Through a Lens Darkly

By Jamie Murphy

It was a coal-black night in March, the kind astronomers like best. At Arizona's Kitt Peak National Observatory, Princeton Astrophysicist Edwin Turner pointed the 158-in. reflecting telescope first at one distant pinpoint of light in the sky, then at a neighboring one. A few hours later, studying the results of his night's labors, Turner could hardly believe his eyes. "It was a big surprise," he says. "But a big surprise is always a clue you might be on the track of something."

Something indeed. After analyzing the light from the distant sources, Turner and seven other scientists concluded that they had apparently found evidence of the most massive object ever detected. That object, they surmise in a report published last week in Nature, could be a huge cluster of galaxies or a black hole far larger than any ever anticipated. More startling, it might be a "cosmic string," a bizarre, hypothetical remnant of the chaotic birth of the universe.

The Kitt Peak telescope had been aimed at what appeared to be two quasars, mysterious, intensely bright bodies so far away that the light they emit travels for billions of years before reaching the earth. Gathered by the telescope's parabolic mirror, the light from each of the quasars was converted into a spectrum, from which a quasar's characteristics and even its distance can be determined. Most scientists believe that each of the some 3,000 known quasars, and thus the spectrum of each, is unique. Says Charles Lawrence, a Caltech astronomer and a co-author of the Nature paper: "Quasar spectra are something like fingerprints, and no two are the same."

But as Turner confirmed, the two spectra recorded at Kitt Peak were virtually identical. This meant that if each were from a different quasar, the two objects would not only have identical chemical properties and temperatures but also would be the same distance (about 5 billion light-years, in this case) away--a highly unlikely coincidence. "If you get matching fingerprints," Turner says, "you could have images from the same quasar."

How does one quasar produce two images? The answer, astronomers say, lies in a "gravitational lens," an immense object with a powerful gravitational field located somewhere between the quasar and the earth. As light from the quasar approaches the object, it is diverted from its original path by the intense field (see diagram) and produces what earthbound observers see as multiple images.

As long ago as 1915, Albert Einstein predicted that as a consequence of his general theory of relativity, light rays would be bent if they passed through the intense gravitational field of a massive object. That prediction was confirmed by British Astronomer Arthur Eddington in 1919, when he traveled to an island off West Africa to observe a total solar eclipse. From there he was able to measure precisely the location of a star that became visible in the suddenly darkened sky near the edge of the sun. Because light from the star was bent by solar gravity as it passed the sun, the apparent position of the star in the sky was slightly displaced from its known position by the amount that Einstein had predicted.

By the 1930s, Einstein and other scientists had recognized the possibility ( of a gravitational lens effect but doubted that it would ever be observed. Then, in 1979, two Britons and an American working at Kitt Peak observed the first lensing phenomenon--two quasar images with virtually identical spectral characteristics. Their conclusion: something, later shown to be a cluster of galaxies, was serving as the gravitational lens, obscuring the actual quasar but bending its light rays to form an image on either side. Since then, five other examples of quasar multiple images have been observed, and intervening lens galaxies found for three of them.

While none of these previous multiple images was separated in the sky by more than seven arc seconds,* the latest dual quasar images are 157 arc seconds, or more than 22 times as far apart. In order to bend light that much, the lens must have the mass of a thousand galaxies. Says Turner: "It was like looking for a cat in the backyard and coming up with an elephant."

So far, however, it is an invisible elephant. Says Co-Author Maarten Schmidt, the Caltech astronomer who 23 years ago discovered that quasars were the most distant and intrinsically brightest objects in the sky: "The fact that we do not see anything there makes it rather challenging. It is difficult to hide an appropriate cluster of mass between us and the quasar."

Rising to the challenge, Turner's team suggests another possible source of the powerful gravitational lens: a black hole at least a thousand times as large as the Milky Way galaxy (which consists of hundreds of billions of stars, including the sun). If that black hole exists, it should be producing dual images of other quasars nearby in the sky, and astronomers have begun to seek them out. Still, astrophysicists find it difficult to explain how so tremendous a black hole could have formed.

The Nature report mentions another intriguing possibility for the lens effect: the cosmic string. This weird one-dimensional creature was derived mathematically by physicists pondering the events that occurred in the first fraction of a second after the Big Bang created the universe. In theory, these strings were either infinitely long or enclosed in a loop and could move at nearly the speed of light. Although infinitesimally thinner than the nucleus of an atom, those that have survived should have formidable gravitational fields. Each mile of length could contain a mass equivalent to that of the earth. As in the case of black holes, strings would theoretically produce double images of other nearby quasars. Explains Schmidt: "Since there are other quasars in the region, one would expect to see some of them doubled as well."

Turner also has reservations. "I give the cosmic string theory less than a fifty-fifty chance of being the answer," he says. "But if it is, it would be extremely exciting. Just think, we would be looking at a fossil of the Big Bang."

FOOTNOTE: *Each degree of arc in the sky is divided into 60 arc minutes and 3,600 arc seconds. The full moon spans a half degree, or 30 arc minutes.

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With reporting by Jon D. Hull/Los Angeles and William Sonzski/New York