Dark Matter Properties "Measured" for First Time, Study Says

James Owen
for National Geographic News
February 13, 2006
How do you measure something you can't see or feel?

If you're a researcher at Cambridge University's Institute of Astronomy in England, the answer is: Use painstaking calculations and a really big telescope.

With the help of data from the world's most advanced optical array, Cambridge scientists announced last week that they may be a step closer to identifying properties of dark matter.

This mysterious yet pervasive substance could be the "cosmic glue" that keeps rotating galaxies from spinning apart.

"Pretty much nothing had been known about dark matter," said Gerry Gilmore, a professor of experimental philosophy and lead researcher on the project.

"All that we knew was that it was transparent and it was heavy. We knew it had weight because it's what holds stars in the sky; without it they'd all fly off into space."

Now the researchers report that, according to their calculations, dark matter travels at around six miles (nine kilometers) per second.

And if it were made of hydrogen atoms—hydrogen being the universe's most abundant element—its temperature would reach 10,000° C (18,000° F)—hotter than the surface of the sun.

Abundant but Elusive

Dark matter is thought to make up almost a quarter of the universe, making it six times more abundant than visible matter, such as planets, stars, and cosmic gases.

The existence of this theoretical substance was first proposed in the 1930s by Swiss astrophysicist Fritz Zwicky.

By studying the rotation of a group of galaxies called the Coma Cluster, Zwicky calculated that the visible mass of the galaxies was 400 times less than the mass needed to explain their gravitational motion.

He concluded that some massive invisible force must also be at work, but scientists have been unable to detect and identify such matter.

In the latest study, the British team used the Very Large Telescope in Acatama, Chile, to gauge the temperature and distribution of dark matter in ten dwarf galaxies around the Milky Way.

"We measured how big a galaxy is and how fast its stars are moving," Gilmore said. "Using Newton's Laws we worked out how much mass there is holding it together."

Each galaxy was found to contain the same amount of dark matter, equivalent to roughly 30 million times the mass of the sun.

"That was the big surprise," Gilmore added. "The galaxies contain very different numbers of stars, so like everybody else we thought they would cover a very wide range of masses, but they don't. There seems to be a minimum mass for a galaxy."

The finding suggests dark matter has a density equivalent to four atoms of hydrogen per cubic centimeter of space, meaning the smallest volume it can be packed into is a cube measuring 1,000 light years along each side.

This in turn implies how fast its particles move—at a speed that doesn't allow dark matter to be compressed any further.

"This property of its minimum speed looks like the first property we've got [for dark matter]," Gilmore added.

The study team's results haven't yet been published and scrutinized by peers. But astrophysicist Bob Nichol from the University of Portsmouth, England, believes the findings represent "a quantum leap in our understanding of dark matter."

The research, Nichol says, "is potentially telling us that you can't squeeze a lot of dark matter into a small space. If you do it will push back, which is where this temperature comes from. It's exceptionally exciting."

Heavier, Colder

Most scientists had assumed previously that dark matter was a denser, relatively cold, sluggish substance.

The new findings suggest dark matter interacts with more than just gravity, Nichol adds.

"There seems to be a pressure to dark matter," he said. "That's telling us there is some interaction either with itself or with the ordinary matter."

Nevertheless, these new clues to the properties of dark matter won't necessarily make it any easier for scientists who are currently trying to record the elusive substance.

"If anything this is going to make it harder for them, because it's easier to find a heavier particle than a lighter-mass particle, as the lighter ones have that much less energy," Gilmore, the study team leader, said.

The best short-term hope, Gilmore says, is the world's largest particle accelerator, the Large Hadron Collider, due to open next year in Geneva, Switzerland.

The eight-billion-U.S.-dollar, 17-mile-long (27-kilometer-long) underground installation aims to smash protons together at the speed of light, recreating conditions that existed a fraction of a second after the Big Bang.

According to Big Bang theory, the universe violently emerged from an enormously dense, hot state and has been expanding ever since. (Read a related National Geographic magazine feature.)

"The stuff we're looking for is the most common form of mass [in the universe], so it must have come straight out of the Big Bang," Gilmore said.

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