LONDON--The universe may not be younger than its oldest stars after all, according to a preliminary analysis of satellite data on star positions. If confirmed, the news could settle one of the most nettlesome quandaries in modern cosmology: Studies of the universe's oldest stars suggest they are as much as 15 billion years old, but observations of how fast the universe is flying apart suggest it is 3 billion to 5 billion years younger than that.
The results, presented here today at a meeting sponsored by Britain's Royal Astronomical Society, come from a new look at data collected by the Hipparcos satellite, launched by the European Space Agency in 1989. The satellite made high-precision measurements of the location and brightness of countless stars before falling silent in June 1993. Because the Hipparcos data had to be processed in one giant block, it wasn't until August of last year that investigators got their first look at it.
Position measurements made by Hipparcos from opposite sides of Earth's orbit around the sun can be parlayed into a measure of distance, much as stereovision allows people to judge depth. And these distance measurements are sparking the excitement, by allowing astronomers to calibrate a longer range cosmic yardstick based on Cepheid variables. Cepheid variables are stars that vary in brightness over a period that depends on their absolute brightness, which lets astronomers use them as "standard candles" to measure vast distances. But to do so, astronomers have to calibrate the Cepheids with known distances. After analyzing the Hipparcos data, a team led by Michael Feast of the University of Cape Town in South Africa concluded that the distances to certain Cepheids are 10% greater than was previously assumed.
That could reduce age estimates for the oldest stars, because the estimates depend on their actual brightness, which has to be inferred from their apparent brightness and their distance. Using the revised Cepheid distance scale, Feast's team recalibrated the oldest stars to be about 11 billion years old. The new Cepheid scale also changes estimates of the cosmic expansion rate, upping the age of the universe to about 13 billion years. Together, the adjustments would do away with the age paradox.
"Anything that is tending to make these time scales agree is, I think, improving our understanding of the universe," says Rod Davies, director of Britain's Jodrell Bank radio telescope. Some astronomers, however, argue that it is too early to start redrawing length scales for the entire universe. A possible problem, points out Floor van Leeuwen of Britain's Royal Greenwich Observatory in Cambridge, is the unique chemical composition of some of the recalibrated Cepheids, which could affect their behavior. "My personal feeling is to say wait a while before drawing those sorts of conclusions," he says.