Astronomers Find Enormous Hole in the Universe
http://www.nrao.edu/pr/2007/coldspot/graphics.shtml
http://www.nrao.edu/pr/2007/coldspot/

National Radio Astronomy Observatory
P.O. Box O
Socorro, NM 87801
http://www.nrao.edu

August 23, 2007
Contacts:

Dave Finley, NRAO
(505) 835-7302
dfinley@nrao.edu

Mark Cassutt, Univ. of Minn.
(612) 624-8038
cassu003@umn.edu

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.

"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.

The 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. .

http://www.nrao.edu/pr/2007/coldspot/
http://www.nrao.edu/pr/2007/coldspot/graphics.shtml