  Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN)
and gamma ray bursts (GRBs) give upper limits on the extragalactic background
light (EBL) from the UV to the mid-IR that are only a little above the lower
limits from observed galaxies. These upper limits now rule out some EBL models
and purported observations, with improved data likely to provide even stronger
constraints. We present EBL calculations both based on multiwavelength
observations of thousands of galaxies and also based on semi-analytic models,
and show that they are consistent with these lower limits from observed
galaxies and with the gamma-ray upper limit constraints. Such comparisons
"close the loop" on cosmological galaxy formation models, since they account
for all the light, including that from galaxies too faint to see. We compare
our results with those of other recent works, and discuss the implications of
these new EBL calculations for gamma ray attenuation. Catching a few GRBs with
groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov
detectors could provide important new constraints on the high-redshift star
formation history of the universe.

Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena