Two groups of astronomers have analyzed light from distant exploding stars to reach a preliminary verdict on the fate of the expanding universe. The longtime rivals have been working independently to learn how the universe's expansion rate has changed over time--whether it has been slowed by gravity, for example. Their new results agree: The expansion has slowed so little that gravity will never be able to stop it. The new results--in press at Nature and under review at Astrophysical Journal Letters--imply that the universe contains far less mass than many theorists had hoped: less than 80% of the critical amount that would be needed to slow its expansion to a halt.
Both groups--one led by Saul Perlmutter of Lawrence Berkeley National Laboratory and the University of California, Berkeley, and the other by Brian Schmidt of Mount Stromlo and Siding Spring Observatory in Australia--probe the cosmos's future by plotting the distance of far-off objects against the speed at which cosmic expansion is sweeping them away from Earth. In nearby parts of the universe--within a few hundred million light years--the resulting line is straight, implying that space is expanding at the same rate everywhere. But objects seen at greater distances, billions of light years away, provide a glimpse into the remote past, since they emitted their light much earlier in cosmic history. So if, for example, gravity has slowed the expansion since then, that should be revealed in the subtle bends of the line.
Measuring how fast an object is flying away from Earth is easy: Just determine the "redshift" of its light, a stretching of its wavelengths analogous to the drop in pitch of a receding train's whistle. Measuring distance is another matter, requiring "standard candles." These are objects that can be seen far out in the universe and that have a roughly constant intrinsic brightness, so that their apparent brightness can be taken as a distance indicator. The exploding stars called type Ia supernovae make good standard candles, and both teams have now identified dozens of type Ia supernovae as much as halfway to the edge of the visible universe. After studying a handful of them from the ground and from the Hubble Space Telescope and carefully analyzing their light, they have plotted them on the telltale line to see how it bends. Both groups find that gravity has slowed the expansion by so little, that the universe falls well short of the critical density.
"If I were a theorist, I'd be getting worried at this stage," says Alexei Filippenko of the Berkeley Lab and a co-author on the Schmidt paper. The reigning cosmic creation theory, known as inflation, traces key features of the universe to a burst of exponential growth in the first fraction of a second after the big bang, and its simplest version predicts a universe that contains just enough matter for gravity to stop the expansion after an infinite time. Filippenko and his colleagues stress that the results still need to be confirmed. But already, says Neta Bahcall of Princeton University, "the results are very exciting and the method is very promising."