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 the formation of emitting astrophysical objects and the Universe’s expansion. It makes EBL one of the fundamental quantities in cosmology. The 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. In this work, we investigate the high-redshift active galaxy 4C +55.17 (z = 0.902), whose unusually hard and stable high-energy spectrum makes it a valuable probe of EBL-induced absorption effects. Using observations extending from GeV to TeV energies, we reconstruct the optical depth associated with gamma-ray propagation and compare the inferred attenuation with predictions from existing EBL models. The results favor relatively low EBL intensities in the optical and infrared bands, consistent with low-level EBL models and suggesting reduced star formation activity and dust contributions over cosmic evolution. We further explore the cosmological implications of the reconstructed optical depth and derive constraints on the Hubble constant in the range H0≈ 64–74 km s−1 Mpc−1, with an average value of H0=69±4 km s−1 Mpc−1. These findings demonstrate the potential of hard-spectrum, high-redshift gamma-ray sources such as 4C +55.17 as cosmological probes for studying EBL evolution and addressing current tensions in cosmological parameter measurements.
Sinitsyna et al. (Sat,) studied this question.