Volume 1, Issue 1
The Mysterious Matter of Dark Matter
Chung-Pei Ma recently formed a string quartet (musical strings, that is) with two Berkeley computer scientists and a psychologist. (Wendy Edelstein photo)
Every day, UC Berkeley astronomy professor Chung-Pei Ma is reminded how little we understand about our universe. Familiar particles like protons and neutrons make up just a tiny fraction of the total mass and energy of the universe, perhaps just one percent. The rest--appropriately dubbed dark matter and dark energy--is literally invisible to us. Ma's research aim is to shine light on this ghostly universe surrounding us. Her efforts could aid scientists in understanding the evolution and destiny of the cosmos.
Dark matter was first proposed in the 1930s when astronomers noticed that the motion of galaxies and clusters of galaxies they observed did not jibe with the visible mass. Since then, dark matter and dark energy have theoretically been implicated in the expansion of the universe.
"The problem is that we don't know what dark matter and dark energy are," Ma says. "And we need more information to theorize about it."
Ma's latest results revolve around dark matter. Currently, she says, there are two primary candidates that may make up the stuff. The first are dead planets or stars that emit no detectable light but do interact with ordinary matter through gravitation.
"Our galaxy might be littered with the dead planets or stars, but the numbers still don't add up," Ma says. "Those big things that you can touch are probably just a small portion of dark matter."
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.
(courtesy Chung-Pei Ma and Ed Bertschinger)
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The second and more popular theory is that most dark matter consists of elementary particles like neutralinos or other theoretical supersymmetric particles. Indeed, researchers at Berkeley are attempting to detect these elementary particles in particle accelerators. According to Ma, using computer simulation to "profile the missing matter," analogous to the way criminal investigators determine the characteristics of an unknown perpetrator, will aid in the design of better detectors.
"Fortunately, we understand the forces behind dark matter, such as gravity," she says. "So we can postulate a number of things and then do large supercomputer simulations of a patch of the universe, maybe just hundreds of millions of light years across."
Already, Ma's simulations have shown that dark matter is not distributed smoothly, like a fog enveloping galaxies and clusters of galaxies. Rather, it clumps together into satellite galaxies, not unlike the luminous formations in the visible universe. This should mean that the dark matter universe somewhat mirrors the visible universe. The rub is that it doesn't, she says. Computer simulations predict that that the clumps of dark matter are far more abundant in a particular region than luminous matter.
To help resolve this conflict and others, Ma and Edmund Bertschinger of the Massachusetts Institute of Technology recently proved that Brownian motion--a phenomenon first explained by Einstein nearly one hundred years ago--can also be employed to model the dynamics of dark matter. Originally used to describe the way a grain of pollen travels through water, Brownian motion refers to the chaotic motion of particles as they're impacted by smaller particles. UC Berkeley professor emeritus of astronomy Ivan King previously applied the theory of Brownian motion to model the movement of stars within clusters, but this latest work, Ma says, "is the first time it has been applied rigorously to large cosmological scales."
To predict Brownian motion and other random phenomena, scientists refer to a standard mathematical formula called the Fokker-Planck equation. Ma and her colleagues are currently attempting to solve the equation for dark matter so that more advanced computer simulations can be created to better understand how the clumps move.
"Through more accurate computer modeling, we can begin to compare theoretical predictions about dark matter with our observations of the universe." Ma says.