Gamma-ray Cosmology and Tests of Fundamental Physics

TL;DR

Gamma-ray observations over cosmological distances provide crucial insights into fundamental physics, cosmic history, and potential new physics beyond the Standard Model, through absorption features and secondary gamma-ray production.

Contribution

This paper reviews recent advances in gamma-ray cosmology, highlighting how gamma-ray propagation constrains cosmological parameters, baryonic matter, and tests of quantum gravity and string theory.

Findings

Gamma-ray absorption features inform on cosmic baryonic matter and cosmological parameters.

Secondary gamma-ray production constrains intergalactic magnetic fields.

Observations limit phenomena like axion-like particles and Lorentz invariance violation.

Abstract

The propagation of gamma-rays over cosmological distances is the subject of extensive theoretical and observational research at GeV and TeV energies. The mean free path of gamma-rays in the cosmic web is limited above 100 GeV due to the production of electrons and positrons on the cosmic optical and infrared backgrounds. Electrons and positrons cool in the intergalactic medium while gyrating in its magnetic fields, which could cause either its global heating or the production of lower-energy secondary gamma-rays. The energy distribution of gamma-rays surviving the cosmological journey carries observed absorption features that gauge the emissivity of baryonic matter over cosmic time, constrain the distance scale of $\Lambda$CDM cosmology, and limit the alterations of the interaction cross section. Competitive constraints are in particular placed on the cosmic star-formation history as well as on phenomena expected from quantum gravity and string theory, such as the coupling to hypothetical axion-like particles or the violation of Lorentz invariance. Recent theoretical and observational advances offer a glimpse of the multi-wavelength and multi-messenger path that the new generation of gamma-ray observatories is about to open.