Astronomers and cosmologists calculate the age of the expanding universe by using the so-called "Hubble Constant," H, a measure of the rate of growth; the smaller its value, the more slowly the universe grew to its present size, and the older it must therefore be.
The observable radius of the universe is about 15 billion light-years, and the long-accepted value for H has been 75 kilometers per second per megaparsec (3.26 million lightyears), suggesting an age of 15 billion years.
Several astronomers now contend that H should be lower, around 50 kps/MPc.
Michael Jones (of the Mullard Radio Astronomy Observatory in Cambridge, England) suggests a value of 39+/-13 kps/MPc, based on the so-called "Sunyaev-Zel'dovich" Effect observed in a single galaxy cluster. He and his group expect to observe several other clusters in the next few years to confirm and refine this value.
Joseph Lear (of the Cambridge University Institute of Astronomy) instead reports a value of 37+/14 kps/MPc, based on the gravitational lensing of light from a distant quasar. He freely admits that, to derive this value, his technique requires more assumptions than Jones'.
Gustave Tammann (of Switzerland's Basel University) offers a value of 52+/-3 kps/MPc, based on observations by the Hubble Space Telescope. His calculation relies on the theory that all "Type 1a" supernovae have the same "absolute brightness," and can thus be used as "standard candles" to measure the distance to far-off galaxies. This, combined with red shift data for those galaxies, yields a value for H.
Finally, Michael Pierce (of the Kitt Peak National Observatory, Arizona) argues for H=85+/-7 kps/MPc, using the "Tully-Fisher" technique, one which relates the spectral line emitted by neutral hydrogen and the brightness of a galaxy to its rotational velocity.
(Abstracted from the 30 October 1993 issue of New Scientist.)
