Superstrings could add gravitational cacophony to universe's chorus (Forwarded)

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Jan. 8, 2007

Superstrings could add gravitational cacophony to universe's chorus

Albert Einstein theorized long ago that moving matter would warp the fabric
of four-dimensional space-time, sending out ripples of gravity called
gravitational waves. No one has observed such a phenomenon so far, but
University of Washington researchers believe it is possible to detect such
waves coming from strange wispy structures called cosmic superstrings.

Many physicists consider a complex and sometimes-controversial premise
called string theory to be a leading candidate to unify their understanding
of the four basic forces of nature -- gravity, electromagnetic, weak and
strong. String theory is sometimes criticized for being untestable or even
unscientific, but some versions now predict an exotic behavior with
observable effects: the formation of cosmic superstrings, narrow tubes of
energy left from the beginning of the universe that have been stretched to
enormous lengths by the expansion of the universe, said UW cosmologist Craig
Hogan.

If the theories are correct, there are countless cosmic superstrings
stretched like a galactic-sized rubber band. They resemble ultra-thin tubes
with some of the vacuum of the early universe preserved inside, Hogan said.
The strings can form into loops that "flop around" and emit gravitational
waves as they decay and eventually disappear.

"They're so light that they can't have any effect on cosmic structure, but
they create this bath of gravitational waves just by decaying," he said.

Theory holds that every time something moves it emits a gravitational wave.
Colliding black holes send out more waves than anything else, typically a
million times more power than is produced by all the galaxies in the
universe. While some gravitational waves could occur at frequencies high
enough that a human theoretically could hear them, many more of the sources
have very low frequencies, 10 to 20 octaves below the range of human
hearing, Hogan said.

"Big masses tend to take a long time to move about, so there are more
sources at lower frequencies," he said. "Sensing these vibrations would add
the soundtrack to the beautiful imagery of astronomy that we are used to
seeing. All this time, we have been watching a silent movie."

A proposed orbiting observatory called the Laser Interferometer Space
Antenna, being developed by the National Aeronautics and Space
Administration, could provide the first measurements of very low frequency
gravitational waves, perhaps the first such measurements at any frequency,
Hogan said. In addition to the expected wave sources, such as binary stars
and black holes, these signals also might include the first direct evidence
of cosmic superstrings.

"If we see some of this background, we will have real physical evidence that
these strings exist," he said.

Calculations for gravitational waves generated by cosmic strings, as well as
the larger rationale for the space antenna mission, are being presented
today at the American Astronomical Society national meeting in Seattle in a
poster by Hogan and Matt DePies, a UW physics doctoral student and visiting
physics lecturer.

An Earth-based project called the Laser Interferometer Gravitational-Wave
Observatory also is attempting to observe gravitational waves, but it is
searching in higher frequencies where Hogan believes waves from superstrings
would be much harder to detect. That's because the background noise would
make it difficult to identify the waves emitted by strings.

"The strings, if they exist, are part of that noise, but we want to listen
in at lower frequencies and try to detect them," he said.

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For more information, contact Hogan at (206) 685-2112 or or DePies at (206)
616-4775.

IMAGE CAPTION:
[http://uwnews.washington.edu/ni/phot...74&spid=29375]
Cosmic superstring loops wiggle and oscillate, producing gravitational
waves, then slowly shrink as they lose energy until they disappear. Credits:
Matt DePies/UW