A team of scientists that includes a University of Florida physicist has suggested that two of the biggest mysteries in particle physics and astrophysics -- the existence of extra time and space dimensions and the composition of an invisible cosmic substance called dark matter -- may be connected.
"For the most part, these two questions have been treated separately in the past, and for the first time we're making a direct link," said Konstantin Matchev, a UF assistant professor of physics. "We're suggesting that the dark matter may be due to extra dimensions."
If correct, the scientists' theory may lead to the discovery of the first concrete evidence of dark matter, an invisible substance that may comprise as much as 30 percent of the universe. Dark matter has never yet been directly or indirectly observed.
Matchev co-authored a paper on the subject that has been widely cited by other scientists since appearing in the journal Physics Review Letters in November. The other authors are Hsin-Chia Cheng and Jonathan Feng, physicists at Harvard University and the University of California at Irvine, respectively.
Scientists have long inferred dark matter is present based on a discrepancy between galaxies' rotational speed and the amount of visible stars within them. In a nutshell, there are not enough stars or visible objects to account for the speed, which means the galaxies must also contain the invisible dark matter. Its composition is unknown.
Extra dimensions are predicted by the superstring theory, which offers a unified description of all of the fundamental particles and forces in nature, including gravity. While this widely accepted theory predicts at least 10 dimensions, however, no one has ever found more than one dimension in time and three in space.
According to one alternative theory, these additional dimensions might be curled up into a ball so small -- significantly smaller than atoms -- that they are difficult or impossible to observe. Matchev said his team believes these dimensions may give rise to heavier versions of known particles, the lightest of which could constitute the elusive dark-matter particle.
"This phenomenon of extra dimensions provides a completely new dark-matter candidate," Matchev said. "We named it Kaluza-Klein dark matter, after the two physicists who first proposed theories with extra dimensions in the early 1920s."
Most important is that Kaluza-Klein dark matter may be detected using a variety of current and future experiments, Matchev said. In addition to dedicated underground searches designed specifically to look for dark-matter particles, Kaluza-Klein particles may give distinct, albeit indirect, signals in numerous other experiments, he said.
For example, an ongoing experiment on the South Pole designed to detect elementary particles called neutrinos -- as well as an antimatter detector set to be placed aboard the International Space Station -- could be used to find these heavier particles.
The South Pole device, known as the Antarctic Muon Neutrino Detector Array, or AMANDA, is designed to detect particles with no electrical charge and no mass created in massive cosmic events such as supernovas.
But this "neutrino telescope" also may pick up telltale high-energy neutrinos necessarily created when dark-matter particles collide where they are most concentrated, at the gravitational centers of stars and planets. The detection of these types of neutrinos from these areas would provide indirect evidence of dark matter, Matchev said.
"Most of the stuff produced by dark-matter particle collisions is probably absorbed in the dense cores of the sun or the Earth, but the neutrinos, being so weakly interacting, escape and may reach our detectors," Matchev said. "So what we're looking for are unusual sources of neutrinos near gravitational centers."
Matchev said scientists also have a separate shot at detecting dark matter in a future antimatter detector, the Alpha Magnetic Spectrometer, slated to reach the International Space Station in 2005. The detector may pick up positrons, the antiparticles of electrons, similarly created when the dark-matter particles collide.
"If we see more positrons than we expect, then we know there is something going on," Matchev said. "What is more, the positron signal is rather unique for Kaluza-Klein dark matter and may thus provide the first evidence of extra dimensions."
Yet another experimental apparatus, the Gamma Ray Large Area Space Telescope, is slated for satellite launch in 2006. This telescope could discover very high-energy photons and help nail down the identity of dark matter, Matchev said.
University of Florida
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