Galaxy study challenges cosmic-age estimate.

Talk about a problem of astronomical proportions.

Last month, researchers announced that they had determined the age of the universe to unprecedented accuracy (SN: 5/29/99, p. 340). Now, a report suggests that a cosmic yardstick used to calculate that age may be shorter than assumed. This could shrink the age of the cosmos by 2 billion years.

The most recent study relies on the Very Long Baseline Array, 10 radio telescopes that span North America. Using this network, researchers have directly measured the distance to the galaxy NGC 4258--independent of any yardstick. "It's hard to go wrong when you're just doing high-school algebra to calculate a distance," says James R. Herrnstein of the National Radio Astronomy Observatory in Socorro, N.M.

His team's estimate, 23.5 million light-years, is the most precise ever measured for a remote galaxy, asserts Herrnstein. That distance is accurate to within 7 percent, he reported June 1 at a meeting in Chicago of the American Astronomical Society.

Another team, however, using the yardstick, puts the distance at 26 million to 28 million light-years. For that measurement, Eyal Maoz of NASA's Ames Research Center in Mountain View, Calif., and his colleagues, including Wendy L. Freedman of the Carnegie Observatories in Pasadena, Calif., used the Hubble Space Telescope to observe about 24 of the stars known as Cepheid variables. Each star's intrinsic brightness is proportional to the rate at which it pulsates. To determine distance, astronomers compare a Cepheid's innate brightness with its appearance in the sky.

NGC 4258 is the first galaxy for which the two methods have been compared, and it's too early to say which will prove correct, Freedman says. But if the error lies with the widely used Cepheid method, the universe may be 15 to 20 percent younger than the 12 billion and 13.5 billion years reported May 25 by Freedman. Those estimates, based on Cepheids and three other distance indicators, are accurate within 10 percent, she says.

Calculating the true brightness of a Cepheid depends ultimately on the distance to one of our galaxy's closest neighbors, the Large Magellanic Cloud. Uncertainty in that distance is the largest source of error in the age calculated by Freedman's group. Recent estimates have come up with a distance 15 to 20 percent less than the one that the team used.

In contrast, the radio telescope--derived distance to NGC 4258 depends primarily on geometry. At the core of this galaxy lies a disk of gas, seen edge-on, whirling around a suspected black hole. The gas includes water masers, clouds of water vapor that emit intense radio waves. Herrnstein's team tracked 30 of these brilliant blobs over a 3-year period to measure how far they traveled across the sky, carried by the disk's rotation.

Like a distant airplane that crawls across the sky, the 30 masers barely seem to move, despite a speed of 3.2 million kilometers per hour, calculated from measurements of other masers in the galaxy. Comparing this speed with the observed change in position, the team calculated a distance that they call "the golden meterstick in the glass case."

Whether that gold will turn out to be brass remains to be seen. The analysis assumes that the masers move in sync with the rest of the gas in the disk. This seems likely because each blob appears to be moving at the same rate, Herrnstein says. Even if the masers move in elliptical orbits, rather than in circles, as the team supposed, a discrepancy with the Cepheid distance still holds, he adds.

David A. Neufeld of Johns Hopkins University in Baltimore, who studies water masers, notes that the measurements amount to "a one-trick pony," since few galaxies are known to contain masers and even fewer can also be easily observed from Earth. Nonetheless, he says, astronomers must explain the difference between the radio and Cepheid measurements before they can assume that they have accurately determined the age of the cosmos.

Freedman says the debate may continue until 2005, when the Space Interferometry Mission measures the distance to the Large Magellanic Cloud.