Astronomers have found a perplexing and enormous hole in the universe, nearly a billion light years across, empty of both normal matter such as stars, galaxies, and gas, and mysterious dark matter.
"Not only has no one ever found a void this big, but we never even expected to find one this size," said Lawrence Rudnick of the University of Minnesota, USA. Rudnick heads a team of astronomers who report the finding in a paper slated for publication in the Astrophysical Journal.
Astronomers have known for years that, on large scales, the universe has voids largely empty of matter. However, most are much smaller than the one found by Rudnick's team.
Finding is "not normal"
"What we've found is not normal, based on either observational studies or on computer simulations of the large-scale evolution of the universe," team member Liliya R. Williams said.
The astronomers drew their conclusion by studying data from the Very Large Array (VLA) radio telescope, in New Mexico. The data revealed a mysterious fall in the number of galaxies in a region of sky in the constellation Eridanus.
The survey that made the finding imaged roughly 82 per cent of the sky visible from the New Mexico site. It consists of 217,446 individual observations that consumed 2,940 hours of telescope time between 1993 and 1997.
"We already knew there was something different about this spot in the sky," said Rudnick. The region had been dubbed the "WMAP Cold Spot," because it stood out in a map of the Cosmic Microwave Background (CMB) radiation made by the Wilkinson Microwave Anisotopy Probe (WMAP) satellite, launched by U.S. space agency NASA in 2001.
The CMB, made up of faint radio waves that are the remnant radiation from the Big Bang, is the earliest "baby picture" available of the universe. Irregularities in the CMB show structures that existed only a few hundred thousand years after the Big Bang.
The WMAP satellite measures temperature differences in the CMB picture accurate to millionths of a degree. The cold region in Eridanus was discovered in 2004. Astronomers wondered if the cold spot was intrinsic to the CMB, and thus indicated some structure in the very early Universe, or whether it could be caused by something more nearby through which the CMB radiation had to pass on its way to Earth.
Now, finding the dearth of galaxies in that region by studying the VLA data has resolved that question.
"Although our surprising results need independent confirmation, the slightly colder temperature of the CMB in this region appears to be caused by a huge hole devoid of nearly all matter roughly 6-10 billion light-years from Earth," Rudnick said.
CMB radiation gains a small amount of energy when it passes through a region of space populated by matter, said Rudnick. However, when it passes through an empty void, it loses a small amount of energy, so appears cooler.
The experts are as yet at a loss to explain the anomaly.