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Scientists propose test of string theory based on neutral hydrogenabsorption (Forwarded)



 
 
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Old January 29th 08, 10:00 PM posted to sci.astro
Andrew Yee
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Default Scientists propose test of string theory based on neutral hydrogenabsorption (Forwarded)

News Bureau
University of Illinois at Urbana-Champaign
Champaign, Illinois

Contact:
James E. Kloeppel, Physical Sciences Editor
217-244-1073

1/28/08

Scientists propose test of string theory based on neutral hydrogen
absorption

CHAMPAIGN, Ill. -- Ancient light absorbed by neutral hydrogen atoms could
be used to test certain predictions of string theory, say cosmologists at
the University of Illinois. Making the measurements, however, would
require a gigantic array of radio telescopes to be built on Earth, in
space or on the moon.

String theory -- a theory whose fundamental building blocks are tiny
one-dimensional filaments called strings -- is the leading contender for a
"theory of everything." Such a theory would unify all four fundamental
forces of nature (the strong and weak nuclear forces, electromagnetism,
and gravity). But finding ways to test string theory has been difficult.

Now, cosmologists at the U. of I. say absorption features in the
21-centimeter spectrum of neutral hydrogen atoms could be used for such a
test.

"High-redshift, 21-centimeter observations provide a rare observational
window in which to test string theory, constrain its parameters and show
whether or not it makes sense to embed a type of inflation -- called brane
inflation -- into string theory," said Benjamin Wandelt, a professor of
physics and of astronomy at the U. of I.

"If we embed brane inflation into string theory, a network of cosmic
strings is predicted to form," Wandelt said. "We can test this prediction
by looking for the impact this cosmic string network would have on the
density of neutral hydrogen in the universe."

Wandelt and graduate student Rishi Khatri describe their proposed test in
a paper accepted for publication in the journal Physical Review Letters.

About 400,000 years after the Big Bang, the universe consisted of a thick
shell of neutral hydrogen atoms (each composed of a single proton orbited
by a single electron) illuminated by what became known as the cosmic
microwave background.

Because neutral hydrogen atoms readily absorb electromagnetic radiation
with a wavelength of 21 centimeters, the cosmic microwave background
carries a signature of density perturbations in the hydrogen shell, which
should be observable today, Wandelt said.

Cosmic strings are filaments of infinite length. Their composition can be
loosely compared to the boundaries of ice crystals in frozen water.

When water in a bowl begins to freeze, ice crystals will grow at different
points in the bowl, with random orientations. When the ice crystals meet,
they usually will not be aligned to one another. The boundary between two
such misaligned crystals is called a discontinuity or a defect.

Cosmic strings are defects in space. A network of strings is predicted by
string theory (and also by other supersymmetric theories known as Grand
Unified Theories, which aspire to unify all known forces of nature except
gravity) to have been produced in the early universe, but has not been
detected so far.

Cosmic strings produce characteristic fluctuations in the gas density
through which they move, a signature of which will be imprinted on the
21-centimeter radiation.

The cosmic string network predicted to occur with brane inflation could be
tested by looking for the corresponding fluctuations in the 21-centimeter
radiation.

Like the cosmic microwave background, the cosmological 21-centimeter
radiation has been stretched as the universe has expanded. Today, this
relic radiation has a wavelength closer to 21 meters, putting it in the
long-wavelength radio portion of the electromagnetic spectrum.

To precisely measure perturbations in the spectra would require an array
of radio telescopes with a collective area of more than 1,000 square
kilometers. Such an array could be built using current technology, Wandelt
said, but would be prohibitively expensive.

If such an enormous array were eventually constructed, measurements of
perturbations in the density of neutral hydrogen atoms could also reveal
the value of string tension, a fundamental parameter in string theory,
Wandelt said. "And that would tell us about the energy scale at which
quantum gravity begins to become important."

Funding was provided by the Alexander von Humboldt Foundation.

To reach Benjamin Wandelt, call 217-333-9374.


 




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