When the universe was slowing down

When the universe was slowing down. As the light from a distant object travels toward Earth, it can pass through clouds of gas that leave an absorption imprint on the light’s spectrum. On a cosmological scale, the so-called Lyman-alpha lines due to intervening neutral atomic hydrogen—which normally emits in the UV and is therefore unobservable by ground-based telescopes—are redshifted into more accessible spectral regions. The farther the object, the more complex the absorption spectrum can be; for quasars the hydrogen lines are so densely packed that they are known as “the Ly-α forest.” With the beacon of a single quasar, absorption-line redshifts in the Ly-α forest can reveal the distribution of neutral hydrogen along the line of sight. As part of the Sloan Digital Sky Survey III, the Baryon Oscillation Spectroscopic Survey (BOSS) team has now acquired spectra for more than 48 000 quasars with redshifts of z...