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Aiming to grasp mysterious dark matter

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For the past quarter century, dark matter has been a mystery we've just had to live with. But the time may be getting close when science can finally unveil what this befuddling stuff is that makes up most of the matter in the universe.

Dark matter can't be seen. Nobody even knows what it is. But it must be there, because without it galaxies would fly apart.

Upcoming experiments on Earth such as the Large Hadron Collider (LHC) particle accelerator in Switzerland, and a new spacecraft called Gaia set to launch in 2011, could be the key to closing the case on one of the biggest unsolved mysteries in science.

A disturbing truth is accepted by most astronomers: There is a lot more stuff in the universe than what we can see. Scientists now think visible matter — all the planets, stars, and galaxies that shine down on us — represents only about 4 percent of the mass-energy budget of the universe, while dark matter and its even more esoteric cousin, dark energy, make up the rest.

"There is no consensus actually at all as to what dark matter is," said Gerard Gilmore, an astronomer at the University of Cambridge who wrote a recent essay for the Dec. 5 issue of the journal Science about the search for dark matter.

A leading hypothesis posits that dark matter is composed of some kind of exotic particle, yet to be detected, that doesn't interact with light, so we can't see it. One such theorized class of particles is called WIMPs (Weakly interacting massive particles), which are thought to be neutral in charge and weigh more than 100 times the mass of a proton.

Atom smasher
The newly-opened LHC, a 17-mile-long underground ring in which sprays of protons speed around and crash into each other, could be the first experiment to detect WIMPS. The particle accelerator officially went online in September 2008, but was halted shortly after due to a fault with its construction — it's due to go back online in the summer of 2009. Since the LHC is the largest and most powerful atom smasher ever built, its collisions could produce the extremely high energies needed to create the elusive particles.

In fact, the LHC will likely create a host of never-before-seen particles, opening up a realm of the universe that physicists have been eager to explore.

"The assumption is, there will be whole families of new types of particles," Gilmore said in a podcast interview with a reporter from Science. "The challenge then is to say, well OK, we now then have a new set of ingredients in our recipe for how nature is put together, but what is the recipe that uses this set of ingredients? I.e., what mix of these particles does nature actually use to create the universe, and how?"

That's where Gaia comes in. The European Space Agency satellite is designed to measure positions and speeds of about 1 billion nearby stars with unprecedented precision. Its vision is so sharp it should be able to discern the equivalent of a shirt button on the surface of the moon as seen from Earth, Gilmore said.

By establishing where things are in our galaxy, the spacecraft will help scientists measure the weight and distribution of mass in the Milky Way in much greater detail than ever before. These measurements are vital for models that attempt to describe how the pull of dark matter has shaped our galaxy.

"What Gaia will do is measure the distances of stuff and measure how they're moving in three dimensions around space to much better precision than we've had before, which will allow us to weigh things on all sorts of scales down to the smallest scales we can find," Gilmore said. "They will tell us to exquisite precision how the dark matter is distributed in space, which is the recipe we need to determine its properties."

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