THE EXTRAGALACTIC BACKGROUND LIGHT, THE HUBBLE CONSTANT, AND ANOMALIES: CONCLUSIONS FROM 20 YEARS OF TeV GAMMA-RAY OBSERVATIONS

Abstract

Ground-based observatories have been collecting 0.2 - 20 TeV gamma rays from blazars for about twenty years. These gamma rays can experience absorption along the line of sight due to interactions with the extragalactic background light (EBL). In this paper, we show that the gamma-ray optical depth can be reduced to the convolution product of an EBL kernel with the EBL intensity, assuming a particular form for the EBL evolution. We extract the absorption signal from the most extensive set of TeV spectra from blazars collected so far and unveil a broad-band EBL spectrum from mid-ultraviolet to far infrared. This spectrum is in good agreement with the accumulated emission of galaxies, constraining unresolved populations of sources. We propose a data-driven estimate of the Hubble constant based on the comparison of local and gamma-ray measurements of the EBL. After setting stringent upper-limits on the redshift of four TeV blazars, we investigate the 106 gamma-ray spectra in our sample and find no significant evidence for anomalies. The intrinsic TeV spectra are not harder than their GeV counterpart, and no spectral upturn is visible at the highest optical depths. Finally, we investigate a modification of the pair-creation threshold due to Lorentz invariance violation. A mild excess prevents us from ruling out an effect at the Planck energy, and we constrain for the first time the energy scale of the modification to values larger than sixty percent of the Planck energy.