Univ. of Minn.
August 23, 2007
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.
Illustration of the
effect of intervening matter in the cosmos on the cosmic microwave
On the right,
the CMB is released shortly after the Big Bang, with tiny ripples in
temperature due to fluctuations in the early Universe.
As this radiation
traverses the Universe, filled with a web of galaxies, clusters,
superclusters and voids, it experiences slight perturbations.
In the direction of
the giant newly-discovered void, the WMAP satellite (top left) sees
a cold spot, while the VLA (bottom left) sees fewer radio galaxies.
CREDIT: Bill Saxton, NRAO/AUI/NSF,
"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
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
"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
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
"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.
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
Data are from NASA's
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,
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
The acceleration of the Universe's expansion, and thus
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.
National Radio Astronomy Observatory
is a facility of the
National Science Foundation,
operated under cooperative agreement by
Associated Universities, Inc.
This research at the University of
Minnesota is supported by individual investigator grants from the
NSF and NASA.