More then six decades ago, Albert Einstein predicted that the gravitational pull of galaxies in deep space would create natural lenses that would bend light and greatly magnify distant objects. Yet the great physicist also predicted that these far-off lenses would be so difficult to find that mankind would never locate one.
In 1979, astronomers confirmed Einstein's theory by finding a distant gravitational lens in the form of a spectacular mirror image of a bright young galaxy, called a quasar. Now, thanks to vastly improved technology, astronomers today are finding more and more gravitational lenses in the skies. Indeed, today researchers working with the Hubble Space Telescope announced that they had located eight more, increasing the total number of known gravitational lenses by almost 20 percent.
Once considered an exotic curio, these lenses are giving scientists a remarkable opportunity to glean answers about the origin, shape, and composition of the universe. And perhaps most intriguing, they even may hint at whether the universe will continue expanding or eventually fall back in on itself.
"Gravitational lenses promise to provide one of the best handles we'll ever get on what the geometry of the universe is and how the components of the universe came to be the way that they are," says Joseph Lehar, an astrophysicist at Harvard University in Cambridge, Mass.
The concept of a gravitational lens is grounded in Einstein's Theory of Relativity, which states that light is affected by gravity. A massive object with strong gravitational pull, such as a galaxy or cluster of galaxies, acts like the lens on a magnifying glass. The gravity of this foreground galaxy bends the light coming from an even more distant object located directly behind it.
This creates a greatly magnified and distorted image of the second distant object. The distorted image often appears as a spectacular array of exotic rings and arcs that are in fact heavenly mirages.
"If you take a wineglass and hold it in the bright light, you would see multiple images. The base of a wineglass works like a lens that bends light and creates multiple images," says Kavan Ratnatunga, an astrophysicist at Carnegie Mellon University in Pittsburgh. "In this case, the galaxy creates multiple images of what's behind it."
Until recently, finding gravitational lenses was nearly impossible. They create images of faint, distant objects billions of miles away that are hard to detect and study even with the most powerful telescopes. Separating the lensed image from much brighter signals emitted by the foreground galaxy is likewise difficult. Furthermore, there are billions of objects in the sky.
But scientists learned that in about 1 of every 400 cases, gravitational lenses accompany quasars that produce radio waves. That - along with improving technology - has allowed researchers to winnow the candidates.
As a result, the number of known gravitational lenses has skyrocketed from a handful a decade ago to more then 50 today- and the pace of discovery is accelerating.
In addition to the discovery of the eight new gravitational lenses announced today by Dr. Ratnatunga and several colleagues, several other research teams are also looking for gravitational lenses. The spate of detections in recent years has vaulted gravitational lenses from anomalies to statistically significant events that can be meaningfully studied. "We've just moved from the truly pioneering stages to the point where we can do serious cosmological tests," says Christopher Impey, a University of Arizona astronomer.
The uses of the information they gather will have profound effects on how humans view the universe.
*By examining how gravitational lenses distort light, scientists can calculate the strength of the gravitational pull of foreground galaxies in these lenses. In turn, they can calculate the mass and distribution of mass in these galaxies - effectively, weigh them and determine their shape and how they evolved.
*Gravitational lenses will provide astronomers with minute details about distant galaxies that could never be gained without a hand from nature.
*Scientists can also measure the movement of extremely distant objects with the lenses. This will give them an idea of the rate at which the universe is expanding, which, in turn, gives clues as to the size and age of the universe.
*Gravitational lenses should provide insight into dark matter - mysterious portions of the universe that emit no detectable electromagnetic radiation but do have mass and therefore gravitational pull.
*Perhaps most important, these natural telescopes might help astrophysicists calculate the total mass of the universe. This is the key to determining the shape of the universe and whether it is infinitely expanding - a so-called open universe.
If scientists find numerous gravitational lenses, this implies that there are more galaxies in the universe to create these lenses and therefore the universe could be infinite. If scientists find a limited number of gravitational lenses, this implies that the universe is finite - or closed - and will, at some point, collapse or hit an equilibrium.
"Ever since people realized that the big-bang theory was something to take seriously, they wanted to know how much mass there is in the universe to figure out whether the universe is closed or open," says Dr. Lehar. "Now we finally might be able to do it."