Revising the age of the universe - again

Hold the press. The age of the universe may not be known as precisely as previously thought.

In May, a team working with the Hubble Space Telescope reported it had achieved the Hubble program's No. 1 goal: measuring the rate at which the universe expands to within 10 percent. That figure suggested that the universe was approximately 12 billion years old - similar to the oldest stars.

Now, a different group of NASA researchers is warning that the Hubble team's cosmic "yardstick" probably is faulty. The expansion rate may be 12 percent faster than the Hubble team measured, meaning that the estimated "age of the universe would decrease by the same amount," says Eyal Maoz of the Ames Research Center in Moffett Field, Calif.

This would bring back a paradox - that the universe's estimated age is slightly younger than the oldest known stars. In the May announcement, NASA noted that an age of 12 billion years "clears up" that "nagging paradox." That may not be true, says Dr. Maoz's team, which includes Wendy Freedman of the Carnegie Observatories in Pasadena, Calif., who also worked with the Hubble Key Project.

This concern surfaced informally during a meeting of the American Astronomical Society in Chicago in June. Now the Maoz group has spelled it out in today's issue of Nature. They do not say for certain that the Hubble team's estimate is wrong. But they have injected new uncertainty into the question of how well the leading goal of the Hubble telescope program has been achieved.

Basically, astronomers are trying to pin down what they call the Hubble constant - a number that reflects the expansion rate of the universe. The farther away a galaxy is, the faster it moves away from us. The Hubble constant specifies how fast a galaxy recedes given its distance.

Astronomers can measure the recession speed fairly accurately. Getting accurate distances is trickier.

Astronomers use what they call Cepheid variable stars to measure distance within a few tens of light years of Earth. These stars brighten and dim with a rhythm that reflects their natural brightness.

To plot distances, scientists use a Cepheid scale: measuring the rhythm of Cepheids in a nearby galaxy, then noting how faint they appear compared with their innate brightness.

The Key Project team used more than 800 Cepheids in 18 galaxies as much as 65 million light years away as part of its effort to determine the Hubble constant to within 10 percent.

Maoz and his colleagues brought these figures into question by showing that the Hubble team's calculations based on Cepheids may be wrong.

The Maoz group used Galaxy M106 - the most distant galaxy scientists have measured accurately. It measured the distance to the galaxy using the Hubble team's Cepheid scale, then compared that with measurements made by astronomer James Herrnstein of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who used more accurate methods.

The numbers did not match. The Maoz group found they had to recalibrate the Cepheid scale to account for the difference. This, in turn, increased the Key Project team's value for the Hubble constant by 12 percent and decreased the team's estimate for the universe's age.

Commenting on this in today's Nature, Bohdan Paczynski of Princeton University in New Jersey notes that there are several sources of possible errors. He observes that "systematic effects and personal judgments dominate published values of [the Hubble constant]."

(c) Copyright 1999. The Christian Science Publishing Society