[Paper Review] Impact of LRG1 and LRG2 in DESI 2024 BAO data on dark energy evolution

Guanine Liu, Yu Wang+1 more|arXiv (Cornell University)|Jul 5, 2024

Geophysics and Gravity Measurements9 citations

TL;DR

The paper analyzes how DESI 2024 BAO data, especially LRG1 and LRG2, influence evidence for dynamical dark energy by comparing early vs late universe parameters and applying model-independent reconstructions of H(z).

ABSTRACT

Recent measurements of baryon acoustic oscillations (BAO) by the Dark Energy Spectroscopic Instrument (DESI) suggest a preference for a dynamic dark energy model over a cosmological constant. This conclusion emerges from the combination of DESI's BAO data with observations of the Cosmic Microwave Background (CMB) and various type Ia supernova (SN Ia) catalogues. The deviation observed in the cosmological constant ($Λ$) reflects a departure from the standard cosmological model. Testing this deviation involves examining the consistency between cosmological parameters derived from early and late-time observations. Specifically, we focus on the matter density parameter $ω_m = Ω_mh^2$ and introduce ${ m ratio}(ω_m)$ to assess consistency, which is defined as the ratio of $ω_m$ values constrained by high and low-redshift measurements. This ratio serves as a metric for quantifying deviations from the $Λ$CDM model. In this paper, we find that the DESI BAO+CMB yields ${ m ratio}(ω_m)=1.0171\pm0.0066$. Upon excluding the LRG1 and LRG2 data in DESI BAO, this ratio adjusts to ${ m ratio}(ω_m)=1.0100\pm0.0082$. This shift, corresponding to a change from $2.6σ$ to $1.2σ$, indicates that the deviation from the $Λ$CDM model is predominantly driven by these two samples from the DESI BAO measurements. To substantiate this conclusion, we utilized two cosmological model-independent methods to reconstruct the cosmic expansion history. Both reconstructions of the Hubble parameter $H(z)$ indicate that the evolving features of dark energy are determined by the combined LRG1 and LRG2 data. Therefore, different methods have reached the same conclusion, namely the importance of accurately measuring the BAO feature in LRG1 and LRG2 data.

Motivation & Objective

• Test consistency between early- and late-time cosmological parameters using DESI 2024 BAO and Planck CMB data.
• Quantify deviations from LambdaCDM via ratio(ωm) and assess the impact of including/excluding DESI LRG1 and LRG2 data.
• Reconstruct the expansion history H(z) in a model-independent way to infer dark energy evolution.
• Evaluate the role of BAO measurements at DESI LRG1 and LRG2 redshifts in driving apparent dynamical dark energy.

Proposed method

• Define ratio(ωm) = ω̃m/ωm to test LambdaCDM consistency between high-z and low-z constraints.
• Combine DESI 2024 BAO data with Planck CMB data to constrain ratio(ωm) and ΛCDM parameters.
• Perform Taylor reconstruction (TR) of H(z) to model-independently infer H(z) from BAO data.
• Perform Chebyshev reconstruction (CR) of w(z) to infer dark energy evolution from BAO, SN Ia, and CMB data.
• Use SN Ia Pantheon+ data and SH0ES H0 prior to break degeneracies in the TR/CR analyses.
• Utilize CMB distance priors (l_a, R, ω_b) to incorporate Planck information in a way independent of dark energy model.

Figure 1 : The BAO measurements $D_{V}/r_{d}$ and $F_{\rm AP}=D_{M}/D_{H}$ at different redshifts are plotted in the top row’s left and right panels, respectively. To enhance visual clarity and reduce the plot’s dynamic range, a scaling of $z^{-2/3}$ has been applied to the left panel and $z^{-1}$ t

Experimental results

Research questions

• RQ1 Does DESI 2024 BAO data imply a departure from LambdaCDM when combined with CMB and SN Ia data?
• RQ2 What is the impact of including or excluding DESI LRG1 and LRG2 BAO measurements on the inferred dark energy evolution?
• RQ3 Are model-independent reconstructions of H(z) and w(z) consistent with the LambdaCDM expectation when LRG1/LRG2 data are removed?
• RQ4 Can the ratio(ωm) statistic reliably quantify deviations from LambdaCDM across data combinations?

Key findings

• DESI BAO+CMB yields ratio(ωm) = 1.0171 ± 0.0066, indicating a 2.6σ tension with ΛCDM.
• Excluding DESI LRG1 and LRG2 data lowers ratio(ωm) to 1.0100 ± 0.0082, reducing tension to 1.2σ.
• Removing LRG2 alone yields ratio(ωm) = 1.0124 ± 0.0071 (≈1.7σ deviation from ΛCDM).
• Taylor and Chebyshev reconstructions of H(z)/w(z) show evolving dark energy driven by LRG1 and LRG2 data, with DESI data alone indicating non-ΛCDM features.
• When LRG1 and LRG2 are removed, DESI BAO results align with non-DESI BAO, consistent with ΛCDM predictions for w = -1.
• Both model-independent reconstructions consistently identify the BAO measurements at DESI LRG1 and LRG2 redshifts as crucial for determining dark energy evolution.

Figure 2 : 68% and 95% credible-interval contours for parameters ${\rm ratio}(\omega_{m})$ and $\Omega_{m}$ obtained under a flat $\Lambda$ CDM framework from fits to the combination of different BAO measurements and CMB data. ”w/o” means this portion of BAO data was excluded from the parameter esti