SAN DIEGO, CALIFORNIA--The graceful patterns of galaxies strewn across space preserve an imprint of the big bang, astronomers have shown. Two international teams verified that sound waves cascading through the infant universe created an expanding set of ripples, where matter tended to congregate as time went on. Today's galaxies clump along those ripples--an effect that matches predictions from other studies of cosmic evolution.
As the newborn universe expanded, matter and energy seethed within a dense cosmic broth. Tiny fluctuations in this broth--likely arising from quantum effects—resulted in some regions having more matter than the rest. When gravity tried to compress these denser regions, the hot particles were already so close that they resisted being squeezed. Instead, they rebounded and then oscillated, sending sound waves reverberating through space. Today, the stretched-out remnants of these waves still pervade the universe as subtle ripples in a faint glow called the cosmic microwave background. Theorists predicted that galaxies would tend to be found on the peaks of these ripples, just as water tends to pile up in ripples spreading on a pond. But astronomers needed to chart the positions of hundreds of thousands of galaxies to be sure.
The search took place from both sides of the planet. In the Northern Hemisphere, the U.S.-led Sloan Digital Sky Survey mapped more than 46,000 bright galaxies across a huge swath of the sky. Analysis revealed that the galaxies have a characteristic separation of 500 million light-years, says astronomer and study leader Daniel Eisenstein of the University of Arizona, Tucson. The pattern matches the expected distances between the peaks of ripples that have expanded since the early universe, Eisenstein notes.
An independent study of 221,000 galaxies by the 2dF Galaxy Redshift Survey in the Southern Hemisphere, conducted by the United Kingdom and Australia, reached the same conclusion. Survey astronomer Richard Ellis, based at the California Institute of Technology in Pasadena, says that as scientists survey galaxies deeper in space, they plan to look for changes in the galaxy patterns that may give clues to the strange forces that are making the cosmos expand faster and faster. Both teams presented their work here on 11 January at a meeting of the American Astronomical Society.
Clustering of galaxies along the vast ripples was predicted more than 30 years ago, so finally seeing the structures is a triumph, says astrophysicist Martin Rees of Cambridge University, U.K. "The concordant picture we have of the universe is hanging together extremely well."