Baryon Acoustic Oscillations in the Ly-α forest of BOSS quasars

Abstract: We report a detection of the baryon acoustic oscillation (BAO) feature in the three-dimensional correlation function of the transmitted flux fraction in the \Lya forest of high-redshift quasars. The study uses 48,640 quasars in the redshift range $2.1\le z \le 3.5$ from the Baryon Oscillation Spectroscopic Survey (BOSS) of the third generation of the Sloan Digital Sky Survey (SDSS-III). At a mean redshift $z=2.3$, we measure the monopole and quadrupole components of the correlation function for separations in the range $20\hMpc<r<200\hMpc$. A peak in the correlation function is seen at a separation equal to $(1.01\pm0.03)$ times the distance expected for the BAO peak within a concordance $\Lambda$CDM cosmology. This first detection of the BAO peak at high redshift, when the universe was strongly matter dominated, results in constraints on the angular diameter distance $\da$ and the expansion rate $H$ at $z=2.3$ that, combined with priors on $H_0$ and the baryon density, require the existence of dark energy. Combined with constraints derived from Cosmic Microwave Background (CMB) observations, this result implies $H(z=2.3)=(224\pm8){\rm km\,s^{-1}Mpc^{-1}}$, indicating that the time derivative of the cosmological scale parameter $\dot{a}=H(z=2.3)/(1+z)$ is significantly greater than that measured with BAO at $z\sim0.5$. This demonstrates that the expansion was decelerating in the range $0.7<z<2.3$, as expected from the matter domination during this epoch. Combined with measurements of $H_0$, one sees the pattern of deceleration followed by acceleration characteristic of a dark-energy dominated universe.

• The paper demonstrates the first detection of a BAO peak in the Lyα forest using 48,640 BOSS quasars, confirming ΛCDM predictions.
• It employs a detailed 3D correlation analysis of the Lyα transmitted flux to measure angular diameter distance and the expansion rate H(z).
• The findings underscore dark energy's influence at high redshifts and encourage advanced simulations for probing early universe dynamics.

Analysis of Baryon Acoustic Oscillations in the Lyα Forest of BOSS Quasars

The research presented in the paper focuses on the detection of the baryon acoustic oscillation (BAO) feature within the Lyα forest of quasars, specifically using data from the Baryon Oscillation Spectroscopic Survey (BOSS) as part of the Sloan Digital Sky Survey (SDSS-III). This study leverages 48,640 quasars over a redshift range of 2.1 to 3.5, providing a unique opportunity to probe cosmic structures in a high-redshift regime, notably at a mean redshift of z = 2.3. By examining the three-dimensional correlation function of the Lyα transmitted flux fraction, the research yields significant constraints on the angular diameter distance $D_A$ and the expansion rate $H(z)$ at this epoch.

Key Findings

• BAO Detection: The research identifies a BAO peak within the Lyα forest, marking the first such observation at high redshift. The detection aligns with the predictions of a concordance $\Lambda$CDM cosmological model, with the peak's separation measured at $(1.01 \pm 0.03)r_s$, where $r_s$ is the sound horizon.
• Implications for Dark Energy: Combining the BAO data with priors from Cosmic Microwave Background (CMB) observations and other cosmological measures implies the necessity of dark energy. The results provide further evidence for the universe's decelerating expansion between redshifts 0.7 and 2.3, moving towards acceleration at lower redshifts.
• Expansion Rate Measurement: At a redshift of 2.3, the study finds $H(z=2.3) = (224 \pm 8)$ km/s/Mpc. This supports a model of the universe where matter dominates expansion dynamics at these high redshifts.

Analytical Approach

The methodology involves assessing the transmitted flux in the Lyα forest, which traces hydrogen density fluctuations. The geographic coverage of the BOSS quasar sampling allows mapping these fluctuations in a three-dimensional framework. The research constructs mock quasar spectra for validating analysis procedures and understanding statistical uncertainties.

Two methods for defining the continuum and mean flux of the quasars were applied independently, confirming the robustness of the finds. The covariance matrix for the correlation function's monopole and quadrupole was rigorously assessed via sub-sampling and bootstrap techniques.

Theoretical and Practical Implications

The results implicate that dark energy models require critical reassessment at high redshifts to accommodate the observed transitions in cosmic acceleration. The BAO signal robustness in the Lyα data offers a promising avenue for refining current cosmological models, providing complementary constraints to lower redshift galaxy surveys.

Practically, the paper demonstrates that the Lyα forest can act as a powerful probe of early universe physics and highlights the necessity of advanced simulations for capturing relevant astrophysical phenomena.

Future Directions

With ongoing data collection in the BOSS project, forthcoming analyses will enhance the precision of high-redshift $D_A$ and $H(z)$ measurements. This study lays the groundwork for the Lyα forest becoming a standard tool in observational cosmology, particularly for high-redshift surveys where galaxy-based BAO measurements falter. Exploring further implications of BAO at these scales could lead to new insights into the cosmic web's evolution and its influence on cosmic inflation models. Additionally, developments in AI could streamline the processing of vast datasets and aid in the subtle detection techniques employed in such studies.

In summary, this research adds a crucial piece to the puzzle of the universe's expansion history and underscores the potential of quasar-based BAO studies in advancing our understanding of cosmological dynamics.