When the cosmic background radiation (right) emitted at about the time of the Big Bang passes the empty spot detected by the VLA (lower left), they change their temperature causing WMAP's unusual readings (left). Illustration: Bill Saxton, NRAO / AUI / NSF, NASA
Reading Astronomers have discovered a huge hole in the universe: in the middle of the constellation Eridanus stretches an area with a diameter of about a billion light years, in which there is neither normal matter such as galaxies, stars or gas nor the enigmatic dark matter. Although scientists have known smaller variants of such vacancies for years, a hole of such gigantic proportions as the one now discovered is so far unique. "It's not just that no one has ever found such a large cavity, we did not even expect to find one of that size, " says Lawrence Rudnick of the University of Minnesota in Minneapolis, describing his find, the college says. In principle, the universe is constructed like a honeycomb. There are areas like galaxies and galaxy clusters, where the density of matter is relatively high, forming the walls of the honeycomb. In between there are more or less large areas, called after the English word for cavity "Voids", in which there is little or very little matter. However, all previously discovered holes are smaller by orders of magnitude than the empty zone in Eridanus located southwest of the constellation Orion.

There were indications of an unusual structure in the region for the first time in 2004, explains study leader Rudnick. At that time, the WMAP satellite had measured the strength and temperature of the cosmic background radiation, the electromagnetic radiation that developed according to popular theory shortly after the Big Bang and today conveys a kind of baby picture of the young universe. Exactly at the point now identified as a hole, the WMAP map found an area where the background radiation temperature was a few millionths of a degree lower than in the surrounding area. This could in principle have two reasons, explains Rudnick: Either it indicates an irregularity in the background radiation itself and thus a phenomenon of the early Universe, or the radiation cools when crossing the region, which rather for an unusual structure in the region speaking.

Apparently the second explanation is correct, Rudnick and his team now conclude by evaluating data from the NRAO VLA Sky Survey, a project in which the "Very Large Array" interferometer in the US state of New Mexico recorded images of the entire visible part of the sky, It can be clearly seen that the number of galaxies in the cooler zone drops significantly compared to the environment. In combination with what was previously known about the energy distribution of the cosmic background radiation, this would only allow the conclusion that there is a gigantic emptiness in the region, explains Rudnick.

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