Posted on 19-12-2003

Science breakthrough of the year: proof of our exploding universe

Tim Radford,

Welcome to the dark side. Around 73% 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 is listed as the breakthrough of the year in the US journal Science today.
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 200bn galaxies, each containing 200bn stars, are detectable by telescopes. But these add up to only 4% 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% 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% 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."

Sir 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 13.7bn 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 3.26m 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." It once again raised profound questions about the nature of the universe: about space, and time, and energy, and matter. And it set the theorists on the hunt first for an explanation, and then for an experiment that would confirm their hypothesis.

So they turned once again to the original evidence for the Big Bang, the cosmic microwave background radiation. This is the original blaze of creation, cooled to minus 270 C - just about 3 C above absolute zero. Several lines of research, including experiments in the Antarctic and from high-flying balloons, began to provide a clearer picture: the universe simply had to consist of something more than just atoms and so-called dark matter.

"But WMAP, with superbly precise data beamed back from a little spacecraft a million miles away, has made the evidence more precise," said Sir Martin, of the Institute of Astronomy at Cambridge.

"The dark energy is spread uniformly through the universe, latent in empty space. Its nature is a mystery. Whereas there's a real chance of learning what the dark matter is within the next five to 10 years, I'd hold out less hope of understanding the dark energy unless or until there's a unified theory that takes us closer to the 'bedrock' of space and time."