaneous
generation of polarization of electromagnetic waves due to mode mixing in the
presence of background magnetic field.We illustrate this by calculations of
propagation in a uniform medium, as well as in a slowly varying background
medium, and finally with resonant mixing. Highly complicated correlations
between different
  Stokes parameters are predicted depending on the parameter regimes.
  The polarization of propagating waves shows interesting and complex
dependence on frequency, the distance of propagation, coupling constants, and
parameters of the background medium such as the plasma density and the magnetic
field strength. For the first time we study the resonant mixing of
electromagnetic waves with the scalar field, which occurs when the background
plasma frequency becomes equal to the mass of the scalar field at some point
along the path. Dynamical effects are found to be considerably enhanced in this
case. We also formulate the condition under which the adiabatic approximation
can be used consistently, and find caveats about comparing different frequency
regimes.
