The Clustering of Galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS): measuring growth rate and geometry with anisotropic clustering (1312.4899v2)

Abstract: We use the observed anisotropic clustering of galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 11 CMASS sample to measure the linear growth rate of structure, the Hubble expansion rate and the comoving distance scale. Our sample covers 8498 ${\rm deg}^2$ and encloses an effective volume of 6.0 ${\rm Gpc}^3$ at an effective redshift of $\bar{z} = 0.57$. We find $f\sigma_8 = 0.441 \pm 0.044$, $H = 93.1 \pm 3.0\ {\mathrm{km}\ \mathrm{s}^{-1} \mathrm{Mpc}^{-1}}$ and $D_{\rm A} = 1380 \pm 23\ {\rm Mpc}$ when fitting the growth and expansion rate simultaneously. When we fix the background expansion to the one predicted by spatially-flat $\Lambda$CDM model in agreement with recent Planck results, we find $f\sigma_8 = 0.447 \pm 0.028$ (6 per cent accuracy). While our measurements are generally consistent with the predictions of $\Lambda$CDM and General Relativity, they mildly favor models in which the strength of gravitational interactions is weaker than what is predicted by General Relativity. Combining our measurements with recent cosmic microwave background data results in tight constraints on basic cosmological parameters and deviations from the standard cosmological model. Separately varying these parameters, we find $w = -0.983 \pm 0.075$ (8 per cent accuracy) and $\gamma = 0.69 \pm 0.11$ (16 per cent accuracy) for the effective equation of state of dark energy and the growth rate index, respectively. Both constraints are in good agreement with the standard model values of $w=-1$ and $\gamma = 0.554$.

• The paper reports a precise measurement of the growth rate parameter fσ8 = 0.441±0.044 using advanced redshift-space distortion models.
• It derives robust estimates for the Hubble expansion rate (H = 93.1±3.0 km/s/Mpc) and angular diameter distance (D_A = 1380±23 Mpc) via the Alcock-Paczynski effect.
• Results align with ΛCDM while hinting at potential deviations in gravity and dark energy when combined with recent CMB data.

Anisotropic Clustering in the BOSS Survey: Implications for Cosmology

The paper by Samushia et al. presents an analysis of the anisotropic clustering of galaxies within the Baryon Oscillation Spectroscopic Survey (BOSS), utilizing data from the Sloan Digital Sky Survey (SDSS-III). This paper aims to extract key cosmological parameters, focusing on the linear growth rate of structure, Hubble expansion rate, and comoving distance scale, using the BOSS Data Release 11 (DR11) CMASS sample which comprises an effective volume of 6 Gpc$^3$ at a mean redshift of $\bar{z} = 0.57$.

Methodology and Key Findings

The analysis employs a comprehensive statistical approach to interpret the galaxy clustering data, facilitated through the streaming model for modeling redshift-space distortions (RSD) and considering geometrical distortions via the Alcock-Paczynski (AP) effect. Key findings include:

• The paper reports a measurement of the growth rate parameter $f\sigma_8 = 0.441 \pm 0.044$, consistent with, though slightly favoring, models that predict a weaker gravity interaction than that proposed by General Relativity.
• The cosmological parameters derived provide estimates for distance measures: $$H = 93.1 \pm 3.0\ \mathrm{km}\ \mathrm{s}^{-1} \mathrm{Mpc}^{-1}$$ and $D_A = 1380 \pm 23\ \mathrm{Mpc}$, when model-fitting the growth and expansion rate simultaneously.
• A compatibility with flat $\Lambda$CDM cosmology is generally observed. However, when combined with recent cosmic microwave background (CMB) data, the paper imposes stringent constraints on deviations from the standard model.
• By varying cosmological parameters, the paper finds $w = -0.983 \pm 0.075$ for the dark energy equation of state and $\gamma = 0.69 \pm 0.11$ for the growth rate index, reflecting consistency with the $\Lambda$CDM predictives of $w = -1$ and $\gamma = 0.554$.

Implications and Speculations

This research enhances our understanding of the growth of cosmic structures and the dynamics of dark energy. The mildly lower values of $f\sigma_8$ suggest potential intriguing aspects in cosmic structure formation that warrant further investigation. As precision in measuring $f\sigma_8$ improves, deviations from GR or $\Lambda$CDM, if substantiated, could stimulate significant advances in the theoretical frameworks of gravity and dark energy.

Future observational developments, including expanded galaxy surveys, will be essential for validating these findings. The methodologies exemplified in this paper, particularly the integrated use of RSD and AP effects combined with precision CMB constraints, set a precedent for future cosmological analyses.

Conclusion

The Samushia et al. paper underscores the powerful combination of large-scale structure data with CMB measurements to test the fundamental predictions of cosmological models. While results align closely with $\Lambda$CDM and GR, they retain an openness to new physics that could subtly adjust our understanding of the universe's fabric. As cosmological probes advance, revelations about the early and late universe could offer unprecedented insights compatible with or beyond the current paradigms.