GAINESVILLE, Fla. — 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.
