Astronomers find gravity's signature.

The Sloan Digital Sky Survey--the largest ever--confirmed the role of gravity in growing structures in the universe, using the result to measure the geometry of the universe precisely. Researchers from New York University, University of Pittsburgh (Pa.), the Massachusetts Institute of Technology, Cambridge, and the United Kingdom's University of Portsmouth detected ripples in the galaxy distribution made by sound waves generated soon after the Big Bang.

These sound waves left their imprint in the Cosmic Microwave Background, remnant radiation from the Big Bang seen when the universe was 400,000 years old. Now the corresponding cosmic ripples can be spotted in the SDSS galaxy maps. Viewing the same ripples in the early universe and the relatively nearby galaxies is smoking-gun evidence that the distribution of galaxies today grew via gravity, astronomers insist.

The early universe was smooth and homogenous, quite a contrast from the clumpy clusters of galaxies observed today. One of the major goals of cosmology is to understand how these structures grew. Present-day galaxies consist of ordinary matter, made up of the atoms of our familiar world. However, astronomers long have known that there is roughly five times more "dark" matter than ordinary or "baryonic" matter. Understanding how gravity causes the clumps that will become galaxies and clusters to grow as the universe expands requires studying the interaction between ordinary and dark matter.

"In the early universe, the interaction between gravity and pressure caused a region of space with more ordinary matter than average to oscillate, sending out waves very much like the ripples in a pond when you throw in a pebble," explains Bob Nichol, an astrophysicist at the Institute of Cosmology & Gravitation at the University of Portsmouth. "These ripples in the matter grew for 1,000,000 years until the universe cooled enough to freeze them in place. What we now see in the SDSS galaxy data is the imprint of these ripples billions of years later."

Gravity's signature likewise could be likened to a ringing bell's resonance in time and space. The last ring gets forever quieter and deeper in tone as the universe expands. It now is so faint as to be detectable only by the most sensitive surveys. The SDSS has measured the tone of this last ring quite accurately.

Comparing the measured value with that predicted gives a yardstick that enables researchers to determine the rate at which the universe expands, which, in turn, depends on the amount of dark matter and energy. Dark energy still is the mysterious force driving the acceleration and expansion of the universe today.