Princeton researchers shedding light on dark matter
Steven Schultz
Princeton NJ -- Something mysterious is hiding in the midst of everything around us -- in the earth, throughout the air and among the stars. It is invisible, yet scientists know it pervades all space and, in fact, makes up 90 percent of matter in the universe.
For nearly 70 years, scientists have struggled to establish anything more than the most cursory hypotheses about the nature of this elusive substance called dark matter.
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Astrophysicists
Jeremiah Ostriker and Paul Bode used supercomputers
to simulate the evolution of the universe and how
it was affected by an elusive substance called dark
matter. They compared one scenario in which dark
matter starts with essentially no temperature and
another in which it has a slightly higher
temperature. The images here show the results for a
large swath of the universe measuring 10 million
light years. In the cold dark matter picture above,
many small galaxies form between large clusters of
galaxies. In the warm dark matter model below, many
fewer small galaxies form, leaving large voids
between the big clusters. The latter more closely
matches actual observations of the
sky.Several Princeton-based groups have proposed competing theories that promise to provide the first clues to the mass, size and other properties of dark matter particles. The results could have profound consequences throughout the field of cosmology.
The advances have generated excitement among cosmologists, in part because they are readily testable. Within the next year, astronomers are expected to have gathered enough data from ongoing sky surveys to declare which of the theories is correct. Although the dark matter itself cannot be seen, the new theories lay out a series of telltale signs to look for in the structure and distribution of nearby galaxies.
"This is one of the key issues in cosmology: What does this dark matter consist of?" said physicist Paul Steinhardt, a proponent of one of the new theories. "I think it's great that we have a number of new ideas that address the problem and that suggest new observations we can make."
From problems to opportunities
The existence of dark matter was proposed by the Swiss astronomer Fritz Zwicky in 1933, and the evidence became stronger in the early 1970s when Jeremiah Ostriker and James Peebles of Princeton calculated that normal material, visible in the form of stars, does not exert enough gravitational pull to keep galaxies from flying apart. In fact, their calculations showed that normal, visible matter makes up only about 10 percent of the mass needed to give galaxies their present structure.
To make up the difference, they proposed the existence of an exotic form of matter that does not absorb or emit light, does not collide with other particles, and has the sole trait of exerting gravitational pull. The proposed material also was assumed to have accumulated from widely distributed particles with essentially no velocity and thus a very low temperature.
Based on the work of many scientists, this cold dark matter theory has become a central tenet of cosmology. It has been supported by indirect observations and has worked remarkably well in explaining how the universe evolved to its present structure from the Big Bang. It accounts for the large-scale elements of the universe -- the distribution and properties of large galaxies and clusters of galaxies.
New observations and calculations, however, have convinced astrophysicists that the cold dark matter theory has significant flaws when it comes to explaining smaller-scale features. Scientists determined, for example, that a universe built of cold dark matter would have a much more cluttered look than is actually observed, with many small galaxies and clusters of stars filling up regions of open, intergalactic space.
"If these problems are real, they present an opportunity," said Ostriker, who has proposed one of the new theories. "They may be giving us a clue. They may be revealing the physical properties of dark matter."
Bumping and boiling
Steinhardt said he first began to give serious consideration to a new theory of dark matter in August 1999 when he and Princeton astrophysicist David Spergel were stuck for two hours on a stalled train outside London. The two had just been to a conference, which Steinhardt had organized, where, for the first time, all the various problems with the cold dark matter theory were laid out at once. They began to look for a common thread.
It was not good enough, they decided, to look at the size of the structures that did not mesh with the cold dark matter theory. Rather, they noticed that it helped to consider the density of matter within the structures -- problems seemed to arise when matter reached a particular range of densities. That insight led Steinhardt and Spergel to consider the possibility that dark matter particles are bigger than previously thought and that bulkiness makes them prone to bumping into each other. So when dark particles get squeezed and begin to clump together to form galaxies, they start to collide with a frequency that tends to break structures apart.
This theory, which was presented in September 1999 and published five months later, would explain, for example, why there are so few dwarf galaxies surrounding our own Milky Way. As dwarf galaxies, pulled by gravity, are drawn toward a large galaxy, like our Milky Way, dark matter in the dwarf galaxy collides with the faster moving dark matter orbiting the large galaxy.
"As the dwarf galaxy moved closer to the big galaxy, its dark matter would be boiled away and the dwarf galaxy would break up," said Steinhardt.
Warm and wiggly
Ostriker and Princeton colleague Paul Bode have a different idea, in which the theory of "cold" dark matter is modified to become "warm" dark matter.
In a paper to be published this summer in the Astrophysical Journal, Ostriker and Bode, along with Neil Turok of Cambridge University, suggest that dark matter particles are likely to be much lighter and to have a history of somewhat higher temperatures than previously believed. This small difference in temperature -- still just a little above zero -- would have a dramatic effect on the way small structures evolve.
Using powerful computers, Ostriker and Bode simulated how the universe evolved from shortly after the Big Bang to the present, and compared the results when they started with dark matter particles of different masses. With heavier, colder particles, small structures began to coalesce about 500 million years after the Big Bang. With lighter, warmer particles, fewer small structures formed, and those that did began to take shape about 1.5 billion years after the Big Bang. Our current 14-billion-year-old universe looks more like one that evolved from the second case rather than the first.
To explain the difference, Ostriker likened the early universe to the surface of a pool table that has lots of dips and irregularities in it. The dark particles are like billiard balls that move quickly when warm and slowly when cold. The balls would tend to settle into the dips if moving slowly, but not if they move quickly.
"It's very analogous to the universe," said Ostriker. "What makes the ball stay in the hole? Gravity. You have gravitational fluctuations in the universe, and the question is: Does dark matter stay in these fluctuations? The hotter it is, the less it will settle."
If the fluctuations are big enough, however, even hot particles will fall in. "So warm dark matter does not much affect the big structures, but it affects the little ones a lot," said Ostriker.
These are not the only ideas at Princeton competing to unseat the standard cold dark matter theory. Astrophysicist Jeremy Goodman has proposed a theory in which dark particles have a repulsive, rather than attractive affect on each other. Renyue Cen, also in the astrophysics department, has suggested a model in which dark matter particles have a pattern of decaying in a way that explains many of the same problems.
Collaboration and planning
Despite their competition, the advocates of the theories have found inspiration in each other's ideas. Ostriker, for example, recently published a paper that used the Steinhardt-Spergel theory and shows how it may answer a different question about why enormous black holes form at the centers of galaxies.
Steinhardt, Ostriker, Spergel and Turok, who was formerly a professor of physics at Princeton, currently are working on a paper that reviews the theories and suggests what observations to perform so that scientists can make the best appraisal of which ideas are correct.
"We all have our personal favorites based on our own prejudices," said Steinhardt. "Fortunately, you solve the problem by observational tests rather than theoretical preferences."
At least part of the data needed to make those checks
will become available over the next year as a result of the
Sloan Digital Sky Survey, a large Princeton-initiated effort
to map 100 million objects across one-quarter of the entire
sky.
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