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—normally in the UV and 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 ≳ 2.0, corresponding to a time more than 10.3 billion years ago, or less than 3.4 billion years after the Big Bang. (For reference, z = 0.0 refers to the current epoch, and even z ≳ 0.1 was long considered "high redshift.") Redshift correlations along different lines of sight then uncover hydrogen's large-scale three-dimensional structure. In the resulting correlation function, the team found a pronounced peak due to "baryon acoustic oscillations" (see the article on BAOs in Physics Today, April 2008, page 44) that constrained the expansion rate at that early time. With further analysis, the BOSS team (63 members from 27 institutions in 9 countries) confirmed that the cosmic expansion switched over at about z = 0.8 (roughly 7 billion years ago), from slowing down in the matter-dominated era to speeding up in the dark-energy-dominated era. (N. G. Busca et al., http://arxiv.org/abs/1211.2616.)—Stephen G. Benka