The Gamma-Ray View of the Extragalactic Background Light
  The Extragalactic Background Light (EBL) from the infrared (IR) through the
ultraviolet (UV) is dominated by emission from stars, either directly or
through absorption and reradiation by dust. It can thus give information on the
star formation history of the universe. However, it is difficult to measure
directly due to foreground radiation fields from the Galaxy and solar system.
Gamma-rays from extragalactic sources at cosmological distances (blazars and
gamma-ray bursts) interact with EBL photons creating electron-positron pairs,
absorbing the gamma-rays. Given the intrinsic gamma-ray spectrum of a source
and its redshift, the EBL can in principle be measured. However, the intrinsic
gamma-ray spectra of blazars and GRBs can vary considerably from source to
source and the from the same source over short timescales. A maximum intrinsic
spectrum can be assumed from theoretical grounds, to give upper limits on the
EBL absorption from blazars at low redshift with very high energy (VHE)
gamma-ray observations with ground-based Atmospheric Cherenkov telescopes. The
Fermi-LAT observations of blazars and GRBs can probe EBL absorption at higher
redshifts. The lower energy portion of the LAT spectrum of these sources is
unattenuated by the EBL, so that extrapolating this to higher energies can give
the maximum intrinsic spectrum for a source. Comparing this to the observed
higher energy LAT spectrum will then give upper limits on the EBL absorption.
For blazars which have been detected by both the Fermi-LAT and at higher
energies by Cherenkov telescopes, combined LAT-VHE observations can put more
stringent constraints on the low redshift EBL. These procedures above can also
be reversed: for sources with an unknown redshift, a given EBL model and
gamma-ray spectrum can lead to an upper limit on the source's redshift.
