Hassan Abdalla Hassan Hamad


The Extragalactic Background Light and gamma-gamma absorption of Very-High-Energy gamma-rays from cosmological gamma-rays sources(pdf)

Very High Energy Gamma Rays from blazars can be absorbed by the Extragalactic Background light, which leads to a high-energy cut-off at the very high energy end of Blazar spectra.The universe should be opaque to VHE gamma rays from sources at high redshift. Recent observations of distant gamma ray sources indicate that the universe is more transparent to VHE-gam.rays than was expected. Possible solutions include the hypothesis that the EBL density is lower overall and/or a reduced EBL opacity results from the expected inhomogeneity of the EBL, in particular if the line of sight to a blazar is passing through large voids in intergalactic space; the existence of exotic Axion Like Particles which very high energy gamma rays can oscillate into in the presence of a magnetic field; and the interactions of extragalactic Ultrahigh Energy Cosmic Rays along the line of sight. In this talk we will review these options, with particular emphasis on our studies of the effect of EBL inhomogeneity