The Imprint of the Extragalactic Background Light in the Gamma-Ray Spectra of Blazars

Abstract
Glow from the Past: Extragalactic background light (EBL) is the integrated radiation from all extragalactic sources in the universe. Foreground emission from our solar system and galaxy makes direct detection of the EBL very difficult. However, it is possible to measure EBL from gamma-ray spectra of distant sources, because gamma-ray photons from these sources interact with the EBL. Ackermann et al. (p. 1190 , published online 1 November; see the Perspective by Bromm ) report a measurement of the EBL based on an attenuation feature seen in the combined spectra of distant active galaxies detected by the Fermi Gamma-Ray Space Telescope. The result puts constraints on the cosmic history of star formation.