Light from distant quasars (red dots at left) is partially absorbed as it passes through clouds of hydrogen gas. A “forest” of hydrogen absorption lines in an individual quasar’s spectrum (inset) pinpoints denser clumps of gas along the line of sight, and the spectra are collected by the telescope’s spectrograph (square at right). The accessible redshift range corresponds on average to about 10 billion years ago. While the Sloan Digital Sky Survey had previously collected spectra from some quasars in this range, by measuring 10 times as many per square degree of sky BOSS can reconstruct a three-dimensional map of the otherwise invisible gas, revealing the large-scale structure of the early universe. (Credit: Illustration by Zosia Rostomian, Lawrence Berkeley National Laboratory; Nic Ross, BOSS Lyman-alpha team, Berkeley Lab; and Springel et al, Virgo Consortium and Max Planck Institute for Astrophysics)

ScienceDaily (Nov. 12, 2012) — BOSS, the Baryon Oscillation Spectroscopic Survey, is mapping a huge volume of space to measure the role of dark energy in the evolution of the universe. BOSS is the largest program of the third Sloan Digital Sky Survey (SDSS-III) and has just announced the first major result of a new mapping technique, based on the spectra of over 48,000 quasars with redshifts up to 3.5, meaning that light left these active galaxies up to 11.5 billion years in the past.

"No technique for dark energy research has been able to probe this ancient era before, a time when matter was still dense enough for gravity to slow the expansion of the universe, and the influence of dark energy hadn't yet been felt," says BOSS principal investigator David Schlegel, an astrophysicist in the Physics Division of the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab). "In our own time, expansion is accelerating because the universe is dominated by dark energy. How dark energy effected the transition from deceleration to acceleration is one of the most challenging questions in cosmology."

Two ways to measure the expanding universe

As an international collaboration, many of whose leading scientists are present or former members of Berkeley Lab, BOSS studies dark energy by mapping baryon acoustic oscillations (BAO) -- the large-scale network of variations in the distribution of visible galaxies and hard-to-see clouds of intergalactic gas, which also reveal impossible-to-see dark matter. The regular spacing of peaks in matter density originated in primordial density variations, whose remnants are visible in the cosmic microwave background radiation. This spacing provides a cosmic ruler for calibrating the rate of expansion wherever BAO can be measured.