Angular BAO Measurements with the DESI DR1 BGS Sample

TL;DR

This study targets transverse (angular) BAO measurements at low redshift using DESI DR1 BGS data, adopting a model-independent angular two-point correlation function and a robust mock-based framework. It employs Landy-Szalay estimation in angular bins, fits a polynomial-plus-Gaussian model to identify the BAO peak via a careful MCMC approach, and corrects for projection effects with both model-dependent and purely statistical methods. The key result is a measured angular BAO scale of $\tilde{\theta}_{BAO}(z_{eff}) = 11.78 \pm 1.12$° for BGS1 and $11.81 \pm 1.20$° for BGS2, corresponding to moderate tensions with CPL and Planck 2018 $\Lambda$CDM expectations but not conclusive given footprint limitations and the low redshift of the sample. This work demonstrates the feasibility and challenges of using angular BAO as a cosmological probe with incomplete survey coverage and motivates fuller DESI DR1+ data for decisive model discrimination, particularly regarding CPL dynamics versus a $\Lambda$CDM baseline.

Abstract

We employ a model-independent approach in both the correlation function estimation and the angular BAO feature estimation by computing the angular two-point correlation function. First, we conducted a series of tests to the available DESI tracers to check their representativeness to angular clustering; the result was that, considering the completeness of the first data release across the footprint, we could only make use of the BGS sample for the effective redshifts 0.21 (BGS1) and 0.25 (BGS2). For a reliable analysis in such low redshift, we consider Lagrangian Perturbation Theory at first order on our mocks, which approximately reproduces the expected non-linearities, and generate the corresponding random catalogues. We use a purely statistical method to correct the projection effects and find that our results show reasonable agreement with the $θ_{\rm BAO}$ expected by the CPL parameters obtained by DESI DR1, being BGS1 $11.78 \pm 1.12$ degrees and BGS2 $11.81 \pm 1.20$ degrees. This means a tension at the $1.5σ$ ($2.6σ$) level for BGS1 (BGS2) CPL parametrization, while a $2σ$ ($3.3σ$) discrepancy within the predicted by $Λ$CDM. We conclude that, with the current sample available, the use of an angular correlation function serving as the BAO probe, although prefers the CPL parametrization, does not provide conclusive results regarding the best cosmological model.

Figures (5)

• Figure 1: BGS example mock. The darker regions indicate the most populated pixels.
• Figure 2: Correlation matrix from the covariance of the 1,000 mock 2PCFs for BGS1 and BGS2.
• Figure 3: BGS Best-fit to the model-free polynomial. Darker: 68% CL; lighter: 95.45% CL (MCMC posterior regions).
• Figure 4: Two-point correlation function estimated from the data (blue) and best-fit model (black) using the model-free polynomial fit sanchez2011tracing.
• Figure 5: $\theta_{\rm BAO}$ with Planck 18 as the dashed-line and the red line is DESI CPL ($\Omega_m=0.3069\pm0.005$, $H_0=67.97 \pm 0.38$, $w_0=-1.79 \pm ^{0.34}_{0.32}$,$w_a=-1.79\pm^{0.48}_{1.0}$). The observed points are in blue.