`Hubble Constant' Helps Calculate Universe's True Age
FOR astronomers seeking the most valuable of cosmic treasures, this is a good time for hunting.
Their long-sought prize is an estimate of the age of the universe accurate to within about 10 percent to 15 percent. Results announced at the end of September encourage the belief that, after six decades of searching, they may be closing in on their goal. Other findings that a Hubble Space Telescope research team is expected to report soon will likely reinforce that belief.
Until now, uncertainties in estimating distances to far-off galaxies have prevented astronomers from coming any closer to the universe's true age than to say it is probably somewhere between 10 billion and 20 billion years. That isn't accurate enough to know what our cosmos is all about.
The late Edwin Hubble - the space telescope's namesake - started the age quest with the discovery in 1929 that the universe is expanding. It is expanding in such a way that the speed with which distant galaxies recede from us is proportional to their distance from us. Divide a galaxy's recessional speed by its distance and you get a number called, appropriately, the Hubble constant. Multiply that number by a standard conversion factor and divide it into one and you get an upper limit on the age of the universe. This so-called Hubble time applies to an empty universe. The true age depends on the universe's structure and matter content. The model cosmologists favor implies an age about two thirds the Hubble time.
Astronomers measure recessional speeds accurately by means of the ``red shift.'' The faster a galaxy recedes, the redder its light appears to be. Distances have been less certain. Hubble constant estimates range from 50 to 100. Lower values imply ages between 14 billion and 20 billion years. But the higher Hubble constant estimates, paradoxically, would make the universe younger than the oldest stars.
Now astronomers are beginning to reduce this uncertainty. They have several new ways of ranging distances. Also they are extending the reach of the most reliable distance gauge - Cepheid variable stars. The rhythm with which these stars alternately brighten and fade directly measures their intrinsic brightness. Knowing that, astronomers gauge their distance by noting how faint they appear to be as seen from Earth.
This has only worked for nearby objects. Now, astronomers are extending the method to the Virgo cluster of galaxies about 49 light-years away. Michael J. Pierce at Kitt Peak National Observatory near Tucson, Ariz., and colleagues get a Hubble constant around 87 using this method according to their report in the Sept. 29 issue of Nature. That would make the universe no older than 11 billion years. Other research teams are using the Hubble telescope for similar measurements. One of them led by Wendy L. Freedman of the Carnegie Observatories in Pasadena, Calif., is expected to report early results shortly.
Meanwhile, Robert Kirshner and colleagues at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the Cerro Tololo Inter-American Observatory in Chile, reported a Hubble constant value of around 73. That would make the universe 14 billion years old at the most - just barely accommodating the oldest stars. These researchers use supernova exploding stars in distant galaxies to gauge distance.
On the other hand, Allan R. Sandage at the Carnegie Observatories has long insisted that the Hubble constant is 50 and that the universe is respectably old. He has another technique for using supernovae to gauge distances. He has been using the Hubble telescope to locate Cepheid stars in other galaxies as a check on his supernova distance-measuring method.
Astronomers are hopeful that studies such as these will reveal the universe's true age within this decade. If it does turn out to be younger than present estimates of stellar ages, scientists will have to rethink their theories of how stars and galaxies form and evolve and of how the universe itself has developed.