Axion-like particles and circular polarisation of active galactic nuclei

Understand

The measurements of the linear polarisation of visible light from quasars give strong evidence for large-scale coherent orientations of their polarisation vectors in some regions of the sky.

• We show that these observations can be explained by the mixing of the photons with very light pseudoscalar (axion-like) particles in extragalactic magnetic fields during their propagation.
• We present a new treatment in terms of wave packets and discuss the circular polarisation.