The largest-ever 3D map of distant galaxies has been constructed with measurements made by the Baryon Oscillation Spectroscopic Survey (BOSS) program of the Sloan Digital Sky Survey-III, headquartered at Lawrence Berkeley National Lab. Shaped by a continuous tug-of-war between dark matter and dark energy, the map revealed by BOSS allows scientists to measure the expansion rate of the universe and thus determine the amount of matter and dark energy that make up the present-day universe. "We have spent five years collecting measurements of 1.2 million galaxies over one quarter of the sky to map out the structure of the universe over a volume of 650 cubic billion light years," says Jeremy Tinker of New York University, one of hundreds of scientists and astronomers who worked on the project. The research was published in Monthly Notices of the Royal Astronomical Society. One slice through the map of the large-scale structure of the universe from the Sloan Digital Sky Survey and its Baryon Oscillation Spectroscopic Survey. Each dot in this picture indicates the position of a galaxy 6 billion years into the past. The image covers about 1/20th of the sky, a slice of the universe 6 billion light-years wide, 4.5 billion light-years high, and 500 million light-years thick. Color indicates distance from Earth, ranging from yellow on the near side of the slice to purple on the far side. Galaxies are highly clustered, revealing superclusters and voids whose presence is seeded in the first fraction of a second after the Big Bang. This image contains 48,741 galaxies, about 3 percent of the full survey dataset. Gray patches are small regions without survey data. Courtesy of Daniel Eisenstein and SDSS-III. BOSS measures the expansion rate of the universe by determining the size of the baryonic acoustic oscillations (BAO) in the 3D distribution of galaxies. The original BAO size is determined by pressure waves that traveled through the young universe up to when it was only 400,000 years old, at which point they became frozen in the matter distribution of the universe. The end result is that galaxies have a slight preference to be separated by a characteristic distance that astronomers call the acoustic scale. The size of the acoustic scale at 13.4 billion years ago has been exquisitely determined from observations of the cosmic microwave background from the light emitted when the pressure waves became frozen. Measuring the distribution of galaxies since that time allows astronomers to measure how dark matter and dark energy have competed to govern the rate of expansion of the universe. "Measuring the acoustic scale across cosmic history gives a direct ruler with which to measure the universe's expansion rate. With BOSS, we have traced the BAO's subtle imprint on the distribution of galaxies spanning a range of time from 2 to 7 billion years ago,” said Ariel Sanchez of the Max-Planck Institute of Extraterrestrial Physics. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration. For more information, visit www.sdss3.org.