Extended Dark Energy analysis using DESI DR2 BAO measurements

TL;DR

This work extends the DESI DR2 BAO dark energy analysis by combining DESI BAO with Planck CMB and multiple SN Ia datasets to test evolving dark energy. It employs both parametric (multiple two-parameter w(z) forms and Chebyshev crossing statistics) and non-parametric (binning and Gaussian Process) reconstructions, consistently finding that a two-parameter w0w_aCDM description captures the data well and that phantom crossing is favored at low redshift. Across methods, the results show deviations from ΛCDM in the late-time expansion history, with Om(z) and q(z) diagnostics supporting dynamical behavior; however, evidence for crossing is not unambiguous and depends on the dataset and method. Model comparisons indicate strong preference for dynamical DE, particularly the mirage direction in the w0–wa plane, while future surveys and cross-checks with Euclid/Rubin will be crucial to confirm these findings and reveal potential underlying physics.

Abstract

We conduct an extended analysis of dark energy constraints, in support of the findings of the DESI DR2 cosmology key paper, including DESI data, Planck CMB observations, and three different supernova compilations. Using a broad range of parametric and non-parametric methods, we explore the dark energy phenomenology and find consistent trends across all approaches, in good agreement with the $w_0w_a$CDM key paper results. Even with the additional flexibility introduced by non-parametric approaches, such as binning and Gaussian Processes, we find that extending $Λ$CDM to include a two-parameter $w(z)$ is sufficient to capture the trends present in the data. Finally, we examine three dark energy classes with distinct dynamics, including quintessence scenarios satisfying $w \geq -1$, to explore what underlying physics can explain such deviations. The current data indicate a clear preference for models that feature a phantom crossing; although alternatives lacking this feature are disfavored, they cannot yet be ruled out. Our analysis confirms that the evidence for dynamical dark energy, particularly at low redshift ($z \lesssim 0.3$), is robust and stable under different modeling choices.

Figures (18)

• Figure 1: Constraints on the parameters $w_0w_a$ from DESI BAO DR2, CMB, and Union3 are illustrated in blue, while the corresponding combination with DESI BAO DR1 is shown in orange. The green line indicates the degeneracy direction associated with calibrated thawing (see \ref{['sec:thaw']}), while the purple line denotes the "mirage" direction (discussed in \ref{['sec:mirage']}) which closely follows the degeneracy direction of the $w_0w_a$ contours.
• Figure 2: Equation of state parameter, $w(z)=P/\rho c^2$, and corresponding normalized dark energy density, $f_\mathrm{DE} (z) \equiv\rho_{\rm DE}(z)/\rho_{\rm DE,0}$, as a function of redshift using the $w_0w_a$ parametrization. The solid and dashed-dotted vertical lines indicate the phantom-crossing ($z_{\rm c}$) and dark energy-matter equality ($z_{\rm eq}$) redshifts, respectively. The horizontal dashed line represents $\Lambda$CDM.
• Figure 3: Evolution of the $Om(z)$ diagnostic and deceleration parameter, $q(z)$, as a function of redshift in the $w_0w_a$CDM model. The solid blue lines correspond to the median, $68\%$, and $95\%$ confidence levels obtained from the DESI+CMB+Union3 combination. The black dashed line depicts the best-fit $\Lambda$CDM for the same data combination. The gray vertical line shows the redshift ($z_{\rm acc}$) corresponding to the onset of cosmic acceleration ($\ddot{a}>0$).
• Figure 4: The dark energy equation of state $w(z)$ for alternative $w_0w_a$ parameterizations — BA, EXP, JBP, and LOG — is illustrated alongside the standard CPL form (shown in blue). The constraints integrate data from DESI, Union3 SNe, and CMB observations, with shaded regions representing $1\sigma$ uncertainty bands. All parameterizations except JBP exhibit consistent phantom crossing near $z \sim 0.5$ and provide a similarly good fit to the data.
• Figure 5: Reconstructions of $w(z)$ and $f_\mathrm{DE}(z)$ using \ref{['eq:crossing_w', 'eq:crossing_fde']} with $N=3$ for the DESI + $(\theta_\ast,\omega_\mathrm{b},\omega_\mathrm{bc})_\mathrm{CMB}$ data combination, with and without the inclusion of Union3. The solid lines correspond to the median, $68\%$, and $95\%$ confidence levels around it. The main reconstructed behavior of DE is in excellent agreement with the different model-agnostic approaches explored in this paper. This confirms that the trend is not driven by the choice of parametrization, and adds to the robustness of the $w_0w_a\rm CDM$ results presented in the main key paper DESI.DR2.BAO.cosmo. The horizontal dashed line represents $\Lambda$CDM.