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BOSS Quasars Unveil a New Era in the Expansion History of the Universe

Light from distant quasars (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). Before BOSS, the Sloan Digital Sky Survey had collected spectra from 10 times fewer quasars (yellow dots) per square degree of sky in the accessible redshift range, which corresponds to about 10 billion years ago. By measuring the spectra from many more quasars in this range (red dots), BOSS can reconstruct a three-dimensional map of the otherwise invisible gas, revealing the large-scale structure of the early universe. (Illustration by Zosia Rostomian, LBNL; Nic Ross, BOSS Lyman-alpha team, LBNL; and Springel et al, Virgo Consortium and Max Planck Institute for Astrophysics) (Click for best resolution)

By collecting tens of thousands of quasar spectra, the Baryon Oscillation Spectroscopic Survey (BOSS) has measured the large-scale structure of the early universe for the first time. Like backlights in the fog, the quasars illuminate clouds of hydrogen gas along the line of sight. No other technique can reach back over 10 billion years to probe structure at a time when the expansion of the universe was still decelerating and dark energy was yet to turn on.

The Evolving Search for the Nature of Dark Energy

Regular variations in the temperature of the cosmic microwave background radiation signal regular variations in mass density at the moment the early Universe became transparent. These density variations gave rise to oscillations in the large-scale clustering of galaxies in the Universe today. The scale of the variations constitute a ruler by which to measure the expansion history of the Universe. (Images courtesy NASA’s Wilkinson Microwave Anisotropy Probe, left, and Sloan Digital Sky Survey, right. Click on image for best resolution)

Baryon acoustic oscillations provides a “standard ruler” for the Universe, a way to measure the details of dark energy.