Sat, Dec 27, 2003 - Page 9 News List

Scientists detect cosmos' dark energy

Researchers are shedding light on the dark side in what has been termed the scientific breakthrough of the year

By Tim Radford  /  THE GUADIAN , LONDON


Welcome to the dark side. Around 73 percent of the universe is made not of matter or radiation but of a mysterious force called dark energy, a kind of gravity in reverse. Dark energy was listed as the breakthrough of the year in the US journal Science on Dec. 19.

The discovery -- in fact a systematic confirmation of a puzzling observation first made five years ago -- paints an even more puzzling picture of an already mysterious universe. Around 200 billion galaxies, each containing 200 billion stars, are detectable by telescopes. But these add up to only 4 percent of the whole cosmos.

Now, on the evidence of a recent space-based probe and a meticulous survey of a million galaxies, astronomers have filled in at least some of the picture.

Around 23 percent of the universe is made up of another substance, called "dark matter." Nobody knows what this undetected stuff could be, but it massively outweighs all the atoms in all the stars in all the galaxies across the whole detectable range of space. The remaining 73 percent is the new discovery: dark energy. This bizarre force seems to be pushing the universe apart at an accelerating rate, when gravitational pull should be making it slow down or contract.

"The implications for these discoveries about the universe are truly stunning," said Don Kennedy, the editor of Science. "Cosmologists have been trying for years to confirm the hypothesis of a dark universe."

Martin Rees, Britain's astronomer royal, called it a "discovery of the first magnitude."

The findings were made by an orbiting observatory called the Wilkinson Microwave Anisotropy Probe (WMAP). This measured tiny fluctuations in the cosmic microwave background, in effect the dying echoes of the Big Bang that launched time, space and matter in a tiny universal fireball.

These painstaking measurements were then backed up by the telescopes of the Sloan Digital Sky Survey, which mapped a million galaxies to see how they clumped together or spread out. Both confirmed that dark energy must exist.

The findings settle a number of arguments about the universe, its age, its expansion rate, and its composition, all at once. Thanks to the two studies, astronomers now believe the age of the universe is 13.7 billion years, plus or minus a few hundred thousand. And its rate of expansion is a bewildering 71km per second per megaparsec. One megaparsec is an astronomical measure, totting up to 3.26 million light years. Something latent in space itself is acting as a form of antigravity, exerting a push on the universe, rather than a pull.

Dark matter was proposed more than 20 years ago when it became clear that all the galaxies behaved as if they were far more massive than they seemed to be. All sorts of explanations -- black holes, brown dwarfs and undetectable particles that are very different from atoms -- have been suggested. None has been confirmed.

But dark matter exists, all the same. The dark energy story began in 1998 when astronomers reported that the most distant galaxies seemed to be receding far faster than calculations predicted. A study of a certain kind of supernova confirmed that they had not been misled -- the universe was indeed expanding ever faster, rather than decelerating.

The discovery that some unexpected and undetectable force was pushing the fabric of space apart seemed to confirm a famous observation decades ago by the British scientist JBS Haldane: "The universe is not only queerer than we suppose. It is queerer than we can suppose."

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