The opacity of the universe for gamma-rays

Dieter Horns (Hamburg Univ.)
DESY Auditorium, 16.45 h

Once stars started to form, the universe has become a complicated place in many aspects - one of them will be discussed here. The continuing injection and re-processing of stellar photons during the past 10 Gigayears has led to a universal optical/infra-red background field. Until now, it has not been possible to directly measure this faint glow of the extra-galactic background light (EBL). However, the EBL has recently been firmly detected in an indirect way: a clear signature of an exponential attenuation has been discovered in the gamma-ray spectra of active galactic nuclei (AGN). The spectral signature is consistent with the effect of pair-production processes. At sufficiently large attenuation, deviations are expected in the case of competing processes that circumvent the absorption. A number of studies indicate the presence of such an anomalous transparency for AGN at TeV energies using energy spectra obtained with ground based imaging Cherenkov telescopes. Recently, additional evidence for anomalous transparency has been reported using lower energy gamma-rays detected with the pair-telescope on-board the Fermi spacecraft. A number of interpretations have been suggested and are confronted with the measurements. A particularly interesting scenario requires the presence of an axion-like particle with a photon coupling and mass well within reach of future laboratory experiments using light-shining-through the wall setups (ALPS-II) or future helioscopes (IAXO).

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