It's official - or at least as official as astronomy can often get: The universe is 12 billion years old, give or take a billion years or so.
The estimate, announced yesterday at NASA in Washington, represents one of the most important achievements by the Hubble Space Telescope since it was launched nine years ago.
For decades, astronomers thought that the universe was between 10 and 20 billion years old. This new estimate will not only help scientists fill in the cosmic time line more confidently, but it also may help solve the mystery of why the expansion of the universe seems to be accelerating.
To researchers, the reduction in uncertainty to a 10 percent margin of error is significant. "We used to disagree by a factor of 2 ... [that's] like being unsure if you have one foot or two," says Robert Kirshner, an astronomer at Harvard University in Cambridge, Mass. "Ten percent is like arguing about one toe. It is big step forward."
"Things are beginning to add up," adds Robert Kennicutt at the University of Arizona in Tucson. "The factor of 2 controversy is over."
Meanwhile, an Australian scientist is publishing an independent age estimate in tomorrow's issue of the journal Science. Charles Lineweaver, an astrophysicist at the University of New South Wales in Sydney, pegs it at 13.4 billion years, with an uncertainty of 1.6 billion years. Given their overlapping margins of error, the two estimates of our universe's age are essentially in agreement.
To arrive at his conclusion, Dr. Lineweaver worked with the published results of many other investigators. The Hubble Space Telescope Key Project Team, however, was assigned the task of making their own measurements of the distance to remote galaxies and determining the speed with which these galaxies are receding from us as part of the universe's expansion.
The favored way of judging distance is to use a so-called standard candle. This is an object whose intrinsic brightness is known. Observers then judge the object's distance by noting how faint it appears compared with that intrinsic brightness.
Astronomers also measure the "red shift" of an object's light to determine how fast it is moving away from us. The faster the object's recession, the redder its light appears. Moreover, the farther away an object is, the faster it is receding because of the expansion of the universe.
The relation between an object's distance and its speed of recession is represented by what astronomers call the Hubble constant. Like the space telescope itself, it is named for astronomer Edwin Hubble who, with several colleagues, discovered the relation seven decades ago. Yet the value of this constant has been the nub of the age controversy.
Astronomers thought the constant was a number between 50 and 100, but couldn't pin it down. To help find the number, the Key Project Team observed 18 galaxies as much as 65 million light-years away. They used the most reliable type of standard candle they know, the Cepheid variable star. (The pulsation rate of Cepheid variable stars accurately reflects their intrinsic brightness.) Observing almost 800 of these stars in the 18 galaxies, the team made some of the most precise distance and red-shift measurements yet obtained.
These data enable the team to assign the Hubble constant a value of 70 with an uncertainty of 10 percent. By combining this constant with the best current estimates of the universe's density, scientists got the 12-billion-year-old age.
Determining the Hubble constant "was a driving force" behind the design of the Hubble telescope, and the No. 1 goal of the mission, says Ed Weiler, NASA associate administrator for space science.
Jeremy Mould, co-leader of the Key Project Team, calls the team's precision results "a legacy ... that will be used in a variety of future research."
Indeed, cosmologists need such precision data to meet their newest challenge. A variety of recent measurements of distant galaxies indicate that the universe's expansion is accelerating - not slowing down, as had been expected. This suggests that the universe may contain a hitherto unsuspected form of energy.
Commenting on this in a review of this century's progress in cosmic knowledge published tomorrow in Science, Princeton University astrophysicist Neta Bachall and several colleagues note that "as the current millennium ends, the past history and present state of the universe are making themselves known."
But they add that the effort to understand what this suspected new form of energy may be is "one of the grand challenges for the millennium to come."