Paradox in Universe's Age
How can the cosmos be younger than the oldest stars?
BOSTON — ASTRONOMERS who study the evolution of the cosmos have a sense of deja vu about their latest work.
Some of it implies that the universe is younger than the oldest stars. Their field has produced this kind of paradox before. When Edwin Hubble started this line of research six decades ago, his first results implied that the universe was younger than Earth.
"Once again, we have the same kind of problem," observes astrophysicist Jacqueline Hewitt of the Massachusetts Institute of Technology. And once again, she and her colleagues are challenged to solve it in the way astronomers solved it in Dr. Hubble's day: They must get a better handle on the distances to cosmic objects.
Hubble had discovered the expanding universe. The farther a galaxy is from Earth the faster it recedes as part of the universal expansion. Hubble found that dividing this speed of recession by the distance to the object always gave the same number.
Astronomers consider this "Hubble constant" to be a basic characteristic of our universe. Multiply the constant by a standard conversion factor and divide it into 1 and you get a rough estimate of the universe's age.
The speed that goes into Hubble's constant is easy to measure. The faster a distant object recedes, the redder its light appears. Judging its distance is trickier. This often is done by estimating the intrinsic brightness of an object and then seeing how dim it appears from Earth.
Hubble got a value of 526 for his constant. That gave the universe an absurdly young age of 2 billion years. Astronomers now think the constant lies between about 50 and 100. But that still means trouble. Dr. Hewitt explains that "for a Hubble constant greater than 50 ... we find that the age of the universe has to be less than or equal to 13 billion years." But, she adds, "the age of the oldest stars in our galaxy [is] coming in at 16 billion years."
Hubble's problem was solved when astronomers realized that he had grossly underestimated distances to cosmic objects. Revised distance estimates gave a smaller Hubble constant of around 50 and a believable age for the universe of 15 billion to 20 billion years. But research is putting upward pressure on the constant. Some estimates run around 80. Robin Ciardullo of Pennsylvania State University, who made one of these high estimates, notes that it implies the universe is only about 8 billion years old. "T hat's absolutely impossible," he says. "It cannot be that young."
Meanwhile, astronomers are revising age estimates of the oldest stars in our Milky Way galaxy. Summarizing that research during the annual meeting of the American Association for the Advancement of Science in Boston earlier this year, Boston University astrophysicist Kenneth Janes explained that, according to modern theories of stellar evolution, those old stars probably are between 16 billion and 17 billion years old. He said that, even allowing for uncertainties in those estimates, "Without some kind o f fundamental new way of looking at stellar evolution, the age of the galaxy has to be 13 billion years or, more likely, substantially older than that."
Astronomers now have the space telescope - named for Hubble - to help them. One of its key missions is to refine what has become the foundation for many cosmic distance scales.
For certain variable stars, called Cepheids, the frequency with which their light varies is a measure of their intrinsic brightness. Astronomers can gauge a galaxy's distance by finding a Cepheid and seeing how dim it appears. This only works for nearby galaxies. But astronomers use the procedure to check out other methods for getting the range to more distant objects. These include such techniques as estimating the intrinsic brightness of exploding stars in a galaxy or estimating a galaxy's overall bril liance.
Wendy Freedman of the Carnegie Institution of Washington Observatories office in Pasadena, Calif., says that she and her colleagues expect to greatly expand the volume of space reachable by the Cepheid method if astronauts can repair the Hubble telescope as planned this December. Even with its flawed mirror, the Hubble telescope has returned some preliminary results. These, together with improved detector technology, have refined the accuracy of Cepheid distance measurements.
Dr. Freedman says, "There's a good case now ... for a high value of the Hubble constant." Astronomers may have to look elsewhere to resolve the paradox of a universe that appears to be younger than its stars.
Dr. Ciardullo suggests that the trouble may lie in assuming the universe to be a simple structure that is more or less the same in all directions. "To my mind, we're really talking about a much more complicated universe than we would like to think," he says.