Thursday, January 31, 2008
Scientists propose test of string theory based on neutral hydrogen absorption
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.