- Berkeley - The "dark matter" that comprises
a still-undetected one-quarter of the universe is not a uniform cosmic
fog, says a University of California, Berkeley, astrophysicist, but instead
forms dense clumps that move about like dust motes dancing in a shaft of
light.
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- In a paper submitted this week to Physical Review D,
Chung-Pei Ma, an associate professor of astronomy at UC Berkeley, and Edmund
Bertschinger of the Massachusetts Institute of Technology (MIT), prove
that the motion of dark matter clumps can be modeled in a way similar to
the Brownian motion of air-borne dust or pollen.
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- Their findings should provide astrophysicists with a
new way to calculate the evolution of this ghost universe of dark matter
and reconcile it with the observable universe, Ma said.
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- Dark matter has been a nagging problem for astronomy
for more than 30 years. Stars within galaxies and galaxies within clusters
move in a way that indicates there is more matter there than we can see.
This unseen matter seems to be in a spherical halo that extends probably
10 times farther than the visible stellar halo around galaxies. Early proposals
that the invisible matter is comprised of burnt-out stars or heavy neutrinos
have not panned out, and the current favorite candidates are exotic particles
variously called neutrilinos, axions or other hypothetical supersymmetric
particles. Because these exotic particles interact with ordinary matter
through gravity only, not via electromagnetic waves, they emit no light.
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- "We're only seeing half of all particles,"
Ma said. "They're too heavy to produce now in accelerators, so half
of the world we don't know about."
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- The picture only got worse four years ago when "dark
energy" was found to be even more prevalent than dark matter. The
cosmic account now pegs dark energy at about 69 percent of the universe,
exotic dark matter at 27 percent, mundane dark matter - dim, unseen stars
- at 3 percent, and what we actually see at a mere 1 percent.
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- Based on computer models of how dark matter would move
under the force of gravity, Ma said that dark matter is not a uniform mist
enveloping clusters of galaxies. Instead, dark matter forms smaller clumps
that look superficially like the galaxies and globular clusters we see
in our luminous universe. The dark matter has a dynamic life independent
of luminous matter, she said.
-
- "The cosmic microwave background shows the early
effects of dark matter clumping, and these clumps grow under gravitational
attraction," she said. "But each of these clumps, the halo around
galaxy clusters, was thought to be smooth. People were intrigued to find
that high-resolution simulations show they are not smooth, but instead
have intricate substructures. The dark world has a dynamic life of its
own."
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- Ma, Bertschinger and UC Berkeley graduate student Michael
Boylan-Kolchin performed some of these simulations themselves. Several
other groups over the past two years have also showed similar clumping.
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- The ghost universe of dark matter is a template for the
visible universe, she said. Dark matter is 25 times more abundant than
mere visible matter, so visible matter should cluster wherever dark matter
clusters.
-
- Therein lies the problem, Ma said. Computer simulations
of the evolution of dark matter predict far more clumps of dark matter
in a region than there are clumps of luminous matter we can see. If luminous
matter follows dark matter, there should be nearly equivalent numbers of
each.
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- "Our galaxy, the Milky Way, has about a dozen satellites,
but in simulations we see thousands of satellites of dark matter,"
she said. "Dark matter in the Milky Way is a dynamic, lively environment
in which thousands of smaller satellites of dark matter clumps are swarming
around a big parent dark matter halo, constantly interacting and disturbing
each other."
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- In addition, astrophysicists modeling the motion of dark
matter were puzzled to see that each clump had a density that peaked in
the center and fell off toward the edges in the exact same way, independent
of its size. This universal density profile, however, appears to be in
conflict with observations of some dwarf
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- galaxies made by Ma's colleague, UC Berkeley professor
of astronomy Leo Blitz, and his research group, among others.
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- Ma hopes that a new way of looking at the motion of dark
matter will resolve these problems and square theory with observation.
In her Physical Review article, discussed at a meeting earlier this year
of the American Physical Society, she proved that the motion of dark matter
can be modeled much like the Brownian motion that botanist Robert Brown
described in 1828 and Albert Einstein explained in a seminal 1905 paper
that helped garner him the 1921 Nobel Prize in Physics.
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- Brownian motion was first described as the zigzag path
traveled by a grain of pollen floating in water, pushed about by water
molecules colliding with it. The phenomenon refers equally to the motion
of dust in air and dense clumps of dark matter in the dark matter universe,
said Ma.
-
- This insight "let's us use a different language,
a different point of view than the standard view," to investigate
the movement and evolution of dark matter, she said.
-
- Other astronomers, such as UC Berkeley emeritus professor
of astronomy Ivan King, have used the theory of Brownian motion to model
the movement of hundreds of thousands of stars within star clusters, but
this, Ma said, "is the first time it has been applied rigorously to
large cosmological scales. The idea is that we don't care exactly where
the clumps are, but rather, how clumps behave statistically in the system,
how they scatter gravitationally."
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- Ma noted that the Brownian motion of clumps is governed
by an equation, the Fokker-Planck equation, that is used to model many
stochastic or random processes, including the stock market. Ma and collaborators
are currently working on solving this equation for cosmological dark matter.
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- "It is surprising and delightful that the evolution
of dark matter, the evolution of clumps, obeys a simple, 90-year-old equation,"
she said.
-
- The work was supported by the National Aeronautics and
Space Administration.
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-
- This story has been adapted from a news release issued
by University Of California - Berkeley.
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