A Universe of Uncertain Age
IN spite of their space-age instruments and subtle theories, astronomers still can't pin down the age of the universe. Indeed, it may be younger than many of them think. George Jacoby and Robin Ciardullo of the Kitt Peak National Observatory near Tucson, Ariz., and Holland Ford of Johns Hopkins University have strengthened one of the somewhat shaky props of these age calculations. As they explain in a recent issue of the Astrophysical Journal, they think they can shave billions of years off the universe's apparent age.
It's worth taking a quick look at some of the details of their work, because it highlights the uncertainties that news reports of cosmic developments tend to gloss over. We often read, for example, that astronomers now estimate that the universe is about 15 billion years old. That word about covers an uncertainty of 40 or 50 percent.
The basic assumption in these age estimates is that our universe originated in the famous Big Bang outburst of energy and has been expanding ever since that event. The expansion is such that, the farther an object - say a galaxy - is from us, the faster it is moving away.
Astronomers then assume a direct relationship between an object's distance and its recessional velocity such that dividing the velocity by the distance always gives the same number. That number is called the Hubble constant after the late Edwin Hubble, who discovered it and who has lent his name to the space telescope now orbiting Earth.
Hubble's constant has the property that, if you invert it - that is, if you divide it into 1 - you get an estimate of the universe's age. This assumes the universe has expanded at its current rate ever since the Big Bang.
The trouble with this neat scheme is that the distance measurements of faraway galaxies have been elusive while their measured velocities are not always what they seem to be. Receding galaxies can be attracted by large concentrations of mass that exist here and there. Their velocity away from us may not be due entirely to the cosmic expansion. It's little wonder that estimates of the Hubble constant vary so widely that the ages they give range from 10 to 20 billion years.
Drs. Jacoby, Ciardullo, and Ford attack this problem with what they believe to be the most accurate distance measurements yet reported. They use what astronomers call a ``standard candle.''
This is an object whose intrinsic brightness is known and is the same in all galaxies. A galaxy's distance can then be estimated by noting how bright this standard object appears when seen from Earth. The three astronomers use the shining ring of gas cast off by many old stars - the so-called planetary nebula - as their ``standard candle.''
Using the new distance measurements, they estimate that the universe is no older than 12 billion years and could be younger. This echoes the estimate Brent Tully of the University of Hawaii reported two years ago. He refined velocity measurements of some relatively nearby galaxies, whose measured velocities he said reflect the attraction of a localized mass concentration as well as the cosmic expansion. He came up with an age for the universe of 10 billion years.
As Jacoby notes, astronomers find the uncertainties in the age estimates ``frustrating.'' He explains that his team has taken a step forward by providing a more accurate way to estimate distance. ``But,'' he adds, ``this problem has bothered astronomers for over 60 years and we can't expect it to go away overnight.'' Remember that the next time a news report - even in this column - cites the universe's age.