Hubble's Keen Eye Puts Age of Universe In Astronomers' Sights
Hubble Space Telescope is fulfilling its promise to help astronomers pin down the age and expansion rate of the universe. These questions have baffled them since Edwin Hubble discovered that expansion over 6-1/2 decades ago. Now, different ways of estimating the rate of expansion and the universe's age are beginning to offer answers.
How different that is from the situation just five years ago. Back then, the best astronomers could do was to come up with estimates that differed by up to 100 percent. Now, the latest numbers from the two leading groups of estimators differ at most by only 25 percent.
Earlier in May, the Hubble Key project group associated with Wendy Freedman of Carnegie Observatories in Pasadena, Calif., said their latest results make the universe 9 billion to12 billion years old. Earlier, in March, the other group, which is associated with Allan Sandage of the Carnegie Observatories, pegged the age at 11 billion to 14 billion years. Both groups are working with the Hubble telescope. And by century's end, these estimators are likely to achieve one of the Hubble telescope's main goals - namely, to measure the rate at which the universe expands within an accuracy of 10 percent.
That would be one of the major astronomical achievements of the 20th century. It would not be a discovery. The discovery was made in 1929, when Edwin Hubble found that the farther other galaxies or clusters of galaxies are from us, they faster they are moving away. In other words, the ratio of the speed of these objects' recession to their distance from Earth is a constant called the Hubble constant. Determining that constant to within 10 percent accuracy would help extract from Edwin Hubble's discovery more accurate knowledge about such important basic facts as our universe's age and size. That is the achievement astronomers now appear to have within their grasp.
They can get a rough age estimate with a formula that uses the Hubble constant. That would apply to an empty universe. They then refine this estimate to take account of how much matter they believe the universe has.
Astronomers have no trouble measuring recession speeds. The faster objects recede, the redder their light appears as seen from Earth. It's getting an accurate assessment of the distances of those objects that has confused Hubble-constant estimates. That's where the repaired Hubble telescope now helps out.
Astronomers generally estimate distance by measuring how dim a "standard" light source appears to be. The dimmer a source of known brightness appears, the farther away it is. Cepheid variable stars are the most reliable "standard candles."
Astronomers use Cepheids to calibrate other standards, such as the peak brightness of what they call Type 1a supernova star explosions. That is, astronomers estimate a distance using the supernova and see how well it matches the estimate based on Cepheids. This lets them fine-tune the supernova distance scale. The problem has been that Cepheids are visible only in nearby galaxies. The space telescope now extends that range tenfold, leading to refined distance estimates and Hubble constant estimates such as those Wendy Freedman reported. Her group, using several distant indicators, now puts the Hubble constant at between 68 and 78. The Sandage group, using supernovae distances only, think it's about 57.
Ground-based telescopes also help. The Lawrence Berkeley National Laboratory in California has developed a computer program that quickly locates Type 1a supernovae in very distant galaxies.
The lab has given a simplified version of this software to schools where students can also collect the images and search for supernovae. This work is expanding rapidly the number of supernovae astronomers have with which to make distant estimates.