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What\'s the big rush?

What's the big rush? 07 Apr 01

Light from the oldest supernova ever seen suggests the Universe is expanding faster and faster

THE most distant supernova ever observed appears to have blown its top when the expansion of the Universe was slowing down. Ironically, this observation boosts the idea that the Universe is filled with "dark energy" that stretches space and is now making it expand faster. Adam Riess of the Space Telescope Science Institute in Baltimore and Peter Nugent of the Lawrence Berkeley National Laboratory in California spotted the explosion in Hubble Space Telescope data.

Because the Universe is expanding, distant stars and galaxies recede from Earth and their light is stretched out, pushing it towards the red end of the spectrum. Hence, assuming the Universe expands predictably, you can judge how far away a star is by the size of its red shift.

Two years ago, two teams of astronomers reported that distant stellar explosions known as type Ia supernovae, which always have the same brightness, appeared about 25 per cent dimmer from Earth than expected from their red shifts. That implied that the expansion of the Universe has accelerated. This is because the supernovae were farther away than they ought to have been if the Universe had been expanding at a steady rate for the billions of years since the stars exploded.

But some researchers have argued that other phenomena might dim distant supernovae. Intergalactic dust might soak up their light, or type Ia supernovae from billions of years ago might not conform to the same standard brightness they do today.

This week's supernova finding seems to have dealt a severe blow to these arguments against an accelerating Universe. The new supernova's red shift implies it is 11 billion light years away, but it is roughly twice as bright as it should be. Hence it must be significantly closer than it would be had the Universe expanded steadily. Neither dust nor changes in supernova brightness can easily explain the brightness of the explosion.

Dark energy can, however. When the Universe was only a few billion years old, galaxies were closer together and the pull of their gravity was strong enough to overcome the push of dark energy and slow the expansion. A supernova that exploded during this period would thus be closer than its red shift suggests. Only after the galaxies grew farther apart did dark energy take over and make the Universe expand faster. So astronomers should see acceleration change to deceleration as they look farther back in time. "This transition from accelerating to decelerating is really the smoking gun for some sort of dark energy," Riess says.

More data is needed to clinch the case, says Ira Wasserman of Cornell University in Ithaca, New York. "I'd be a little reluctant to put too much faith in one supernova," he says. But the lone observation is enough to rule out other ideas, says Michael Turner of the University of Chicago: "This supernova has driven a stake through the heart of more conventional explanations that try to avoid cosmic speed-up."

Adrian Cho

From New Scientist magazine, vol 170 issue 2285, 07/04/2001, page 6