Further evidence of a youthful universe.

The conundrum continues.

Yet another set of observations indicates that the universe--as described by a popular cosmological model--appears to be younger than its oldest stars. The new study puts the age of the cosmos at 8.4 billion to 10.6 billion years, younger than the 13 billion to 16 billion years estimated for elderly stars.

Like the findings that made headlines a year ago, the new work relied on the Hubble Space Telescope to obtain the distance to a faraway cluster of galaxies. Combining that distance with the speed at which this cluster recedes from Earth, researchers determined the Hubble constant, which measures the expansion rate and age of the cosmos (SN: 10/8/94, p.232).

A team led by Nial R. Tanvir of the University of Cambridge in England used a two-step method to estimate the constant. First, they observed a type of "standard candle"--stars known as Cepheid variables--to find the distance to the spiral galaxy M96 in the Leo cluster of galaxies. Even at 37 million light-years, M96 lies too close to the Milky Way for its velocity to reflect cosmic expansion unadulterated by the gravitational tug of other galaxies. But the team used the Leo distance as a stepping-stone to the more remote Coma cluster.

To obtain the Coma distance, the researchers relied on a unique property of elliptical galaxies, they report in the Sept. 7 Nature. Astronomers have long known that the bigger an elliptical galaxy, the greater its spread of stellar velocities. But the exact relationship between the two remained uncertain. Previous observations had hinted that the spiral galaxy M96 lies near the center of the Leo cluster, where the ellipticals gather. This coincidence enabled the team to use the distance to M96 to calibrate for the first time the relationship between the size of elliptical galaxies and their velocity spreads.

Applying this calibration to the elliptical galaxies in the Coma cluster, the team found a distance of about 345 million light-years and a Hubble constant between 61 and 77 kilometers per second per megaparsec (1 parsec is 3.26 light-years). In models in which the universe has just enough matter to keep from expanding forever, this corresponds to an age of about 9.5 billion years.

The discrepancy between this age and the age of old stars suggests that astronomers have come to a crossroads. They must either embrace a more complex cosmological model or reexamine how they estimate stellar ages. However, cautions theorist David N. Schramm of the University of Chicago, "You have to be very careful about [drawing conclusions] because all of the [Hubble constant] measurements have huge systematic errors."