The Extragalactic Background Light and the Gamma-ray Opacity of the
  Universe
  The extragalactic background light (EBL) is one of the fundamental
observational quantities in cosmology. All energy releases from resolved and
unresolved extragalactic sources, and the light from any truly diffuse
background, excluding the cosmic microwave background (CMB), contribute to its
intensity and spectral energy distribution. It therefore plays a crucial role
in cosmological tests for the formation and evolution of stellar objects and
galaxies, and for setting limits on exotic energy releases in the universe. The
EBL also plays an important role in the propagation of very high energy
gamma-rays which are attenuated en route to Earth by pair producing gamma-gamma
interactions with the EBL and CMB. The EBL affects the spectrum of the sources,
predominantly blazars, in the ~10 GeV to 10 TeV energy regime. Knowledge of the
EBL intensity and spectrum will allow the determination of the intrinsic blazar
spectrum in a crucial energy regime that can be used to test particle
acceleration mechanisms and VHE gamma-ray production models. Conversely,
knowledge of the intrinsic gamma-ray spectrum and the detection of blazars at
increasingly higher redshifts will set strong limits on the EBL and its
evolution. This paper reviews the latest developments in the determination of
the EBL and its impact on the current understanding of the origin and
production mechanisms of gamma-rays in blazars, and on energy releases in the
universe. The review concludes with a summary and future directions in
Cherenkov Telescope Array techniques and in infrared ground-based and space
observatories that will greatly improve our knowledge of the EBL and the origin
and production of very high energy gamma-rays.
