The Extragalactic Background Light: The Emission from All Stars and Black Holes

Abstract

Intergalactic space is filled with the light produced by all the stars and accreting compact objects that populated the observable Universe throughout the whole cosmic history. This relic cosmic background from IR to UV is called the diffuse extragalactic background light (EBL). Knowledge of the EBL and its cosmic evolution represents a key issue in astrophysics since it helps unveiling the nature of star formation and galaxy evolution. Direct measurements of the EBL are difficult due to bright local foregrounds, in particular zodiacal light. As a consequence indirect measurements constitute a mandatory complement to them. A powerful approach for probing these diffuse radiation fields in the UV to far-IR bands is through gamma‐gamma absorption of high-energy photons. Pair production (e + e ) from high-energy photons interacting with ultraviolet to infrared EBL photons is effective at attenuating gamma rays with energy above 10 GeV. This process introduces an attenuation in the spectra of gamma-ray sources above a critical energy. The major effect is a redshift- and energy-dependent dimming of the gamma-ray flux of extragalactic sources such as blazars and gamma-ray bursts (GRBs), which can be used to infer limits on the EBL intensity. All recent measurements favor models with a minimal level of EBL, consistent with the estimated contribution from resolved galaxies.