Photon propagator for axion electrodynamics

Understand

The axion modified electrodynamics is usually used as a model for description of possible violation of Lorentz invariance in field theory.

• The low-energy manifestation of Lorentz violation can hopefully be observed in experiments with the electromagnetic waves.
• It justifies the importance of study how a small axion addition can modify the wave propagation.
• Although a constant axion does not contribute to the dispersion relation at all, even a slowly varying axion field destroys the light cone structure.