Indications for a low opacity universe from Fermi-LAT data.

γ-rays undergo pair production with photons of the extragalactic background light (EBL) in the ultra-violet to infrared wavelength band and, thus, the intrinsic photon flux is attenuated. For sources at cosmological distances, the absorption can be significant above 10 GeV, i.e., within the energy range of the large area telescope (LAT) on board the Fermi satellite. Here, photons in the first 39 months of Fermi-LAT data are analyzed which are associated with extragalactic sources with known redshift. Under the assumption of different EBL models, the expected number of photons is compared with the observed number at large optical depth. The background of unassociated photons is self-consistently derived from the Fermi-LAT data. As a result, the predictions for three recent EBL models are in conflict with the data at a 3 to 4σ level, which indicates a lower opacity of the Universe for high energy γ-rays than previously thought. [ABSTRACT FROM AUTHOR]