In section 3. Cosmology
Measurement of the EBL spectral energy distribution using the VHE gamma-ray spectra of H.E.S.S. blazars
The low-energy spectrum of extragalactic background light (EBL) has already been inferred from measurements at low energy, but this study presents a novel recovery of the ELB using very high energy observations in the range of 0.2 to nearly 20 TeV using the H.E.S.S. imaging atmospheric Cherenkov telescope array. Using a single array provides better control of systematic uncertainties, that are extensvely discussed. The authors use a set of nine blazars, including those displaying different activity states, spanning almost a decade in redshift, 0.03 < z 0.29. Systematic deviations in the (relatively simple and monotonic) intrinsic spectral shape are produced by the interaction of the VHE source photons and the cosmologically ambient light. Although a model function is required for the redshift evolution, the recovered shape is not based on a spectral model. The authors extract the restframe spectrum, in four optical and infrared bands between 0.5 and 100 microns (Fig. 5), and the redshift evolution of the gamma-ray horizon (the effective VHE photosphere) (Fig. 6).