WE NOW know exactly when the expansion of the universe started to speed up. 鈥淚t happened five billion years ago,鈥� reveals Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland. 鈥淭he universe stopped slowing down and began to accelerate.鈥� The finding rules out certain theories about what鈥檚 causing the acceleration.
It was originally assumed that the expansion of the universe had been slowing down ever since a burst of inflation shortly after the big bang, around 14 billion years ago. But in 1998, astronomers scouring distant galaxies for stellar explosions called type 1a supernovae found that light from these stars was dimmer than they expected, suggesting that the expansion is actually speeding up. They called the mysterious force driving space apart 鈥渄ark energy鈥�.
Since then, further observations of supernovae have suggested that the universe鈥檚 expansion was slowing down until dark energy took over at some point and it started to speed up. That fits the idea that dark energy is some kind of antigravity force. When the universe was small and dense, gravity would have been much stronger than it is now, keeping the universe鈥檚 expansion in check. But as space expanded, gravity would have got weaker while dark energy got stronger, until it was strong enough to override gravity and cause space to expand faster.
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To work out exactly what dark energy is, scientists wanted to know when this happened, and how fast the acceleration took off. 鈥淲e needed to know just when the universe stepped on the gas,鈥� says astrophysicist Michael Turner of the University of Chicago.
Riess and his team used the Hubble Space Telescope to study 42 supernovae, including 6 that were between 9 and 11 billion light years away. Last week they announced at a cosmology conference in Cleveland, Ohio, that the more distant supernovae were brighter than expected, confirming that the universe鈥檚 expansion was slowing down at that point. Riess plotted the brightness of the supernovae against their age, and calculates that the universe must have started speeding up just 5 billion years ago, in what he calls a 鈥渃osmic jerk鈥�.
The team also worked out the strength of the repulsive effect more accurately than has been possible before. Astronomers usually measure dark energy鈥檚 repulsiveness using a single number, w, which is the ratio of the dark energy鈥檚 pressure to its energy density.
The new data suggest that the effect is relatively strong, with w between 鈭�0.9 and 鈭�1.2, says Saul Perlmutter of Lawrence Berkeley National Laboratory, who also presented results at the conference. That rules out certain theories for dark energy, notably the suggestion that as the early universe cooled, space split into regions with boundaries possessing negative gravity. This theory predicted that w would be just 鈭�0.67.
There are many theories still in the running which predict stronger effects, however, including the idea that quantum fluctuations in space-time push space apart, or that a kind of antigravity field pervades space. More supernova data will help narrow it down still further, but for now the theories are still vague. 鈥淲e are in search of ideas,鈥� says Turner.