U of M astronomers find gaping hole in the
Universe: Dark energy reveals its power by the dawn's (of the
universe) early light
Contacts: Dave Finley, public information officer, (505)
835-7302, dfinley@nrao.edu
Mark Cassutt, University of Minnesota News Service, (612)
624-8038
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MINNEAPOLIS / ST. PAUL ( 8/23/2007 ) --University of Minnesota
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, as well as 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 astronomy professor. Rudnick, along with
grad student Shea Brown and associate professor Liliya 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 study of the NVSS data showed a remarkable drop in the
number of galaxies in a region of sky in the constellation Eridanus,
southwest of Orion.
"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 lower 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 lower temperature in the Big
Bang's remnant radiation as seen from Earth?
The answer lies in dark energy, which became a dominant force in
the Universe very recently, when the Universe was already
three-quarters of the size it is today. Dark energy works opposite
gravity and is speeding up the expansion of the Universe. Thanks to
dark energy, CMB photons that pass through a large void just before
arriving at Earth have less energy than those that pass through an
area with a normal distribution of matter in the last leg of their
journey.
In a simple expansion of the universe, without dark energy,
photons approaching a large mass -- such as a supercluster of
galaxies -- pick up energy from its gravity. As they pull away, the
gravity saps their energy, and they wind up with the same energy as
when they started.
But photons passing through matter-rich space when dark energy
became dominant don't fall back to their original energy level. Dark
energy counteracts the influence of gravity and so the large masses
don't sap as much energy from the photons as they pull away. Thus,
these photons arrive at Earth with a slightly higher energy, or
temperature, than they would in a dark energy-free Universe.
Conversely, photons passing through a large void experience a
loss of energy. 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 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.
Images and graphics are available at
www.nrao.edu/pr/2007/coldspot/graphics.shtml
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