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by Elizabeth Pierce
As much as science has advanced in the past few centuries, we still have much to learn about the way our universe works. Several researchers in the School of Physics and Astronomy are tackling one of the biggest problems of all.
Because the gravity exuded by the mass of visible objects in the universe does not explain the motions of galaxies and other astronomical bodies, scientists theorize that there must be another kind of matter in a form we cannot detect using current methods. Called "dark matter," this matter is believed to make up roughly 80% of the mass of the universe.
The fact that scientists cannot positively identify so much of the universe leaves some researchers uneasy. “It’s embarrassing to admit in class,” says Evan Skillman, an astronomy professor at the University of Minnesota. “If anyone discovers [dark matter], there will be a huge sigh of relief.”
Several theories attempt to explain what makes up the mass of dark matter. Among the candidates: white dwarf stars, neutrinos, weakly interacting massive particles (WIMPs), and various other exotic particles. There are also suggestions that dark matter does not exist. Some scientists argue that we simply do not understand gravity, so what appears to be discrepancies in the motion of matter located in the outer parts of galaxies may be the unknown effects of gravity.
An alternative to Newton’s gravitational theory is Modified Newtonian Dynamics, which suggests that as you get farther from the center of a mass, gravity gets weaker at a greater rate than regular Newtonian laws predict. The theory which works well for galaxies and clusters, but breaks down when applied to dwarf galaxies.
Skillman, an observational astronomer, focuses on dwarf galaxies. He is currently studying a nearby dwarf galaxy, named NGC 6822, which is approximately 1.63 million light years away from Earth, close by astronomical standards.
Using radio measurements taken by the Very Large Array in New Mexico, Skillman has accurate measurements of the galaxy's rotation. When the rotational velocity of points in the galaxy are plotted against their radius from its center, there is no evidence supporting the theory of Modified Newtonian Dynamics. That is, the detectable mass in the galaxy is not enough to account for its movement and rotation.
“Something has to be there,” agrees Heidi Brandenburg, a research assistant on the project and an undergraduate physics, astrophysics and computer science major. That something is the presence of dark matter.
Shaul Hanany, an observational cosmologist, calls the search for dark matter “the most important scientific goal for astrophysics for this decade.”
His research is much different than Skillman's. Using balloons which rise to an altitude of 120,000 feet and carry a payload of scientific instruments, Hanany studies cosmic microwave background radiation, the radiation which was released when the universe was much younger.
“It is the glow from the ashes of the Big Bang,” Hanany explains. Studying this radiation allows astrophysicists to observe directly the universe at its earliest observable point in time, when it was roughly 1/100,000 of its present age.
Data from the balloon flights are used to create detailed graphs of the fluctuations in cosmic background radiation. The results of the research give Hanany a better idea of how much dark matter is present and a general idea of what it is composed of.
“It will not tell us exactly what it is," says Hanany, "but it will tell us whether [dark matter] is primarily made up of WIMPs or another particle.”
As an experimentalist, Hanany builds his own equipment in order to measure the universe. He is currently in Palestine, Texas, launching one of his balloon projects, and construction of another balloon, which will be launched from Sicily this summer, is also nearing completion.
While Skillman and Hanany use advanced equipment in their quest for dark matter, Keith Olive uses pencil and paper. Olive, a theoretical physicist, focuses on the particle composition of dark matter. He believes dark matter is one stableparticle, possibly a variety of WIMP called the neutralino, whose properties have already been predicted.
According to Olive, this mysterious particle can either be discovered directly, by creating dark matter or similar, more easily created, particles in an accelerator, or indirectly, by observing their interactions with other atoms in neutrino detectors like the one being built in a Minnesota mine (see related story).
Olive himself does not work with particle accelerators or detectors. “What I do is interpret the results, if they get any, and use current experimental results to constrain the theory,” he says.
If dark matter particles are ever discovered, besides relieving some embarrassment, it will help answer many questions in cosmology. “Dark matter is an important component of how galaxies form,” Skillman says. “We will learn how dark matter collapses to form galaxies.” Olive and Hanany are also hoping to learn about galaxy formation and the evolution of our universe.
The observational, experimental and theoretical research being conducted at the University of Minnesota will help answer one of today’s greatest scientific questions: What is dark matter? The truth about dark matter is out there, and someday Skillman, Hanany, Olive and their colleagues hope to find it.
