May 25, 2013
Scientists are hot on the tail of one of nature?s most elusive substances, the mysterious dark matter that is thought to make up the bulk of the universe. Many scientists think dark matter might even be hiding right under our noses here on Earth.
Dark matter is especially tricky to find because of its dark nature. In fact, scientists don?t know what it is. It doesn?t emit or reflect any light, so the most powerful telescopes have no hope of spying it directly. It has been thought to exist since the 1970s based on observations of gravity?s effects on large-scales, such as among and between galaxies ? regular matter can?t account for the amount of gravity at work.
And dark matter doesn?t often interact with most other matter, scientists theorize. One idea is that it flies right through the Earth, your house, and your body without bouncing off atoms.
Some scientists have taken to underground searches in hopes of catching just a few out of the multitude of dark matter particles in a rare instance of actually bouncing off of a regular particle.
?They?re just streaming right though us, and every once in while there?s an interaction,? said Angela Reisseter of the University of Minnesota, a member of a project called the Cryogenic Dark Matter Search (CDMS). She spoke this month at the meeting of the American Physical Society in Washington, D.C.
Dark matter detected?or was it?
In a recent issue of the journal science Express, Reisseter and her colleagues reported finding two possible events that may or may not be dark matter impacts on their detectors.
?Our previous results have been no, no, no,? Reisseter said. ?This is our first maybe ? that is all it is.?
The CDMS is buried in a mine in Minnesota underneath about 766 yards (700 meters) of rock, plastic, lead, copper and other materials designed to stop everything but dark matter from reaching the experiment ? thus cosmic rays and other particles that might be confused for dark matter particles will mostly be eliminated.
The detectors themselves are basically small, hockey-puck shaped blocks of the elements germanium and silicon. If the nucleus from one of the germanium or silicon atoms is hit by a dark matter particle, then it will rebound and send a signal to the detector.
However the researchers can?t be absolutely sure that the two signals they measured were dark matter and not some other particle, which they call background. Two signals is just too few to be confident of, they said, because their calculations predicted about one false event from background.
?If it was one, we?d say ?Oh, it?s the background.? If it was three you start to say ?Oh, it?s a signal,?? Reisseter said. ?We can?t call it background and we can?t call it signal.?
The CDMS team intends to keep running their experiment at ever more sensitive levels in hopes that a more substantial signal is discovered.
Dark matter hunt goes on
Other attempts to track down dark matter on Earth have focused on powerful particle accelerators that speed up subatomic particles to close to the speed of light and then smash them together, hoping the incredibly high collision energies create exotic particles, including dark matter.
But even with our increasingly powerful atom smashers, no sign of dark matter has yet been spotted.
?You have to ask why would this be?? said Sarah Eno of the University of Maryland. ?Why would the particle that makes up most of the matter in the universe never have been seen in our accelerators??
One reason could be that they just aren?t powerful enough. Scientists aren?t sure how massive the dark matter particle might be, and certain possibilities require extremely high energies to create them in the laboratory. Or it might be impossible find at any accelerator.
?We don?t know for a fact that the dark mater particle is a particle we would be able to produce and detect,? Eno said.
The best hope now could be a new particle accelerator ? the Large Hedron Collider (LHC) near Geneva, Switzerland ? that is the largest ever built. It opened recently and isn?t yet running at full speed. When it does, many are holding out hope that dark matter will finally be pinned down.
?It could be that now we have this new machine we?ll finally have enough energy to make this dark matter particle and see it in our collisions,? Eno said. She is a member of the compact Muon Solenoid experiment team at the LHC.
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