The Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Cosmological Implications from two Decades of Spectroscopic Surveys at the Apache Point observatory (2007.08991v2)

Abstract: We present the cosmological implications from final measurements of clustering using galaxies, quasars, and Ly$\alpha$ forests from the completed Sloan Digital Sky Survey (SDSS) lineage of experiments in large-scale structure. These experiments, composed of data from SDSS, SDSS-II, BOSS, and eBOSS, offer independent measurements of baryon acoustic oscillation (BAO) measurements of angular-diameter distances and Hubble distances relative to the sound horizon, $r_d$, from eight different samples and six measurements of the growth rate parameter, $f\sigma_8$, from redshift-space distortions (RSD). This composite sample is the most constraining of its kind and allows us to perform a comprehensive assessment of the cosmological model after two decades of dedicated spectroscopic observation. We show that the BAO data alone are able to rule out dark-energy-free models at more than eight standard deviations in an extension to the flat, $\Lambda$CDM model that allows for curvature. When combined with Planck Cosmic Microwave Background (CMB) measurements of temperature and polarization the BAO data provide nearly an order of magnitude improvement on curvature constraints. The RSD measurements indicate a growth rate that is consistent with predictions from Planck primary data and with General Relativity. When combining the results of SDSS BAO and RSD with external data, all multiple-parameter extensions remain consistent with a $\Lambda$CDM model. Regardless of cosmological model, the precision on $\Omega_\Lambda$, $H_0$, and $\sigma_8$, remains at roughly 1\%, showing changes of less than 0.6\% in the central values between models. The inverse distance ladder measurement under a o$w_0w_a$CDM yields $H_0= 68.20 \pm 0.81 \, \rm km\, s^{-1} Mpc^{-1}$, remaining in tension with several direct determination methods. (abridged)

• The paper delivers the final cosmological analysis from SDSS-IV eBOSS data, rigorously measuring BAO and RSD to constrain dark energy and cosmic curvature.
• It demonstrates that BAO measurements independently yield an 8-sigma detection of dark energy effects and consistently indicate a lower Hubble constant than local estimates.
• It establishes a 95% upper limit on neutrino masses below 0.111 eV, reinforcing the ΛCDM framework and compatibility with General Relativity.

Overview of "The Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Cosmological Implications from two Decades of Spectroscopic Surveys at the Apache Point Observatory"

The paper presents the final cosmological analysis derived from the Sloan Digital Sky Survey (SDSS) lineage, which includes data from SDSS, SDSS-II, BOSS, and eBOSS. This two-decade-long project aimed to measure large-scale structure and improve our understanding of cosmology, focusing on baryon acoustic oscillations (BAO) and redshift-space distortions (RSD). The comprehensive data set, which utilizes galaxies, quasars, and Lyman-alpha forests, enables stringent tests of the cosmological model, particularly through the measurement of the Hubble parameter, dark energy density, and the sum of neutrino masses.

Key Findings

1. BAO and Expansion History: The BAO data alone, without the use of CMB measurements, constrain the dark energy density parameter to more than eight standard deviations, supporting evidence of cosmic acceleration. The addition of BAO to CMB data improves constraints on cosmic curvature significantly, nearly by an order of magnitude.
2. Hubble Constant ($H_0$) Tension: BAO measurements provide estimates of $H_0$ that remain robust against different assumptions of the cosmological model, emphasizing a lower value of $H_0$ compared to local Universe measurements. This tension persists even when using different methodologies free from CMB data.
3. RSD and Growth Rate: The RSD data from BOSS and eBOSS significantly refine constraints on the growth rate of structure, fσ8, and improve precision on the parameters of modified gravity models. These findings indicate compatibility with General Relativity when combined with weak lensing data from other surveys.
4. Neutrino Mass: The combined data give a 95% upper limit on the sum of neutrino masses, $\sum m_\nu$, of less than 0.111 eV, helping to distinguish between normal and inverted neutrino mass hierarchies.

Methodology

The analysis utilizes comprehensive statistical techniques to extract cosmological parameters from the vast data set. It involves a detailed mapping of the observed galaxy distributions onto a fiducial cosmological model and employs sophisticated modeling of statistical and systematic uncertainties. The BAO signature is extracted through isotropic and anisotropic fits to the galaxy clustering data, and RSD analyses provide insight into the growth of structures over cosmic time.

Implications

The results from this paper solidify the $\Lambda$CDM as a robust model for understanding cosmic expansion and structure formation, while also positing stringent limits on deviations such as non-zero curvature or a time-varying dark energy equation of state. The improved precision over the last two decades underscores the synergy between diverse cosmological probes and heralds tighter constraints on fundamental physics, even as discrepancies in $H_0$ demand further investigation.

Future Perspectives

Looking to the future, the methodologies and data obtained from SDSS set a high standard for upcoming projects like DESI and Euclid, which will further enhance our understanding of the Universe. The findings encourage continued exploration into the diversity of astrophysical mechanisms at play in cosmology and hint at the necessity for new physics to reconcile current tensions in cosmological measurements.