Dark Energy After DESI DR2: Observational Status, Reconstructions, and Physical Models

TL;DR

This paper reviews the observational status of dark energy after DESI DR2, analyzing data combinations, diagnostics, and reconstructions to understand cosmic acceleration and constrain physical models.

Contribution

It introduces new diagnostics and systematic maps to interpret DESI DR2 data and explores the phenomenology of dark energy models in light of recent observations.

Findings

DESI DR2 provides percent-level BAO measurements up to z~2.5.

Allowing evolving dark energy improves fit in some datasets.

Diagnostics for systematics and model constraints are developed.

Abstract

We review late-time cosmic acceleration after DESI Data Release 2 (DR2), emphasizing the interplay between Type Ia supernovae (SNe Ia), anisotropic BAO, CMB calibration, and perturbation-sensitive probes (RSD and weak lensing). DESI DR2 delivers percent-level BAO distance ratios over $0\lesssim z\lesssim2.5$, including a Ly$\alpha$-forest anchor at $z_{\rm eff}=2.33$. In CMB-calibrated combinations, flat $\Lambda$CDM exhibits a mild parameter mismatch, while allowing evolving dark energy (e.g.\ CPL $w_0$--$w_a$) can improve the fit; the preference is dataset-dependent and is particularly sensitive to redshift-dependent SN calibration/selection residuals at the few$\times10^{-2}$\,mag level. To sharpen likelihood-level interpretation, we provide two diagnostics: (i) an $r_d$-independent BAO-shape observable, $F_{\rm AP}(z)\equiv D_{\rm M}(z)/D_{\rm H}(z)$, constructed directly from published $(D_{\rm M}/r_d,\,D_{\rm H}/r_d)$ with covariance propagation; and (ii) a linear-response map from SN Hubble-diagram systematics $\delta\mu(z)$ to induced biases in $(w_0,w_a)$, yielding an explicit calibration requirement for DESI-era claims of evolving $w(z)$. We synthesize parametric and non-parametric reconstructions of $w(z)$ and $\rho_{\rm DE}(z)$ and map the resulting phenomenology to microphysical dark-energy and modified-gravity models subject to perturbation stability and gravitational-wave propagation constraints.