IRVINE, CALIFORNIA--Giant walls of galaxies, hundreds of millions of light-years long, may have crisscrossed the universe when it was just 15% of its present age. If confirmed, the finding, revealed here earlier this week at a National Academy of Sciences colloquium, would suggest that the universe has far less mass than many theorists predict. It may even indicate that the universe's largest features were there from the beginning.
A team led by Charles Steidel of the California Institute of Technology (Caltech) found these structures by exploiting a shortcut for picking out galaxies at huge distances. The observers aimed the giant 10-meter Keck telescope in Hawaii at distant objects whose ultraviolet light spectrum is partly blotted out by interstellar hydrogen. The telescope measured how much each object's light has been redshifted--displaced toward the long-wavelength end of the spectrum--by the expansion of the universe. The redshift indicates the galaxy's distance and hence its age. As of last month, the team had cataloged 168 galaxies at redshifts of between 2.8 and 3.5, which translates to perhaps 2 billion years after the big bang--early days in a universe that is now at least 12 billion years old. The researchers quickly noticed that the numbers of these galaxies varied widely in different "fields" scattered around the sky. These density spikes probably mark places where the Keck's line of sight is piercing great sheets of galaxies.
"It's a fantastic result," says Neta Bahcall of Princeton University. All of this evidence for early large-scale structure could support a scenario in which cosmic clumpiness originated a few hundred thousand years after the big bang, long before any galaxies formed and while the universe was still a sea of hot, ionized gas. If this hot soup of matter had the right ingredients, the clumps could have turned into the great walls of galaxies persisting all the way up to the present.
But it's also troubling for some theorists. "The more you push this back in time, the harder pressed the theorists become," says Judith Cohen of Caltech, who has seen similar structures at more modest distances. The matter-rich universe they have pictured should evolve rapidly over time, spurred by gravitational forces, so that its early architecture should look nothing like today's. "Any time the early universe starts looking more like today's, it favors a low [density]," says Bahcall. One thing is clear: Observers will have to explore still greater distances and earlier times to see whether the eerie familiarity of the early universe really does go back all the way to the beginning.