For the past five-billion years, the expansion of the universe has been powered by a mysterious repulsive force known as "dark energy." Now, thanks to a new technique for measuring the three-dimensional structure of the distant universe, scientists in an international team within the Sloan Digital Sky Survey (SDSS-III), including an astronomer at Penn State University, have made the first measurement of the rate of this cosmic expansion as it was just three-billion years after the Big Bang.
"Observations in the past 15 years have revealed that the expansion rate of the universe is accelerating," said Donald Schneider, Distinguished Professor of Astronomy and Astrophysics at Penn State, a coauthor of the study. "Most cosmological models predict that when the universe was young, dark energy had little influence on the expansion; at that time the evolution of the large-scale structure of the universe was dominated by gravitation, which is an attractive force that acted to slow the expansion. The new SDSS-III observations are an important probe of this early era." Schneider is the Sloan Digital Sky Survey's survey coordinator and scientific publications coordinator.
The above graph shows how the universe's expansion rate has changed over the last 10-billion years. Until recently, three-dimensional maps by BOSS and other surveys were able to measure the regular distribution of galaxies back to only about five-and-a-half-billion years ago, a time when the expansion of the universe was already accelerating. The numbers along the bottom of the graph show the time in the universe's past, in billions of years. The vertical scale (y-axis) shows the expansion rate of the universe; higher means the universe was expanding faster.These older measurements appear as data points toward the right of the graph. The new SDSS-III measurements, shown as the data point to the far left, have now probed the structure of the early universe at a time when expansion was still slowing down.