Gravity's Telescopes

July 25, 2008

Among the plethora of memorable characters in Philip Pullman's His Dark Materials trilogy is Mary Malone, a fictional Oxford physicist. In the third installment, The Amber Spyglass, she finds herself lost in a parallel world, and ingeniously constructs her own primitive telescope lenses out of a lacquer made from tree sap, fitting the lenses on either end of a bamboo tube so she can see farther in every direction to get her bearings.

It's the same basic principle a German-born eyeglass manufacturer named Hans Lippershey employed in the 1590s when he opened a spectacles shop in the Netherlands. Legend has it that Lippershey noticed some children playing with the lenses in his shop, exclaiming that when they looked through two lenses, a weather vane on a nearby church appeared to be larger and clearer. History has not seen fit to record the names of those precocious children, but their play led to the invention of the first telescope (a kijker or "looker").

We have trouble seeing objects in the distance because they just don't take up enough space on the eye's retina for the retinal sensor to detect them. A bigger eye would collect more light from a distant object, creating a brighter image, and then magnify part of that image so that it stretches over more of the retina. So a telescope is just a bigger eye.

Scientists have been improving on those rudimentary first instruments to build ever-bigger "eyes" ever since. And sometimes the cosmos itself gives us a hand. Today, astrophysicists are using "gravitational telescopes" to make very faint objects in the farthest reaches of the visible universe appear brighter, so they can be more easily studied.

The unlikely source of these "gravitational telescopes" is Einstein's theory of general relativity. Gravity isn't so much a force, Einstein reasoned, as a curvature n the fabric of space-time caused by the presence of mass (or energy). Light also follows that curvature, a prediction that was spectacularly confirmed in May 1919 during a solar eclipse. Two separate scientific expeditions -- one on an island off west Africa, the other in Brazil -- photographed stars near the eclipsed sun and found that their light was deflected, just as Einstein had predicted.

The warping of space-time can also give rise to a kind of "mirage" effect, known as gravitational lensing. When two galaxies line up precisely in the sky, one right behind the other, the gravitational field of the nearer galaxy will distort the image of the more distant galaxy. It's a little different from a conventional lens, in that the most bending of light occurs closest to the center of a gravitational lens; in a regular telescopic lens, the greatest bending occurs furthest from the center.

Einstein predicted this effect in the 1930s, but the first empirical observation wasn't made until 1979, when a team of astronomers observed what they initially thought were identical twin quasars. It turned out to be a single quasar viewed through a gravitational lens. Such an effect is known as an Einstein ring if the source, lens, and observer are all aligned. Einstein ring images can be as much as 30 times brighter than a distant galaxy would appear to be without this lensing effect. There is even an extremely rare formation known as Einstein's Cross, in which four images of the same distant quasar appear around a foreground galaxy because of a strong gravitational lensing effect.

Because gravitational lenses concentrate the light from objects seen behind them, those more distant, faint objects will appear bright and larger when viewed through this warping of space-time. Voila! We have a built-in gravitational telescope. In 2004, for example, Caltech researchers exploited the strong gravitational lensing of the Abell 2218 cluster of galaxies to detect the most distant galaxy yet known.

This week, another team of astronomers announced results from the largest-ever single collection of "gravitational lens" galaxies -- some 70 in all -- as part of a project known as the Sloan Lens ACS (SLACS) Survey. First, they identified all the gravitational lenses using data collected by the Sloan Digital Sky Survey. Then they studied images of all these "Einstein rings" taken by the Hubble Space Telescope. They measured the apparent size of the Einstein rings, and then measured the distances between the two galaxies of the aligned pair producing each ring. By combining the two measurements, they could deduce the mass of the nearer galaxy.

What did they learn? Well, the results helped settle a long-standing debate over what the amount of matter (mass) in galaxies has to do with their brightness. That doesn't sound too earth-shattering unless you figure in the presence of mysterious "dark matter": the astronomers were able to infer that there does seem to be dark matter contributing to the overall mass of the galaxies. In fact, the more mass a galaxy has, the larger the fraction of dark matter will be, relative to mass contributed by ordinary matter, like stars.

Of course, most of us are more concerned with watching our own weight, rather than weighing galaxies, but at least we don't have dark matter creeping up on us and expanding our waistlines on the sly. And from our terrestrial vantage point, we can appreciate the eye-popping images provided by gravity's telescopes.

Photos: (top) Hans Lippershey, Source: Wikimedia Commons. (middle) Einstein's cross. Source: NASA/European Space Agency. (bottom) Hubble Space Telescope image of one of the SLACS gravitational lenses, with the lensed background galaxy enhanced in blue. Source: NASA/European Space Agency.