Baryon oscillation spectra for all

By professional astronomy standards, the 2.5 m telescope at Apache Point Observatory is quite small. More than 50 research telescopes are larger and many are located at much better sites. Apache Point Observatory is also a little too close to city lights – the atmospheric turbulence that dominates the sharpness of focus is about two times worse than at the best sites on Earth – and summer monsoons shut down the observatory for two months each year.

Yet, the Sloan Digital Sky Survey (SDSS), using this telescope, has produced the most highly cited data set in the history of astronomy (Trimble and Ceja 2008; Madrid and Macchetto 2009). Its success is rooted in the combination of high-quality, multipurpose data and open access for everyone: SDSS has obtained 5-filter images of about a quarter of the sky, spectra of 2.4 million objects and has made them publicly available on a yearly basis, even as the survey continues.

SDSS-III launched its ninth data release (DR9) on 31 July. This is the first release to include data from the upgraded spectrographs of the Baryon Oscillation Spectroscopic Survey (BOSS) – the largest of the four subsurveys of SDSS-III. By measuring more distant galaxies, these spectra probe a larger volume of the universe than all previous surveys combined.

BOSS has already published its flagship measurement of baryon acoustic oscillations (BAO) to constrain dark energy using these data (Anderson et al. 2012). BAO are the leftover imprint of primordial matter-density fluctuations that froze out as the universe expanded, leaving correlations in the distances between galaxies. The size scale of these correlations acts as a “standard ruler” to measure the expansion of the universe, complementing the “standard candles” of Type Ia supernovae that led to the discovery of the accelerating expansion of the universe.

Another major BOSS analysis using these data is still in progress. In principle, BAO can also be measured by using bright, distant quasars as backlights and measuring the “Lyman alpha forest” absorption in the spectra as intervening neutral hydrogen absorbs the quasars’ light. The wavelength of the absorption traces the red shift of the hydrogen and the amount of absorption traces its density. Thus, this also measures the structure of matter – including BAO – but at much further distances than is possible with galaxies. BOSS has the first data set with enough quasars to make this measurement and the collaboration is nearing completion of this analysis. However, the final results are not yet published and now the data are public for anyone else to try this.

Are there any surprises in the results? Not yet. BOSS has the most accurate BAO measurements yet, with distances measured to 1.7%, but the results are consistent with the “ΛCDM” cosmological standard model, which includes a dark-energy cosmological constant (Λ) and cold dark matter (CDM). But DR9 contains only about a third of the full BOSS survey and BOSS has already finished observations for data release 10 (DR10), due to be released in July 2013. DR10 will also include the first data from APOGEE, another SDSS-III subsurvey that probes the dynamical structure and chemical history of the Milky Way.