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The Baryon Oscillation Spectroscopic Survey (BOSS) is designed to measure the scale of baryon acoustic oscillations (BAO) in the clustering of matter over a larger volume than the combined efforts of all previous spectroscopic surveys of large-scale structure. BOSS uses 1.5 million luminous galaxies as faint as i = 19.9 over 10,000 deg 2 to measure BAO to redshifts z < 0.7. Observations of neutral hydrogen in the Lyα forest in more than 150,000 quasar spectra (g < 22) will constrain BAO over the redshift range 2.15 < z < 3.5. Early results from BOSS include the first detection of the large-scale three-dimensional clustering of the Lyα forest and a strong detection from the Data Release 9 data set of the BAO in the clustering of massive galaxies at an effective redshift z = 0.57. We project that BOSS will yield measurements of the angular diameter distance d A to an accuracy of 1.0% at redshifts z = 0.3 and z = 0.57 and measurements of H (z) to 1.8% and 1.7% at the same redshifts. Forecasts for Lyα forest constraints predict a measurement of an overall dilation factor that scales the highly degenerate D A (z) and H −1 (z) parameters to an accuracy of 1.9% at z ∼ 2.5 when the survey is complete. Here, we provide an overview of the selection of spectroscopic targets, planning of observations, and analysis of data and data quality of BOSS.

We combine the Ly-α forest power spectrum (LYA) from the Sloan Digital Sky Survey (SDSS) and high resolution spectra with cosmic microwave background (CMB) including 3-year WMAP, and supernovae (SN) and galaxy clustering constraints to derive new constraints on cosmological parameters. The existing LYA power spectrum analysis is supplemented by constraints on the mean flux decrement derived using a principle component analysis for quasar continua, which improves the LYA constraints on the linear power. We find some tension between the WMAP3 and LYA power spectrum amplitudes, at the ∼ 2σ level, which is partially alleviated by the inclusion of other observations: we find σ8 = 0.85 ± 0.02 compared to σ8 = 0.80 ± 0.03 without LYA. For the slope we find ns = 0.965 ± 0.012. We find no evidence for the running of the spectral index in the combined analysis, dn/d ln k = −(1.5 ± 1.2) × 10 −2 , in agreement with inflation. The limits on the sum of neutrino masses are significantly improved: mν < 0.17eV at 95% (< 0.32eV at 99.9%). This result, when combined with atmospheric and solar neutrino mixing constraints, requires that the neutrino masses cannot be degenerate, m3/m1 > 1.3 (95% c.l.). Assuming a thermalized fourth neutrino we find ms < 0.26eV at 95% c.l. and such neutrino cannot be an explanation for the LSND results. In the limits of massless neutrinos we obtain the effective number of neutrinos−2.5 and N eff ν = 3.04 is allowed only at 2.4 sigma. The constraint on the dark energy equation of state is w = −1.04 ± 0.06. The constraint on curvature is Ω k = −0.003 ± 0.006. Cosmic strings limits are Gµ < 2.3 × 10 −7 at 95% c.l. and correlated isocurvature models are also tightly constrained.PACS numbers: 98.80.Jk, 98.80.Cq