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 鈥渄ark energy鈥� 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.
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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鈥檚 supernova finding seems to have dealt a severe blow to these
arguments against an accelerating Universe. The new supernova鈥檚 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. 鈥淭his transition from
accelerating to decelerating is really the smoking gun for some sort of dark
energy,鈥� Riess says.
More data is needed to clinch the case, says Ira Wasserman of Cornell
University in Ithaca, New York. 鈥淚鈥檇 be a little reluctant to put too much faith
in one supernova,鈥� he says. But the lone observation is enough to rule out other
ideas, says Michael Turner of the University of Chicago: 鈥淭his supernova has
driven a stake through the heart of more conventional explanations that try to
avoid cosmic speed-up.鈥�