Cosmological implications of baryon acoustic oscillation (BAO) measurements (1411.1074v3)

Abstract: We derive constraints on cosmological parameters and tests of dark energy models from the combination of baryon acoustic oscillation (BAO) measurements with cosmic microwave background (CMB) and Type Ia supernova (SN) data. We take advantage of high-precision BAO measurements from galaxy clustering and the Ly-alpha forest (LyaF) in the BOSS survey of SDSS-III. BAO data alone yield a high confidence detection of dark energy, and in combination with the CMB angular acoustic scale they further imply a nearly flat universe. Combining BAO and SN data into an "inverse distance ladder" yields a 1.7% measurement of $H_0=67.3 \pm1.1$ km/s/Mpc. This measurement assumes standard pre-recombination physics but is insensitive to assumptions about dark energy or space curvature, so agreement with CMB-based estimates that assume a flat LCDM cosmology is an important corroboration of this minimal cosmological model. For open LCDM, our BAO+SN+CMB combination yields $\Omega_m=0.301 \pm 0.008$ and curvature $\Omega_k=-0.003 \pm 0.003$. When we allow more general forms of evolving dark energy, the BAO+SN+CMB parameter constraints remain consistent with flat LCDM. While the overall $\chi^2$ of model fits is satisfactory, the LyaF BAO measurements are in moderate (2-2.5 sigma) tension with model predictions. Models with early dark energy that tracks the dominant energy component at high redshifts remain consistent with our constraints. Expansion history alone yields an upper limit of 0.56 eV on the summed mass of neutrino species, improving to 0.26 eV if we include Planck CMB lensing. Standard dark energy models constrained by our data predict a level of matter clustering that is high compared to most, but not all, observational estimates. (Abridged)

• The paper demonstrates that BAO data independently confirm dark energy and support a nearly flat universe when combined with CMB observations.
• The methodology integrates BAO, SN, and CMB measurements, yielding precise estimates of H0 (67.3±1.1 km/s/Mpc), matter density, and spatial curvature.
• Key findings include tight constraints on neutrino masses and identification of minor tensions with LyaF BAO data, suggesting areas for further investigation.

Overview of Cosmological Implications of Baryon Acoustic Oscillation (BAO) Measurements

This paper investigates the constraints placed on cosmological parameters by combining baryon acoustic oscillation (BAO) measurements with data from the cosmic microwave background (CMB) and reanalyzed Type Ia supernovae (SN). The primary aim is to provide insights into dark energy models and test the assumption of a flat universe.

Data and Methodology

The authors utilize high-precision BAO data from the Sloan Digital Sky Survey-III (SDSS-III) Baryon Oscillation Spectroscopic Survey (BOSS) involving galaxy clustering and the Lyman-$\alpha$ forest (LyaF). By treating the BAO scale as an uncalibrated standard ruler, the authors first evaluate how BAO data alone can detect dark energy. When combined with CMB data, these observations support a nearly flat universe model.

Key Findings

1. Dark Energy Detection: BAO data independently confirm the presence of dark energy, which is further corroborated by combining it with CMB data, indicating a nearly flat universe dominated by dark energy.
2. Hubble Constant ($H_0$) Measurement: Integrating BAO and SN data via an inverse distance ladder approach allows an $H_0$ measurement of $67.3 \pm 1.1$ km/s/Mpc, assuming standard pre-recombination physics.
3. Matter Density and Curvature: The combined BAO, SN, and CMB data yield precise estimates for matter density ($\Omega_m = 0.301 \pm 0.008$) and spatial curvature ($\Omega_k = -0.003 \pm 0.003$). This supports the assumption of a flat universe in the standard $\Lambda$CDM cosmological model.
4. Constraints on Neutrino Mass and Extra Relativistic Species: The analysis places upper limits on the summed mass of the neutrino species, particularly $\sum m_\nu < 0.56$ eV, tightening to $\sum m_\nu < 0.25$ eV when including CMB lensing data. For models allowing extra relativistic species, the effective number of neutrino species, $N_{\rm eff}$, is constrained to $3.43 \pm 0.26$.
5. LyaF BAO Tension: There is moderate tension between the LyaF BAO measurements and model predictions, suggesting potential inconsistencies or unexplained systematic errors.

Implications and Future Directions

This comprehensive analysis consolidates the current understanding of dark energy effects and supports the $\Lambda$CDM model, with its implications for cosmology being strong but not definitive. The minor tension with LyaF BAO measurements indicates a need for further investigation, possibly hinting at future modifications or extensions of the standard model. Continued advancements in CMB measurements and emerging large-scale surveys will likely address these tensions and refine other cosmological parameter estimates. The results also confirm the utility of combined cosmological probes in constraining the properties of dark energy, neutrino masses, and cosmic curvature, setting the stage for upcoming observational initiatives.