What's the time on the cosmic clock? Getting a handle on the universe's age could help settle the question of whether the universe will keep expanding or collapse back in on itself
PINNING down a precise age for the universe has been one of astronomy's tougher tasks. Researchers say that getting an accurate handle on the universe's age could - at least in principle - help settle the question of whether the universe will continue to expand forever or eventually collapse back in on itself in what has come to be called the ``Big Crunch.''
Astronomers say it's remarkable that current estimates of 10 billion to 20 billion years differ by only a factor of two. But discussions heat up when someone weighs in with a new estimate that leans to one extreme.
Which is precisely what Harvey R. Butcher, director of the Kapteyn Astronomical Institute in the Netherlands has done - at least indirectly. By studying the relative abundance of two elements in the atmospheres of the sun and 20 sunlike stars of various ages, he has concluded that the Milky Way galaxy is no more than 9.6 billion years old. Add to that about 1 billion years, which cosmologists estimate to be the time between the ``Big Bang'' that one theory says spawned the universe to the formation of galaxies, and Dr. Butcher's research would imply that the universe is about 11 billion years old.
Butcher selected two chemical elements to use as his cosmic clock: neodymium, which doesn't decay into lighter elements over time, and a radioactive form of thorium that has a half-life of 14 billion years, comparable to the suspected age of the galaxy. In older stars, the ratio of thorium to neodymium should be smaller than in younger stars, because the thorium has had time to decay. Indeed, Butcher says that he expected the thorium-to-neodymium ratio in the older stars to be one-half to one-third that in the younger ones.
In fact, he found that the ratio between the two elements was essentially the same, regardless of whether the age of the star - using other dating techniques - was 600 million or 19 billion years. Then, by making some assumptions about the initial abundance of the two elements as well as their production rates within the stars, he arrived at his maximum age for the galaxy.
``I'm enthusiastic about his results,'' says William A. Fowler, a Nobel Prize-winning physicist at the California Institute of Technology. Dr. Fowler, who won his prize for pioneering research into the way stars create chemical elements, says that he has used three radioactive isotopes - one of thorium and two of uranium - to arrive at an age for the universe of 10 billion years ``that's uncertain to a couple of billion years.''
Others, such as David N. Schramm of the University of Chicago, say that Butcher's conclusions go beyond those his data actually allow. ``His experimental work is fine,'' he says, but adds that in his view, Butcher's results only serve to put limits on models trying to explain how galaxies evolve, not on the age of the universe.
For his own part, while Butcher expresses a great deal of confidence in his data, he is also calling for additional research.
Butcher's method is one of three used to give an age to the universe. The two other measures include:
Hubble age. Using distances and the velocities at which galaxies are moving away from us, astronomers can calculate the universe's rate of expansion, known as the Hubble constant. This can be used not only to estimate the age of the universe, but also to help calculate whether or not the mass in the universe is dense enough to generate the gravity needed to eventually reverse the universe's expansion. Estimates of the Hubble constant vary by a factor of two, yielding a Hubble age ranging from 10 billion to 20 billion years.
Oldest star age. This method, based on decades of research into stellar evolution, yields age estimates from 13 billion to 16 billion years, says Kenneth Bresher, a Boston University astrophysicist.
Indeed, it's over this estimate that claims for a younger universe run into opposition. The rationale: You can't have a universe younger than the oldest objects in it.
What will it take to resolve the differences?
Each method has its unique problems, and solutions depend on what notions one holds of the physical processes involved.
At least as far as Hubble age is concerned, astronomer Allan Sandage says that the Hubble space telescope, scheduled for launch in 1989, should go a long way toward boosting the accuracy of distance measurements, the main source of uncertainty. As a result, the gap in estimates of the Hubble constant should narrow, leading to a smaller gap in estimates of the Hubble age.
That would be a useful step toward using ages to test various models of the universe. The Hubble constant doesn't take into account the effects of gravity on the expansion rate of the universe. Hence, age estimates based strictly on the Hubble constant would represent the age of an infinitely expanding universe.
But, says MIT physicist Alan Guth, any cosmological model of the universe includes the effects of gravity in its calculations. An ``open'' universe has some collective gravitational pull, but not enough to halt its expansion. A ``flat'' universe has just enough gravity to continuously slow the rate of expansion, although the rate will never reach zero. A ``closed'' universe has enough gravity to ultimately reverse the expansion and collapse the universe.
If one had a precise Hubble age and a fairly widely accepted observed age - using radioactive dating or the age of the oldest stars - then comparing the observed age of the universe with the Hubble age would, in effect, adjust it for the effects of gravity. Astronomers say that if the observed age of the universe comes close to matching the Hubble age, then the universe is open. If the observed age is two-thirds the Hubble age, the universe is flat. If the observed age is any smaller than two-thirds the Hubble age, that would herald the eventual coming of the Big Crunch.
Age estimates vary over time
Late in the 18th century, scientists were estimating the age of the Earth and the universe (thought to have been created at the same time) in terms of hundreds of thousands of years. It wasn't until after the turn of the century that scientists began thinking in terms of billions of years. By 1960, the scientific community had pretty much settled on 4.6 billion years as the age of the Earth. But estimates of the universe's age have continued to vary widely in the 10 to 20 billion-year range.