Breaks in gamma-ray spectra of distant blazars and transparency of the Universe

Understand

The expected opacity of the intergalactic space limits the mean free path of TeV gamma rays to dozens of Megaparsecs.

• However, TeV photons from numerous more distant sources have been detected.
• This might be interpreted, in each particular case, in terms of hardening of the emitted spectrum caused by presently unknown mechanisms at work in the sources.
• Here we show that this interpretation is not supported by the analysis of the ensemble of all observed sources.