Embers of the Distant Past

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Science  30 Nov 2012:
Vol. 338, Issue 6111, pp. 1160-1161
DOI: 10.1126/science.1231128

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Modern cosmology has come a long way in fulfilling its grand goal of reconstructing the entire history of the universe. The emergence of cosmic structure is governed by dark matter whose gravitational attraction led to the condensation of gas into galaxies, stars, and black holes. Whereas the evolution of the dark matter component is well understood (1, 2), cosmic star and black hole formation is still subject to large uncertainties. The combined energy production from all stars and black holes, releasing radiation when gas is falling close to their event horizon, however, is preserved in a pervasive cosmic radiation background—the extragalactic background light (EBL). This radiation comprises wavelengths from ultraviolet (UV) to far-infrared (IR) and provides us with an independent check on how many stars and active black holes have existed. On page 1190 of this issue, Ackermann et al. (3) present a new measurement of the EBL with the Fermi Space Telescope, based on the attenuation of distant sources of gamma-ray photons when they travel through the sea of lower-energy, background EBL photons.