Axion-like particles and the propagation of gamma rays over astronomical distances

Understand

In this mini-review, possible manifestations of mixing between axion-like particles (ALPs) and energetic photons propagating over astronomical distances are considered.

• We discuss the evidence for the anomalous transparency of the Universe from observations of ensembles of distant gamma-ray sources, present the general formalism for the ALP-photon mixing and explain how this mechanism may remove the anomaly.
• We present relevant values of ALP parameters and discuss future ways to verify the scenario and to discover the particle in question.