The Diffuse Extra-galactic Gamma-ray Background
Observations with previous gamma-ray experiments have shown that there is significant background radiation at high-galactic latitudes. This diffuse radiation is believed to be extragalactic in origin. GLAST should be able to determine whether or not this extragalactic background radiation is entirely comprised of unresolved sources, that were too faint to be detected by the earlier experiments, or if the radiation is truly diffuse, and a remnant of some earlier time in the evolution of the universe. In the northern hemisphere of the figure below, a simulation shows what the sky would look like if the extragalactic background were entirely comprised of individual sources (assumed to be blazars). The southern hemisphere of the figure illustrates a different scenario in which half of the background flux is due to blazars while the other half is truly diffuse, and hence primordial, radiation.
Extragalactic Background Light
The Extragalactic Background Light (EBL) is light at long wavelengths (primarily near-ultraviolet, optical and near-infrared) produced by galaxies which are in the process of forming stars. When traveling cosmological distances, gamma-rays from more distant active galaxies will be attenuated by interacting with the photons that make up the EBL. Studying the gamma-ray spectra of very distant active galaxies can therefore tell scientists about the star-forming galaxies formed at early epochs. Understanding the order in which galaxies formed, and the age of the universe at which different types of galaxies were forming, allows scientists to test their current models for the structure and formation of the universe. Galaxies form differently in universes that contain different types of dark matter. By studying the effects of the EBL on the high-energy spectra of blazars, GLAST may be able to tell whether the dark matter in our universe is primarily hot or cold.