Professor Abraham Loeb
Harvard-Smithsonian Center for Astrophysics
Cambridge, MA 02138
Tel: 617-496-6808; Fax: 617-495-7093
Professor Eli Waxman
Weizmann Institute of Science
Tel: +972-8-946-4260; Fax: +972-8-946-5110
James Cornell
Harvard-Smithsonian Center for Astrophysics
Cambridge, MA 02138
Tel: 617-495-7462; Fax: 617-495-7468
Release No.: 00-13

CAMBRIDGE, MA — The faint veil of gamma rays draped over the entire sky — long a puzzle for astronomical theorists — may actually be leftover energy from the cosmic construction project that created the large-scale structure in the Universe.
Massive shock waves, triggered by gravity during the formation of large- scale structures, such as the observed sheets and filaments of galaxies, were sufficiently powerful to produce today’s observed background radiation, according to a model proposed by Abraham Loeb of the Harvard- Smithsonian Center for Astrophysics and Eli Waxman of Israel’s Weizmann Institute.
In an article in the 5/11/00 edition of the journal Nature, Loeb and Waxman suggest 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.
The origin of the diffuse and pervasive background of gamma-ray radiation 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 flux.
As recent results from the BOOMERANG experiment confirm, the Universe started from a nearly smooth initial state, but small density fluctuations in cosmic matter grew larger over time due to the effects of gravity. As the overdense regions condensed into large structures — such as filaments, sheets, and clusters of galaxies — the cosmic gas was shocked to a temperature of about ten million degrees.
These shock waves must have also produced relativistic electrons, say Loeb and Waxman, since X-ray and gamma-ray observations by modern telescopes have demonstrated the existence of such electrons in similar shock waves surrounding supernova remnants. Assuming that the physics of shock acceleration can be scaled up to intergalactic distances, Loeb and Waxman argue that similar high-energy electrons were produced in the intergalactic medium.
In the Loeb-Waxman model, the gamma-ray background was created
primarily in those regions where dense filaments and sheets channeled gas from converging flows in the intergalactic medium. The hottest, most powerful shocks occurred at the intersections of these filaments, especially where they encompassed emerging clusters of galaxies.
Although rich young clusters are rare and make up only a fraction of the overall background radiation, they probably contain the strongest shock waves, say Loeb and Waxman, and thus should produce the strongest fluctuations in the diffuse background.
Direct detection of such shock waves would be the best way to test the model, say Loeb and Waxman. The first step, however, is to measure the smoothness of the background radiation on the sky with much greater precision. Although the level of precision needed is beyond the capabilities of existing telescopes, it will easily become accessible with the future Gamma-Ray Large Area Space Telescope (GLAST), which is planned for launch in the year 2005.