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 on average 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, Lawrence Berkeley National Laboratory; Nic Ross, BOSS Lyman-alpha team, Berkeley Lab; and Springel et al, Virgo Consortium and Max Planck Institute for Astrophysics