Gamma rays may be left over from cosmic construction project

The origin of the diffuse and pervasive background of gamma-ray radiation that exists over the universe has been one of the great unsolved mysteries in cosmology. Even the known population of powerful extragalactic gamma-ray sources, called “blazars,” can account for no more than a quarter of the gamma-ray background radiation that is observed. According to a model proposed by Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics and Eli Waxman of Israel’s Weizmann Institute, massive shock waves, triggered by gravity during the formation of large-scale structures such as galaxies, are sufficiently powerful to account for the background radiation. In an article in the May 11, 2000, edition of the journal Nature, Loeb and Waxman suggested that the gravity-induced shock waves generated a population of highly-relativistic electrons, which, in turn, scattered the equally pervasive microwave background, itself a remnant of the Big Bang, pumping up a fraction of the microwave photons to gamma-ray energies, thus producing the all-sky gamma-ray background seen in today’s universe.