Considerable theoretical and observational study of gamma-ray propagation over cosmic distances is in progress. The generation of electrons and positrons on the UV-to-infrared cosmic backgrounds limits the mean free path of gamma rays at energies above 100 GeV. As they gyrate in the intergalactic magnetic fields, the secondary electrons and positrons cool, possibly causing a global warming of the cosmos or more likely the formation of lower-energy tertiary gamma rays. By probing magnetic and photon fields in the darkest corners of the universe, gamma-ray observations of active galactic nuclei and gamma-ray bursts with Fermi-LAT, H.E.S.S., MAGIC and VERITAS have enabled the exploration of uncharted territories. In this talk, I will review recent measurements of gamma-ray cosmology and tie them to the latest multi-wavelength and multi-messenger constraints. Beyond Pluto's orbit, ongoing observations of the darkest patches of the sky particularly put us on the spot. Have we really missed half of the visible light in the Universe? The upcoming Cherenkov Telescope Array and joint studies of all extragalactic gamma-ray observations collected to date may solve this modern version of Olbers' paradox and reveal some surprises…
---
It will be held in person in room A. Berthelot. For those who cannot attend in person, there will also be a zoom connection, available at this link : https://cern.zoom.us/j/68399739111?pwd=SGErcHBQYmVLWFZTVE9jdm5xSmdEdz09
