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Tuesday, December 1, 1998 Published at 18:03 GMT


Through a cosmic lens

By BBC News Online Science Editor Dr David Whitehouse

An international team of astronomers have measured how fast the Universe is expanding and from that its age. They have found it to be larger and older than previously thought.

The rate at which the Universe is expanding is measured by a number called the Hubble Constant. It can also be used to calculate how big and how old the Universe is.

The researchers used a method that avoids the built-in uncertainties in other techniques and calculated the age to be about 15 billion years old.

Astronomers have been arguing over the Hubble Constant for decades. Measuring it accurately was one of the reasons why the Hubble Space Telescope was built.

American astronomer

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," said Dr Jim Lovell, leader of the team that made the measurement. "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."

To measure the Hubble constant, Dr Lovell looked at distant objects in space.

Distant quasar

"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."

The process that makes the mirage is called gravitational lensing, and was predicted by Einstein.

Light that ends up on 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."

Gravity lens

The technique sounds simple but it has taken until now to use it successfully.

This "gravity lens" is called PKS 1830-211 and was discovered in 1991 by astronomers from the 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 lens. 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.

Our value of the Hubble constant is about 20% lower than the one got with the Hubble Space Telescope in 1994."

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Simulation of a Gravitational Lens (Jim Lovell)

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