The transparency of the universe for very high energy gamma-rays

Understand

The propagation of very high energy gamma-rays ($E>100$~GeV) over cosmological distances is suppressed by pair-production processes with the ubiquitous extra-galactic soft photon background, mainly in the optical to near infra-red.

• The detailed spectroscopy of gamma-ray emitting blazars has revealed the signature of this absorption process leading to a meaningful measurement of the background photon field which is linked to the star-forming history of the universe.
• Deviations from the expected absorption have been claimed in the past.
• Here the status of the observations is summarized, an update on the search for the persisting anomalous transparency is given and discussed.