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

TL;DR

This paper reviews how axion-like particles could explain the universe's unexpected transparency to gamma rays from distant sources, discussing the formalism, evidence, and future verification methods.

Contribution

It provides a comprehensive overview of ALP-photon mixing as a solution to gamma-ray transparency anomalies and discusses potential experimental tests.

Findings

ALP-photon mixing can explain gamma-ray transparency anomalies

Relevant ALP parameter ranges are identified

Future experiments could verify the ALP scenario

Abstract

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.