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 "antigravity" 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's equations of general relativity ("To infinity and beyond", New Scientist, 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's expansion will cease-and maybe even reverse until all matter collapses in a "big crunch".
"This is definitely a warning flag," says Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
Last year's 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 "type Ia" 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-which 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's expansion was forcing the light to travel a greater distance.
But a nagging doubt remained. Might distant supernovae be less powerful-and therefore intrinsically less bright-than those nearby? Adam Riess of the University of California at Berkeley, a member of Schmidt's 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's 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. "Or it could be something different about the explosion itself, although that would be hard to understand," 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. "It's hard to make an iron-clad case for it when you see something like this," says Riess, who has sent a paper on the findings to The Astronomical Journal.
Perlmutter is keeping an open mind. "It's very premature to draw a conclusion," he says. "The analysis has not been completed." But unless the discrepancy vanishes, cosmologists will be back to what they do best: arguing about the ultimate fate of the Universe.
Author - Hazel Muir
New Scientist issue 17th July 1999
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