1 December 1998
The expansion is measured as a number called the Hubble Constant. It is used to calculate how big and how old the Universe is.
The observing team used a new method that avoids the built-in uncertainties in older methods and found the universe to be 15 billion years old.
Astronomers have been wrangling over the value of the Hubble Constant -- a number that indicates the expansion rate of the universe and hence its age -- for several decades. Measuring it is a key problem that the Hubble Space Telescope was built to solve.
"Our value is about 20% lower than the one got with the Hubble Space Telescope in 1994," said Dr Jim Lovell, leader of the team that made the measurement.
The Hubble Constant is named after American astronomer Edwin Hubble, who in the 1920s found that other galaxies were moving away from ours.
"The Universe as a whole is expanding, like a lump of bread dough rising," Dr Lovell explained. "Like currants in the dough, the galaxies are all moving away from each other."
"The further away a galaxy is, the faster it is moving away from us. The Hubble Constant links the galaxy's distance with its speed."
"Measuring the speed at which a galaxy is receding is fairly easy," said CSIRO's Dr David Jauncey, another member of the observing team. "But accurately measuring its distance is very hard."
The usual way to measure a galaxy's distance has been to look for objects whose intrinsic brightness is known. "The further off these things are, the fainter they look, and as we know how bright they are intrinsically, we can calculate their distances," said Dr Jauncey. "The usual objects to use have been rare, special pulsating stars called Cepheids, or some kinds of exploding stars (supernovae)."
In 1994 a team used the Hubble Space Telescope to measure the brightnesses of 20 Cepheids in a galaxy called M100. From these, they calculated its distance was 56 million light-years and from that they got a figure for the Hubble Constant.
Dr Lovell used a different, more direct method.
"We have been looking at light from a very distant quasar -- a galaxy with an extremely bright centre," he said.
"Quasars usually look like small spots but this one looks like a ring. Its image has been distorted by the gravity of a galaxy lying between us and the quasar. We're seeing a sort of mirage of the distant quasar," explained Dr Lovell.
The process that makes the mirage is called gravitational lensing, and was predicted by Einstein.
Light that ends up in one side of the ring has travelled along a different path to the light on the other side, he said.
"These paths are different lengths. When the quasar puts out more radiation, one side of the ring varies before the other. We put that time-lag together with other information we know about the system -- redshifts and angles -- and out pops the Hubble Constant."
The technique sounds simple but it has taken until now to use it successfully, says Dr Lovell, because the lens has to meet a number of conditions. "It has to be strong enough to see clearly, you have to be able to measure the distances to both the background quasar and the lensing galaxy, and the quasar has to vary over time in just the right way.
What we've got is one of the good ones -- a so-called 'golden lens', the type everyone has been looking for," Dr Lovell said.
This 'golden lens' is called PKS1830-211 and was discovered in 1991 by astronomers from the CSIRO's Australia Telescope National Facility (ATNF), the University of Tasmania and NASA's Jet Propulsion Laboratory. It is the strongest gravitational lens ever found, 10 times brighter than any other known. It is 14 billion light-years away while the galaxy distorting its image is 8 billion light-years away.
"As more good lens systems become known, more groups are trying to measure the Hubble Constant in this way," said Dr Lovell.
The twist is that the Hubble Constant is probably not constant at all.
"We think it has varied over the lifetime of the Universe," said Dr Jauncey. "From other work it seems that the Universe is now expanding faster than it used to. The value we've measured is for a particular point in the Universe's history, and will help us to build up a picture of how the rate of expansion has changed."
General information plus still images
A short run-down on the Hubble Constant
An animation of a gravitational lens prepared by Jim Lovell (in Australia)
The same animation as above, in Japan