May 26, 1999
Hubble Telescope Yields Data for Recalculating Age of Universe
By JOHN NOBLE WILFORD
fter eight years of measurements by the Hubble Space Telescope, astronomers think they have finally established a reliable value to the expansion rate of the universe, the indispensable number required for determining the age, size and fate of the universe.
It is a number that scientists have sought ever since 1929, when Edwin P. Hubble discovered that the galaxies are flying outward at velocities that increase with distance. Estimates of the expansion rate, also known as the Hubble constant, are fundamental to all theories of cosmic evolution. But they have fluctuated wildly between numbers that would put the age of the universe anywhere from 10 billion to 20 billion years.
By the new calculations, announced Tuesday, the universe has been expanding for at least 12 billion years since its theorized explosive creation in the Big Bang. Depending on the density of cosmic matter and the possible existence of a mysterious form of vacuum energy, the age of the universe could be closer to 13.5 billion or even 15 billion years.
Astronomers said they had 90 percent confidence in the reliability of the basic measurement underlying these age estimates. The number of the Hubble constant, they concluded, is 70, plus or minus 7. That means that a galaxy appears to be flying away at a rate of 70 kilometers per second per megaparsec, or 160,000 miles per hour faster for every 3.26 million light-years away from Earth.
A team of scientists led by Wendy L. Freedman of the Carnegie Observatories in Pasadena, Calif., determined the expansion rate by using the Earth-orbiting Hubble telescope in measuring distances to 800 stars of known brightness, called Cepheid stars, in 18 galaxies. Measuring the expansion rate was the telescope's top scientific objective.
"After all these years, we are finally entering an era of precision cosmology," Freedman said in announcing the findings at NASA in Washington. "Now we can more reliably address the broader picture of the universe's origin, evolution and destiny."
Only a decade ago, astronomers argued over wide differences in expansion-rate estimates, which resulted in a range of cosmic ages running from 10 billion to 20 billion years. The first tentative results by Freedman's group, reported five years ago, seemed to defy logic. An expansion rate of 80 yielded age estimates ranging from 8 billion to 12 billion years, which made the universe younger than its oldest stars.
Freedman said in an interview that the lower value for the Hubble constant, as well as the team's confidence in the research, stemmed from an increase in observational data through different methods and a careful analysis of possible sources of uncertainty or error.
Now the estimates by rival teams are beginning to converge on numbers that lead to ages of the universe similar to or greater than the ages of the stars. But the controversy has not entirely ended.
"We used to disagree by a factor of two; now we are just as passionate about 10 percent," said Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "A factor of two is like being unsure if you have one foot or two. Ten percent is like arguing about one toe. It's a big step forward."
Freedman's adversary in the Hubble constant wars has been Allan R. Sandage, also at the Carnegie Observatories. His observations of distances to certain types of exploding stars have consistently led to lower values for the Hubble constant, usually less than 60, and thus greater ages for the universe, from 15 billion to 20 billion years.
At a recent science conference in Baltimore, Sandage, referring to his own results, reported: "At a 99 percent confidence level, the value is between 53 and 65. The curves are going to intersect in 2006."
Kirshner remarked, "We are no longer talking about a real conflict."
As Freedman and others observed, the margins for error in the two sets of Hubble constant estimates are such that they are not only drawing closer to each other, but are also beginning to overlap.
Michael S. Turner, a theoretical astrophysicist at the University of Chicago, said the refined value for the Hubble constant represented "the first act in a grand cosmological drama that's going to play out in the next two decades."
In the drama, cosmologists will be trying to answer some profound questions about the amount and nature of matter constituting the universe, the possibility that a strange form of energy is speeding up the cosmic expansion, what was the Big Bang itself and how will it all end. Uncertainties about the density of matter in the universe and the existence of a mysterious energy, called the cosmological constant, contribute to the wide range of age estimates drawn from the newly determined value of the Hubble constant.
Freedman, for example, explained an estimated age of 12 billion years was valid even if the universe is relatively lightweight, as recent observations suggest. If there is indeed a cosmological constant, a kind of antigravity force acting to speed up cosmic expansion, that would increase the age estimate, perhaps up to 15 billion.
The expansion and age of the universe were also subjects of two articles in the issue of the journal Science that is being published on Friday.
In an analysis of recent observations and other research, Charles H. Lineweaver, an astrophysicist at the University of New South Wales in Sydney, Australia, put the age for the universe at 13.4 billion years, plus or minus 1.6 billion. The estimate took into account low values for the mass density of the universe and the presence of a "missing energy" source like the cosmological constant.
In a review of "the state of the universe," Dr. Neta A. Bahcall of Princeton University and three colleagues noted growing evidence that is "forcing us to consider the possibility that some cosmic dark energy exists that opposes the self-attraction of matter and causes the expansion of the universe to accelerate."
Such findings suggest the future in store for the universe. It will probably keep on expanding, almost to the point of vanishing. As Turner of Chicago said, "The universe will become a bleaker and bleaker place," and as the galaxies speed away from one another and the stars grow dimmer, "in 400 to 500 billion years, we're only going to be able to see a few neighboring galaxies."