posted on Friday, December 18, 2009 05:30 AM
For 80 years, it has eluded the finest minds in science. But tonight it appeared that the hunt may be over for dark matter, the mysterious and invisible substance that accounts for three-quarters of the mass of the universe.
In a series of coordinated announcements at several US laboratories, researchers said they believed they had captured dark matter in a defunct iron ore mine half a mile underground. The claim, if confirmed next year, will rank as one the most spectacular discoveries in physics in the past century.
Tantalising glimpses of dark matter particles were picked up by highly sensitive detectors at the bottom of the Soudan mine in Minnesota, the scientists said.
Dan Bauer, head of the Cryogenic Dark Matter Search (CDMS), said the group had spotted two particles with all the expected characteristics of dark matter. There is a one in four chance that the result is due to some other effect in the underground detectors, Bauer told a seminar at the Fermi National Accelerator Laboratory, near Chicago.
Rumours that Bauer's group was on the verge of making an announcement surfaced on physicists' blogs a few weeks ago. Though tentative, tonight's results triggered an immediate wave of excitement in the science community.
"If they have a real signal, it's a seriously big deal. The scale on which people are looking for dark matter is vast," said Gerry Gilmore at Cambridge University's institute of astronomy. "Dark matter is what created the structure of the universe and is essentially what holds it together. When ordinary matter falls into lumps of dark matter it turns into galaxies, stars, planets and people. Without it, we wouldn't be here," Gilmore said.
Scientists have debated the existence of dark matter since 1933, when the Swiss astronomer Fritz Zwicky argued that a distant cluster of galaxies would fall apart were it not for the gravitational pull of some vast but invisible cosmic substance. It was named dark matter because it does not reflect or absorb light, making it impossible to observe with telescopes.
Last year, the Hubble telescope photographed indirect evidence in the form of a ghostly halo around a distant galaxy, caused by clumps of dark matter bending light from stars as it passed by. A year before that, scientists led by the British astronomer Richard Massey, at the California Institute of Technology, published the first 3D map of dark matter, which revealed how it clung around galaxies and held clusters of them together.
Dark matter is likely to be made up of a variety of invisible particles that not only explain the missing mass of the universe, but shed light on some of the most profound mysteries in science.
Some dark matter particles could explain why ordinary matter is not radioactive, while others may help scientists understand why time – so far as we know – always runs forward.
"The real impact of this is psychological, in that it shows we're getting close to being able to do a whole new kind of physics," Gilmore said. "We know there are properties of the universe that should correspond to new families of particles. One of the great mysteries is why time only goes in one direction, and one candidate to explain that is a dark matter particle."
Many scientists believe dark matter particles will turn out to be proof of a theory called supersymmetry, which predicts that every kind of particle in the universe is paired with a heavier twin. Finding evidence for supersymmetry is one of the major goals of the Large Hadron Collider at Cern, in Switzerland.
Dark matter particles are peculiar because they pass through objects as if they were not there. Their aloof nature has led scientists to name them weakly interacting massive particles, or Wimps. Vast amounts of these are thought to be constantly moving through the Earth and everything on it, us included, as the solar system spins around our galaxy.
The detectors at the Soudan mine are buried underground to shield them from other kinds of particles that bombard Earth from space. To detect dark matter, scientists have to wait for the extremely rare occasion when a dark matter particle knocks into an atomic nucleus in the detector and makes it vibrate.
Detectors in the mine will be upgraded in the new year before the search for more dark matter continues, Bauer said.
The hunt for dark matter
What is dark matter?
The night sky might seem full of stars and planets, but what we see is only 4% of the stuff of the universe. Some three-quarters is dark matter, an invisible substance that scientists believe is there because of the gravitational force it exerts.
What does dark matter do?
Dark matter stretches throughout space where it attracts ordinary matter that coalesces into galaxies of billions of stars and planets. It forms a kind of cosmic skeleton that gives the universe its structure. Many scientists believe they will find a family of invisible dark matter particles, each of which plays a different role in nature. Some may even explain why time always goes in the same direction.
Who came up with the idea?
The Swiss astronomer Fritz Zwicky postulated dark matter in 1933. He noticed that a distant cluster of galaxies would fall apart were it not for the extra gravitational pull of some mysterious unseen mass in space. Astronomers verified his prediction by showing that stars swirling around distant galaxies zipped around so fast they must be held in place by extra gravitational forces.
Does everyone believe in dark matter?
A minority of astronomers and physicists dismiss dark matter as a fudge. Instead, they suspect that the strength of gravity varies from place to place, in a way that explains why stars do not hurtle out of spinning galaxies. The theory is known as Modified Newtonian Dynamics (Mond).
Ian Sample, science correspondent guardian.co.uk, Thursday 17 December 2009