The Search For The History Of The Universe's Light Emission

A cartoon of the effects of the diffuse extragalactic background light (EBL) on the gamma-ray emission from a distant quasar, before reaching the Earth. The gamma-rays are partly absorbed by colliding with the EBL photons produced by all the stars and galaxies in the Universe. If the density of EBL photons is high (upper graph), absorption is high and the highest energy gamma-rays are lost. So the distribution of measured energies (spectrum) is strongly changed. If instead the density is low (lower graph), absorption is less and the spectrum is not changed as much. (Credit: HESS Collaboration)