11-18-2012, 08:44 AM
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BOSS Quasars Unveil a New Era in the Expansion History of the Universe
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.
Tens of thousands of targets for surveying the Lyman-alpha forest of BOSS quasars were selected, and their exact positions plotted, based on imaging and photometric data (the intensity of their light in different colors) from previous phases of the Sloan Digital Sky Survey.
Until recently, three-dimensional maps by BOSS and other surveys were able to measure the regular distribution of galaxies back to an average of only about five and a half billion years ago, a time when the expansion of the universe was already accelerating. BOSS’s quasar measurements (red circle, left), by measuring the distribution of intergalactic gas, have now probed the structure of the early universe at a time when expansion was still slowing under the influence of gravity. The quasar data gives new access to the transition from deceleration to acceleration caused by dark energy.
All generalizations are dangerous.