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