The Extragalactic Background Light and the Gamma-ray Opacity of the Universe

TL;DR

This paper reviews the significance of the extragalactic background light (EBL) in cosmology, its impact on gamma-ray propagation, and recent advances in observational techniques to better understand the universe's energy releases and gamma-ray sources.

Contribution

It provides a comprehensive review of recent developments in measuring the EBL and its effects on gamma-ray astronomy, highlighting future observational prospects.

Findings

EBL influences gamma-ray attenuation in the 10 GeV to 10 TeV range.

Recent observational techniques have improved constraints on EBL intensity and spectrum.

Future telescopes will enhance understanding of gamma-ray sources and cosmic energy releases.

Abstract

The extragalactic background light (EBL) is one of the fundamental observational quantities in cosmology. All energy releases from resolved and unresolved extragalactic sources, and the light from any truly diffuse background, excluding the cosmic microwave background (CMB), contribute to its intensity and spectral energy distribution. It therefore plays a crucial role in cosmological tests for the formation and evolution of stellar objects and galaxies, and for setting limits on exotic energy releases in the universe. The EBL also plays an important role in the propagation of very high energy gamma-rays which are attenuated en route to Earth by pair producing gamma-gamma interactions with the EBL and CMB. The EBL affects the spectrum of the sources, predominantly blazars, in the ~10 GeV to 10 TeV energy regime. Knowledge of the EBL intensity and spectrum will allow the determination of the intrinsic blazar spectrum in a crucial energy regime that can be used to test particle acceleration mechanisms and VHE gamma-ray production models. Conversely, knowledge of the intrinsic gamma-ray spectrum and the detection of blazars at increasingly higher redshifts will set strong limits on the EBL and its evolution. This paper reviews the latest developments in the determination of the EBL and its impact on the current understanding of the origin and production mechanisms of gamma-rays in blazars, and on energy releases in the universe. The review concludes with a summary and future directions in Cherenkov Telescope Array techniques and in infrared ground-based and space observatories that will greatly improve our knowledge of the EBL and the origin and production of very high energy gamma-rays.