Gravity at play: A double lens

From Austin, Texas, at a meeting of the American Astronomical Society

GRAVITY’S RINGS. A rare double Einstein ring is part of the gravitational lens system SDSSJ0946+1006. Two concentric partial ring-like structures are revealed after subtracting the glare from the central, foreground galaxy. R. Gavazzi, Treu, NASA, ESA, and the SLACS team

Astronomers have for the first time spied an extremely rare, double cosmic mirage.

As first predicted by Albert Einstein’s theory of general relativity, a massive foreground galaxy acts like a lens, bending or distorting the light from a galaxy that lies farther away. In most cases, the light is bent into an arc. But in rare instances, when the background galaxy is exactly aligned with the foreground galaxy as observed from Earth, the light is distorted into a perfect circle known as an Einstein ring. About 50 such rings have been detected.

Now, researchers have discovered the first double Einstein ring. The concentric rings are generated when two separate background galaxies line up exactly with the same foreground body. The odds of seeing such an alignment are about 1 in 10,000, says Tommaso Treu of the University of California, Santa Barbara.

He and his colleagues first found evidence of what appeared to be a single Einstein ring in data from the Sloan Digital Sky Survey, which has mapped a million galaxies over one-quarter of the sky. But when Treu’s team took a much closer look at the mirage with the Hubble Space Telescope, the researchers found they were seeing double.

The foreground galaxy, or lens, lies 3 billion light-years from Earth, while the inner and outer rings are images of galaxies that lie farther away—6 billion and 11 billion light-years—and hail from an earlier epoch of cosmic history.

The twin rings represent more than a rare cosmic coincidence, Treu notes. By measuring the distance to each ring and its relative radius, astronomers can determine the amount of mass in the universe at different times. If they’re lucky enough to find 50 double rings with future large-scale surveys, researchers would have a new measure of dark energy, the mysterious substance that’s revving up the rate at which the universe is expanding.

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