Voids and Supervoids
Cosmic voids, and supervoids, are large volumes of space that are devoid of matter. This includes normal matter, in the form of galaxies, and dark matter. Initially, astronomers were not sure if the voids contained dark matter, even though there were no galaxies, but recent observations show that the halos of dark matter are not present. The filamentary structure of galactic superclusters surrounds the voids. While space is mostly empty, voids are large volumes, tens of megaparsecs across. The largest supervoids found by Rudnick and his colleagues is nearly a billion light-years across . The larger known voids include the Boötes Supervoid, and the Northern and Southern Local Supervoids. To explain the cold spot in the cosmic microwave background (CMB), some astronomers propose a huge supervoid, tentatively dubbed the Eridanus or Great supervoid. The Capricornus void is another disputed void, but would be around 230 Mpc across.
Hole in Universe revealed by its effect
on Cosmic Microwave Background radiation.
CREDIT: Bill Saxton, NRAO/AUI/NSF, NASA
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Astronomers have found an enormous hole in the Universe, nearly a billion light-years across, empty of both normal matter such as stars, galaxies, and gas, and the mysterious, unseen “dark matter.” While earlier studies have shown holes, or voids, in the large-scale structure of the Universe, this new discovery dwarfs them all.
“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. Rudnick, along with Shea Brown and Liliya R. Williams, also of the University of Minnesota, reported their findings in a paper accepted 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 of these voids are much smaller than the one found by Rudnick and his colleagues. In addition, the number of discovered voids decreases as the size increases.
Left: A 25-degree region of the Cosmic Microwave Background emission around the region of the WMAP cold spot (circled). The colors represent very small variations (parts in 100,000) around the average temperature of 2.7 degrees above absolute zero, with blue colors being colder. Data are from NASA’s WMAP satellite.
Right: A heavily smoothed portion of the NRAO Very Large Array Sky Survey (NVSS) showing the blended emission from radio galaxies along each path. Blue colors represent brightnesses approximately 20% below the average.
CREDIT: Rudnick et al., NRAO/AUI/NSF, NASA
“What we’ve found is not normal, based on either observational studies or on computer simulations of the large-scale evolution of the Universe,” Williams said.
The astronomers drew their conclusion by studying data from the NRAO VLA Sky Survey (NVSS), a project that imaged the entire sky visible to the Very Large Array (VLA) radio telescope, part of the National Science Foundation’s National Radio Astronomy Observatory (NRAO). Their careful study of the NVSS data showed a remarkable drop in the number of galaxies in a region of sky in the constellation Eridanus.
“We already knew there was something different about this spot in the sky,” Rudnick said. 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 NASA in 2001. The CMB, 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 measured temperature differences in the CMB that are only 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 had to pass on its way to Earth. Finding the dearth of galaxies in that region by studying NVSS data 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.
How does a lack of matter cause a cooler temperature in the Big Bang’s remnant radiation as seen from Earth?
Photons of the CMB gain a small amount of energy when they pass through a region of space populated by matter. This effect is caused by the enigmatic “dark energy” that is accelerating the expansion of the Universe. This gain in photon energy makes the CMB appear slightly warmer in that direction. When the photons pass through an empty void, they lose a small amount of energy from this effect, and so the CMB radiation passing through such a region appears cooler.
The acceleration of the Universe’s expansion, and thus dark energy, were discovered less than a decade ago. The physical properties of dark energy are unknown, though it is by far the most abundant form of energy in the Universe today. Learning its nature is one of the most fundamental current problems in astrophysics.
The NVSS imaged the roughly 82 percent of the sky visible from the New Mexico site of the VLA. The survey consists of 217,446 individual observations that consumed 2,940 hours of telescope time between 1993 and 1997. A set of 2,326 images was produced from the data, and these images are available via the NRAO Web site. The survey also produced a catalog of more than 1.8 million individual objects identifiable in the images. The NVSS has been cited in more than 1,200 scientific papers.
NASA’s WMAP satellite, using microwave amplifiers produced by NRAO’s Central Development Laboratory, has yielded a wealth of new information about the age and history of the Universe, the emergence of the first stars, and the composition of the Universe. WMAP results have been extensively cited by scientists in a wide variety of astrophysical specialties.