Space Science division, Naval Research Laboratory
The gamma-ray determination of the Universe's star formation history
The light emitted by all galaxies over the history of the Universe produces the extragalactic background light (EBL) at ultraviolet through the optical and into the infrared. Gamma rays emitted at cosmic distances are absorbed by the EBL photons, producing electron positron pairs, leaving an imprint on the gamma-ray spectra of distance sources. I discuss using the Fermi Large Area Telescope (LAT) to measure the absorption features in the gamma-ray spectra of a large sample of active galactic nuclei and reconstruct the evolution of the EBL. This allows for the reconstruction of the star formation history of the Universe throughout cosmic time with results and accuracy comparable to galaxy surveys. Gamma-ray upper limits on the EBL at the epoch of re-ionization suggests that the upcoming James Webb Space Telescope will resolve most of the faint galaxies at this epoch.