By JOHN NOBLE WILFORD
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
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
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
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
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