Extragalactic Background Light: Inventory of light throughout the cosmic history

TL;DR

This paper reviews the measurement of the Extragalactic Background Light, highlighting recent progress and discussing unresolved components beyond known galaxies and stars, which may include unknown sources.

Contribution

It provides an updated overview of EBL measurements and emphasizes the existence of an unexplained component beyond resolved galaxies.

Findings

Resolved galaxies contribute significantly to EBL

Current measurements suggest an additional unexplained EBL component

Unresolved sources may include faint galaxies, diffuse gas, or unknown phenomena

Abstract

The Extragalactic Background Light (EBL) stands for the mean surface brightness of the sky as we would see it from a representative vantage point in the intergalactic space outside of our Milky Way Galaxy. Averaged over the whole 4 pi solid angle it represents the collective light from all luminous matter radiated throughout the cosmic history. Part of the EBL is resolved into galaxies that, with the increasing detecting power of giant telescopes and sensitive detectors, are seen to deeper and deeper limiting magnitudes. This resolved part is now known to contribute a substantial or even the major part of the EBL. There still remains, however, the challenge of finding out to what extent galaxies too faint or too diffuse to be discerned individually, individual stars or emission by gas outside the galaxies, or more speculatively, some hitherto unknown light sources such as decaying elementary particles are accounting for the remaining EBL. We review the recent progress that has been made in the measurement of EBL. The current photometric results suggest that there is, beyond the resolved galaxies, an EBL component that cannot be explained by diffuse galaxy halos or intergalactic stars.