The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological analysis of the DR12 galaxy sample (1607.03155v1)

Abstract: We present cosmological results from the final galaxy clustering data set of the Baryon Oscillation Spectroscopic Survey, part of the Sloan Digital Sky Survey III. Our combined galaxy sample comprises 1.2 million massive galaxies over an effective area of 9329 deg^2 and volume of 18.7 Gpc^3, divided into three partially overlapping redshift slices centred at effective redshifts 0.38, 0.51, and 0.61. We measure the angular diameter distance DM and Hubble parameter H from the baryon acoustic oscillation (BAO) method after applying reconstruction to reduce non-linear effects on the BAO feature. Using the anisotropic clustering of the pre-reconstruction density field, we measure the product DM*H from the Alcock-Paczynski (AP) effect and the growth of structure, quantified by f{\sigma}8(z), from redshift-space distortions (RSD). We combine measurements presented in seven companion papers into a set of consensus values and likelihoods, obtaining constraints that are tighter and more robust than those from any one method. Combined with Planck 2015 cosmic microwave background measurements, our distance scale measurements simultaneously imply curvature {\Omega}_K =0.0003+/-0.0026 and a dark energy equation of state parameter w = -1.01+/-0.06, in strong affirmation of the spatially flat cold dark matter model with a cosmological constant ({\Lambda}CDM). Our RSD measurements of f{\sigma}_8, at 6 per cent precision, are similarly consistent with this model. When combined with supernova Ia data, we find H0 = 67.3+/-1.0 km/s/Mpc even for our most general dark energy model, in tension with some direct measurements. Adding extra relativistic species as a degree of freedom loosens the constraint only slightly, to H0 = 67.8+/-1.2 km/s/Mpc. Assuming flat {\Lambda}CDM we find {\Omega}_m = 0.310+/-0.005 and H0 = 67.6+/-0.5 km/s/Mpc, and we find a 95% upper limit of 0.16 eV/c^2 on the neutrino mass sum.

• The paper employs BAO and redshift-space distortion techniques on 1.2M galaxies to derive high-precision cosmological parameters like ΩK and w.
• The study uses post-reconstruction BAO measurements and joint analysis with Planck CMB data to robustly support a spatially flat ΛCDM cosmology.
• The analysis of the structure growth rate (fσ8) across three redshift slices establishes benchmarks for future surveys probing dark energy and cosmic expansion.

Cosmological Analysis of BOSS Galaxies

The paper, "The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological analysis of the DR12 galaxy sample," provides a comprehensive analysis of large-scale galaxy clustering data from the Baryon Oscillation Spectroscopic Survey (BOSS) as part of the Sloan Digital Sky Survey III (SDSS-III). This dataset comprises a massive galaxy sample cataloged in the twelfth data release (DR12), covering a significant volume of the universe and serving as a crucial tool in contemporary cosmology.

Data and Methodology

The BOSS DR12 galaxy sample consists of 1.2 million massive galaxies spanning an effective area of approximately 9,329 square degrees and a volume of 18.7 cubic gigaparsecs. The analysis divides these galaxies into three overlapping redshift slices centered at effective redshifts 0.38, 0.51, and 0.61. The paper's primary goal is to analyze galaxy clustering statistics using various techniques, including the measurement of the baryon acoustic oscillation (BAO) scale and the characterization of redshift-space distortions (RSD).

Post-reconstruction BAO techniques are employed to sharpen the acoustic feature and improve distance measurements related to the angular diameter distance $D_M$ and the Hubble parameter $H$. The paper harnesses the Alcock-Paczynski (AP) effect and the growth rate of structures, quantified by $f\sigma_8(z)$, to break degeneracies between these parameters, providing precise constraints on cosmological models.

Results

The analysis reveals tightly constrained cosmological parameters. The combination of galaxy clustering results with Planck 2015 cosmic microwave background (CMB) measurements strongly supports a spatially flat cold dark matter model with a cosmological constant ($\Lambda$CDM). Specifically, the combined dataset allows for the measurement of cosmic parameters such as the curvature, $\Omega_K=0.0003\pm0.0026$, and the equation-of-state parameter, $w=-1.01\pm0.06$. These measurements affirm the consistency of a spatially flat universe with dark energy described by a cosmological constant.

Furthermore, the method allows for the extraction of the growth rate of cosmic structures, with $f\sigma_8$ measured to 6% precision across the three redshift slices. When combined with Type Ia supernova data, the results yield a present-day Hubble constant of $H_0=67.3\pm1.0$, though in mild tension with some other direct measurements.

Implications and Future Directions

The findings have significant implications for our understanding of dark energy and the overall geometry of the universe. The precise measurements of $D_M$ and $H$, along with checks on the growth of structure, provide robust cross-validation for the prevalent cosmological model. These results will serve as a benchmark for future spectroscopic galaxy surveys designed to further elucidate the characteristics of dark energy, the mass scale of neutrinos, and possible extensions to General Relativity on cosmological scales.

The paper showcases the synergy between different cosmological probes and illustrates the potential of large-scale structure surveys to answer fundamental questions about the universe. While the BAO feature provides a cornerstone for distance measurements, the ability to account for RSD adds depth to our understanding of structure growth and the dynamic evolution of the universe.

Future advancements in survey capabilities, including extended redshift coverage and improved volume, promise to further tighten cosmological constraints and potentially reveal new physics beyond the current model. With ongoing and upcoming surveys, the approach demonstrated in this research sets the stage for next-generation explorations of cosmic history and the fate of the universe.