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"Dark matter" forms dense clumps in ghost universe (Forwarded)



 
 
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Old November 21st 03, 05:41 PM
Andrew Yee
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Default "Dark matter" forms dense clumps in ghost universe (Forwarded)

Media Relations
University of California-Berkeley

"Dark matter" forms dense clumps in ghost universe
By Robert Sanders, Media Relations

05 November 2003

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.

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.

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.

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.

"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."

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.

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."

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.

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.

"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."

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
galaxies made by Ma's colleague, UC Berkeley professor of astronomy Leo Blitz,
and his research group, among others.

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.

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."

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.

"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.

MOVIE CAPTION:
[http://astron.berkeley.edu/~cpma/movie.avi]
Computer simulation of the initial Hubble expansion and subsequent formation of
a galaxy-size halo of dark matter over the last 13.5 billion years -- 99 percent
of the lifetime of the universe. The simulation shows an intricate pattern of
swarming dark matter clumps, some of which may not host luminous matter such as
stars and gas. (Credit: Chung-Pei Ma, Ed Bertschinger)

 




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