|These multiple blue smoke rings are actually a single distant galaxy whose image has been split and magnified by curved space around the nearer cluster of yellow galaxies.|
Photograph courtesy of W. Colley/NASA
For once, nature did not throw us a curveball. Last spring, two teams studying microwave radiation left over from the Big Bang reported that the overall shape of the universe is flat. In other words, parallel beams of light travel in straight lines and never meet, just like parallel lines in high school geometry class. That finding comes as a bit of a relief, considering that some odd cosmological models had predicted the universe could have the form of a doughnut or a saddle.
But don't throw away the aspirin just yet. This flat-space business applies only to the cosmos as a whole. According to Albert Einstein's general theory of relativity, every mass creates its own little disturbance, like a dimpled chad on an enormous voting card. Warped space is very much present in local areas including our own solar system.
The sun, for instance, noticeably bends any light rays passing close by its surface. Scientists watching a solar eclipse in 1919 saw that stars near the sun in the sky seemed slightly displaced by the solar gravity, apparently proving relativity and making Einstein a celebrity. In the late 1970s, NASA engineers monitoring signals from the Viking
spacecraft on Mars confirmed this effect. When the planet passed behind the sun's edge, the radio waves had to follow the curvature of space around the sun. They therefore took a longer route to Earth and arrived late, as if Mars had suddenly jumped 18 miles farther away.
During the 1990s, astronomers uncovered some truly colossal warps in deep space. In one case, a primeval blue galaxy appears in five different places in the sky, its image split by the steep curvature around a foreground galaxy cluster. In another, a group of galaxies known as Abell 2218 acts like a lens, squeezing and magnifying the images of more distant galaxies into long, thin arcs. There's a lot to be learned from these twisted scenes. The amount of bending reveals the mass of the intervening galaxies, while gravitational amplification brings dim background objects into plain view.
More than a decade ago, Princeton University astrophysicist Bohdan Paczynski suggested that more modest instances of curved space could be used to detect dark matter, mysterious clumps of invisible material thought to make up much of the mass of our galaxy. If an unseen object (dark matter or just dim ordinary stuff) were to pass in front of a more distant star, its space-warp would act as a "microlens," temporarily amplifying the star's brightness and identifying the dark interloper.
Such events would happen rarely, so Paczynski and others set up computerized surveys to check the light of millions of stars in crowded regions of the galaxy. In 1993, these efforts resulted in the first observed microlensing events and have turned up hundreds more since. Most of the results have been negatives: no clear sign of exotic new material and no indication of planets around other stars. But last year one survey uncovered tentative evidence of solitary black holes drifting through our galaxy.
And this past spring, researchers using the Very Large Telescope in Chile hit space-warp pay dirt. They watched as a double star passed in front of a more distant one, creating an unusual dual super-lens. Normally stars appear as nothing but specks, even through the largest telescopes. But the super-lens magnified the background star to the point that the scientists could examine its surface strip by strip, like a CAT scan. In this way, they obtained the first detailed view of a star other than the sun.
Andy Gould, an astronomer at Ohio State University, has more ambitious projects in mind. Using NASA's upcoming $1 billion Space Interferometry Mission, he wants to use gravitational lensing to study neighboring stars, brown dwarfs, neutron stars, and black holes a truly mind-bending endeavor.
For more about the Very Large Telescope, the largest optical telescope in the world, visit the European Southern Observatory's Web site at www.eso.org/projects/vlt
Students for the Exploration and Development of Space explain more about gravitational lensing around galaxy cluster Abell 2218; see www.seds.org/hst/A2218.html
For a diagram that explains gravitational lensing, check out imagine.gsfc.nasa.gov/docs/features/news/grav_lens.html