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Cosmic γ-ray background from structure formation in the intergalactic medium


The Universe is filled with a diffuse background of γ-ray radiation1, the origin of which remains one of the unsolved puzzles of cosmology. Less than one-quarter of the γ-ray flux can be attributed to unresolved discrete sources2,3, such as active galactic nuclei; the remainder appears to constitute a truly diffuse background. Here we show that the shock waves induced by gravity in the gas of the intergalactic medium, during the formation of large-scale structures like filaments and sheets of galaxies, produce a population of highly relativistic electrons. These electrons scatter a small fraction of the cosmic microwave background photons in the local Universe up to γ-ray energies, thereby providing the γ-ray background. The predicted diffuse flux agrees with the observed background across more than four orders of magnitude in photon energy, and the model predicts that the γ-ray background, though generated locally, is isotropic to better than five per cent on angular scales larger than a degree. Moreover, the agreement between the predicted and observed background fluxes implies a mean cosmological density of baryons that is consistent with Big Bang nucleosynthesis.

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Figure 1: Spectrum of the unresolved γ-ray background.

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This work was supported in part by the Israel-US BSF and by NSF. A.L. thanks the Weizmann Institute for hospitality during the work. E.W. is the incumbent of the Beracha foundation career development chair.

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Correspondence to Abraham Loeb.

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Loeb, A., Waxman, E. Cosmic γ-ray background from structure formation in the intergalactic medium. Nature 405, 156–158 (2000).

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