Speaker
Description
Extragalactic background light (EBL), formed by the light radiated and re-radiated by stars, galaxies, and active galactic nuclei throughout the evolution of the Universe, brings the imprint of the history of the rate of formation of emitting astrophysical objects and the universe expansion. It makes EBL one of the fundamental quantities in cosmology. Optical depth for high-energy emission from the distant active galactic nuclei provides a constraint for the EBL density that is clear from the foreground galactic and other emissions, and, therefore, for the cosmological parameters. Implications of the unique hard high-energy spectrum active galaxy 4C +55.17 at high redshift for determining the EBL density, and also as an instrument for resolving the Hubble tension, are illustrated. Derived parameters may be applied to further models of the formation of stars and galaxies, and the expansion of the Universe.