IF YOU thought the fate of the Universe was certain, think again. Last year,
in a blaze of publicity, it was announced that the Universe would expand forever
in the grip of an exotic 鈥渁ntigravity鈥� force. But now the evidence is in
doubt.
The conclusion depended on the observation that exploding stars in distant
galaxies seem fainter than expected. This suggested that the expansion of the
Universe is speeding up because of a mysterious force, represented by a
cosmological constant in Einstein鈥檚 equations of general relativity
(鈥淭o infinity and beyond鈥�, 快猫短视频, 11 April 1998, p 26).
But that holds true only if these distant supernovae are inherently as bright
as the stellar explosions happening nearby. The new findings hint that this may
not be true, and reopen the possibility that the Universe鈥檚 expansion will
cease鈥攁nd maybe even reverse until all matter collapses in a 鈥渂ig crunch鈥�.
鈥淭his is definitely a warning flag,鈥� says Robert Kirshner of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
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Last year鈥檚 conclusions came from teams led by Brian Schmidt of the Mount
Stromlo and Siding Springs Observatories in Australia and Saul Perlmutter of the
Lawrence Berkeley Laboratory in California. They studied 鈥渢ype Ia鈥� supernovae.
These occur when matter from one star falls onto a companion white dwarf star,
which then becomes unstable and explodes.
In galaxies near ours, the explosions grow brighter over about 20 days, then
fade over the following months. But the peak intrinsic brightness is always
roughly the same鈥攚hich means that how bright an explosion appears should
depend only on how far away it lies. So when supernovae billions of light years
away were revealed to be consistently fainter than expected, astronomers
concluded that an acceleration in the rate of the Universe鈥檚 expansion was
forcing the light to travel a greater distance.
But a nagging doubt remained. Might distant supernovae be less
powerful鈥攁nd therefore intrinsically less bright鈥攖han those nearby?
Adam Riess of the University of California at Berkeley, a member of Schmidt鈥檚
team, decided to check. If the peak brightness of the distant supernovae is
different, the time they take to reach this point should also be different. So
Riess and his colleagues studied the rate at which 10 nearby explosions
brightened and compared it with provisional data from Perlmutter鈥檚 team for over
30 distant supernovae. They found that the distant supernovae lit up more
rapidly, reaching peak brightness around two days earlier.
Why distant supernovae are different is unclear. They could have different
compositions. 鈥淥r it could be something different about the explosion itself,
although that would be hard to understand,鈥� says Riess.
Nor is it certain that the faster brightening means that the brightness of
distant supernovae is different from those nearby. But if those 5 billion light
years away are less than 75 per cent as luminous as nearby ones, the case for
accelerating expansion would collapse. 鈥淚t鈥檚 hard to make an iron-clad case for
it when you see something like this,鈥� says Riess, who has sent a paper on the
findings to The Astronomical Journal.
Perlmutter is keeping an open mind. 鈥淚t鈥檚 very premature to draw a
conclusion,鈥� he says. 鈥淭he analysis has not been completed.鈥� But unless the
discrepancy vanishes, cosmologists will be back to what they do best: arguing
about the ultimate fate of the Universe.
