By DAVID HICKSON and TIM THWAITES Two teams of radio astronomers from five countries have found a distant radio source with features which could help determine how big and how old the universe is. The 35 astronomers reported their discovery of the ‘peculiar’ object in the 11 July issue of Nature (vol 352, p 132)/ The radio source appears as an unusual elliptical ring-like structure, within which lie two brighter and highly similar components. These characteristics suggest that radio waves are being split and focused or bent as they pass through a strong gravitational field between the source and observers. The effect, like light passing through a lens, is known as an Einstein ring, because it was first predicted by Albert Einstein from his general theory of relativity. Einstein rings are rare – only three have been found to date – because they demand the alignment of a source of radiation directly behind a gravitational body when observed from Earth. But if the intervening gravitational body can be identified optically, by measuring characteristics of the radio waves from the source, astronomers have a means of calculating the body’s velocity and mass with great accuracy. And this in turn will provide a better picture of the scale of the universe in space and time. The first complete Einstein ring was found by a team of British and American astronomers in 1988. Analysis of the emission from a radio source MG 1131+0456 revealed that it took the form of an elliptical ring, leading to the conclusion that it came from a distant galaxy, and had been focused by the gravitational pull of a nearer one. A second ring was found a year later by American researchers, who photographed the two lobes of a quasar, one much larger than the other, leading again to the assumption that its image had been magnified by an intervening galaxy. PKS1830-211 was first picked up in the mid-60s by the large Australian radio telescope at Parkes in western New South Wales. But its special qualities only became apparent in 1988 when a group of astronomers linked seven Australian radio telescopes across more than 3000 kilometers from Perth to Hobart to give high resolution radio images. The Australian results were confirmed by the US Very Large Array radio telescope in New Mexico. The source is about 100 times brighter than either of the other two rings. Indeed, it is among the six brightest flat-spectrum radio sources in the sky, even though gravitational lensing only occurs once in every few thousand radio sources. Although the radio source lies in the direction of the centre of our own galaxy, various factors – including a remote possibility of a black hole with the required characteristics to act as a gravitational lens – imply that both the lens and the source producing the radio image are ‘almost certainly extragalactic’, the scientists say. The authors of the Nature paper admit to being puzzled by their observations. ‘Is PKS1830-211 merely a particularly unusual chance alignment with a large amplification, or are there indeed more strong Einstein rings and gravitational lenses to be found?’ they conclude. ‘If so, where are they and how are they to be recognised?