The 6dF Galaxy Survey: Baryon Acoustic Oscillations and the Local Hubble Constant (1106.3366v1)

Abstract: We analyse the large-scale correlation function of the 6dF Galaxy Survey (6dFGS) and detect a Baryon Acoustic Oscillation (BAO) signal. The 6dFGS BAO detection allows us to constrain the distance-redshift relation at z_{\rm eff} = 0.106. We achieve a distance measure of D_V(z_{\rm eff}) = 456\pm27 Mpc and a measurement of the distance ratio, r_s(z_d)/D_V(z_{\rm eff}) = 0.336\pm0.015 (4.5% precision), where r_s(z_d) is the sound horizon at the drag epoch z_d. The low effective redshift of 6dFGS makes it a competitive and independent alternative to Cepheids and low-z supernovae in constraining the Hubble constant. We find a Hubble constant of H_0 = 67\pm3.2 km s^{-1} Mpc^{-1} (4.8% precision) that depends only on the WMAP-7 calibration of the sound horizon and on the galaxy clustering in 6dFGS. Compared to earlier BAO studies at higher redshift, our analysis is less dependent on other cosmological parameters. The sensitivity to H_0 can be used to break the degeneracy between the dark energy equation of state parameter w and H_0 in the CMB data. We determine that w = -0.97\pm0.13, using only WMAP-7 and BAO data from both 6dFGS and \citet{Percival:2009xn}. We also discuss predictions for the large scale correlation function of two future wide-angle surveys: the WALLABY blind H{\sc I} survey (with the Australian SKA Pathfinder, ASKAP), and the proposed TAIPAN all-southern-sky optical galaxy survey with the UK Schmidt Telescope (UKST). We find that both surveys are very likely to yield detections of the BAO peak, making WALLABY the first radio galaxy survey to do so. We also predict that TAIPAN has the potential to constrain the Hubble constant with 3% precision.

• The paper identifies a clear BAO signature in the 6dF Galaxy Survey, achieving a 5.9% precision in distance measurement and a 4.8% determination of H0 using WMAP-7 calibrations.
• The methodology leverages the large-scale correlation function at an effective redshift of 0.106, offering an independent and competitive cosmological probe.
• The findings validate the ΛCDM model and provide robust priors for dark energy and curvature, while motivating future surveys like WALLABY and TAIPAN for even finer constraints.

Insights into Baryon Acoustic Oscillations and the Local Hubble Constant from the 6dF Galaxy Survey

The paper discusses the analysis and implications of detecting a Baryon Acoustic Oscillation (BAO) signal from the 6dF Galaxy Survey (6dFGS), a significant endeavor in cosmological research focused on understanding the Universe's expansion dynamics and composition. The detection of BAOs is critical because they serve as a "standard ruler" for measuring cosmological distances, thus allowing the determination of various cosmological parameters, including the Hubble constant ($H_0$), which describes the universe's expansion rate.

The 6dFGS covers a vast area—almost half the sky—and focuses on low-redshift galaxies, which offers a distinct perspective compared to other surveys operating at higher redshifts, such as SDSS or 2dFGRS. The analysis centers around the large-scale correlation function derived from the survey, with an effective redshift ($z_{\rm eff}$) of 0.106. Importantly, this work provides novel constraints on the distance-redshift relation and improves the understanding of low-redshift cosmology.

Summary of Numerical Results and Parameter Constraints

One pivotal result from the analysis is the determination of the distance $D_V(z_{\rm eff})$ to the effective redshift with a 5.9% precision, leading to $D_V(z_{\rm eff}) = 456\pm27$ Mpc. The paper further constrains the Hubble constant to $H_0 = 67\pm3.2$ km s$^{-1}$ Mpc$^{-1}$, with a remarkable precision of 4.8%, relying solely on WMAP-7 calibrations. Compared to higher-redshift BAO analyses, this makes the 6dFGS less sensitive to extrapolated cosmological parameters, thus providing a relatively independent measure of $H_0$. Furthermore, the paper claims the potential to constrain the dark energy equation of state parameter $w = -0.97\pm0.13$ when combined with other datasets.

Theoretical and Practical Implications

The findings have substantial implications:

1. Independent Validation: The low effective redshift makes 6dFGS an independent and competitive alternative to other methods like Cepheids and low-$z$ supernovae, used to constrain the Hubble constant, due to its reduced sensitivity to assumptions about dark energy ($w$) and curvature ($\Omega_k$).
2. Cosmological Constraints: The derived measures can serve as priors for other analyses, reducing parameter degeneracies in, for example, cosmic microwave background (CMB) data. This is especially relevant in constraining the equation of state parameter for dark energy, thereby advancing understanding in this critical area.
3. Forward-Looking Perspectives: The detection of the BAO signal within 6dFGS aligns with the predictions of $\Lambda$CDM, reinforcing the standard model of cosmology. The authors also discuss future prospects for surveys like WALLABY and TAIPAN, which promise even more precise constraints on the Hubble constant and possibly more detailed knowledge of the Universe’s large-scale structure.
4. Systematic Checks: The capability to cross-check results with other low redshift distance indicators allows for an evaluation of systematic uncertainties associated with different methodologies, enhancing the robustness of cosmological conclusions.

Future Developments

Looking ahead, the detailed predictions for upcoming surveys such as WALLABY and TAIPAN highlight the potential to further refine cosmological parameter constraints, especially the Hubble constant, to even higher precision. These surveys may also utilize multiple tracer analyses to maximize the information garnered from large-scale structures, building a more comprehensive picture of cosmic history and evolution.

Conclusion

The 6dFGS provides pivotal measurements in an under-explored regime, offering a clearer picture of low redshift cosmology and a refined measure of cosmic distances and expansion rates. The survey's success underscores the continued need for broad, all-sky analyses and the calibration of standard rulers, cementing its contributions to precision cosmology and the ongoing quest to decode the universe’s most profound mysteries.