The Clustering of the SDSS DR7 Main Galaxy Sample I: A 4 per cent Distance Measure at z=0.15 (1409.3242v2)

Abstract: We create a sample of spectroscopically identified galaxies with $z < 0.2$ from the Sloan Digital Sky Survey (SDSS) Data Release 7, covering 6813 deg$^2$. Galaxies are chosen to sample the highest mass haloes, with an effective bias of 1.5, allowing us to construct 1000 mock galaxy catalogs (described in Paper II), which we use to estimate statistical errors and test our methods. We use an estimate of the gravitational potential to "reconstruct" the linear density fluctuations, enhancing the Baryon Acoustic Oscillation (BAO) signal in the measured correlation function and power spectrum. Fitting to these measurements, we determine $D_{V}(z_{\rm eff}=0.15) = (664\pm25)(r_d/r_{d,{\rm fid}})$ Mpc; this is a better than 4 per cent distance measurement. This "fills the gap" in BAO distance ladder between previously measured local and higher redshift measurements, and affords significant improvement in constraining the properties of dark energy. Combining our measurement with other BAO measurements from BOSS and 6dFGS galaxy samples provides a 15 per cent improvement in the determination of the equation of state of dark energy and the value of the Hubble parameter at $z=0$ ($H_0$). Our measurement is fully consistent with the Planck results and the $\Lambda$CDM concordance cosmology, but increases the tension between Planck$+$BAO $H_0$ determinations and direct $H_0$ measurements.

• The paper achieves a 4% BAO distance measurement at z=0.15 using improved reconstruction techniques on SDSS DR7 galaxy data.
• It selects galaxies in high-mass dark matter halos to build reliable mock catalogs for precise statistical error estimation.
• The refined measurement bridges BAO constraints across redshifts, enhancing tests of dark energy and alleviating H0 tensions.

Analyzing the SDSS DR7 Main Galaxy Sample: Precision BAO Measurements at Low Redshift

The paper entitled "The Clustering of the SDSS DR7 Main Galaxy Sample I: A 4 per cent Distance Measure at $z=0.15$" by A. J. Ross et al. presents an in-depth analysis of the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) main galaxy sample, specifically focusing on measuring the Baryon Acoustic Oscillation (BAO) scale at a low redshift of $z=0.15$. This paper leverages improved analysis techniques, including reconstruction, to enhance the signal of BAO in the data, resulting in a highly precise measurement that serves as a crucial addition to the BAO distance ladder.

Methodology and Sample Selection

The authors construct a sample from the SDSS DR7 of galaxies with redshifts $z < 0.2$, targeting those residing in high-mass dark matter halos. This selection enables them to generate reliable mock catalogs for statistical error estimation, contributing to the precision of the BAO measurement. The reconstruction technique, known for improving BAO feature visibility, was applied, showcasing its efficacy in enhancing the precision of clustering measurements.

Crucially, the analysis focuses on utilizing information from the large-scale structure to 'reconstruct' the linear density fluctuations, applying it successfully to SDSS data to achieve a BAO distance measure with better than 4% precision at an effective redshift of $z_{\rm eff}=0.15$. This effort fills a consequential gap in the BAO distance ladder, bridging local and higher redshift constraints, thereby offering significant contributions to our understanding of the universe's expansion history and dark energy properties.

Key Results and Implications

The BAO measurement, $D_{V}(z_{\rm eff}=0.15) = 664\pm25$, where $D_V$ is the volume-averaged distance, indicates a high precision level that supports cross-verification with other cosmological models and datasets. Ross et al. demonstrate that this measurement is consistent with the $\Lambda$CDM cosmology provided by Planck data but slightly increases the existing tension between the BAO-based $H_0$ estimations and direct $H_0$ measurements.

In synthesizing these results with BAO measurements from other surveys such as the Baryon Oscillation Spectroscopic Survey (BOSS) and the 6-degree Field Galaxy Survey (6dFGS), the paper achieves a notable 15% improvement in constraining the dark energy equation of state parameter, $w$, and $H_0$. This outcome underscores the importance of leveraging BAO as a cosmic ruler for precision cosmology.

Theoretical and Practical Implications

This paper's implications extend both practically and theoretically. Practically, this refined measurement at low redshift enhances the robustness of the BAO distance ladder, which is pivotal in constraining dark energy models. Theoretically, the ability to partially reconcile $H_0$ tensions while providing consistent results with $\Lambda$CDM showcases the model-independent power of BAO and emphasizes the necessity for further studies incorporating diverse datasets and methodologies.

Future Directions

The results from the DR7 galaxy clustering set the stage for future explorations into cosmic acceleration's intricacies. Data from subsequent surveys, including SDSS-IV and other forthcoming missions, will likely build on these findings to further elucidate the nature of dark energy and refine cosmological models. Expansion into three-dimensional analyses exploiting anisotropies, along with improvements in reconstruction techniques, may offer even deeper insights into universe structure and dynamics.

In summary, the research by Ross et al. is a critical step forward in cosmological precision measurements, reinforcing the utility of BAO studies in unraveling the mysteries of cosmic expansion and dark energy. Future research efforts will expand on this foundation, leveraging larger and deeper datasets to further comprehend the fundamental forces shaping our universe.