http://www.guardian.co.uk/international/story/0,3604,1110212,00.html?79%3A+International+news+%2D+guardian
Science breakthrough of the year: proof of our exploding universe
Tim Radford, science editor
Friday December 19, 2003
The Guardian
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."
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