Tests of Evolving Dark Energy with Geometric Probes of the Late-Time Universe

TL;DR

This work probes the robustness of DESI's indication for time-varying dark energy by separating geometric information from growth in weak lensing through a growth parameter $\Omega_{\rm m}^{\rm growth}$ and combining with BAO, SN, and primary CMB data. The method marginalizes growth effects to focus on late-time geometry, finding that the $w_0w_a$CDM preference persists, though the inferred significance depends on SN calibration at low redshift. The results show that excluding $z<0.1$ SN data reduces the evidence for evolving dark energy to about $2\sigma$, while high-redshift SN data further lowers or alters the significance; LSST-Y1 cosmic shear is projected to offer competitive constraints. The study emphasizes the importance of robust low-$z$ SN samples and demonstrates that geometry-focused analyses with growth marginalization remain a powerful approach for testing dark energy dynamics across cosmological probes.

Abstract

Recent results from the Dark Energy Spectroscopic Instrument (DESI) have shown a strong statistical preference for a time-evolving dark energy model over $Λ$CDM when combining BAO, CMB, and supernova (SN) data. We investigate the robustness of this conclusion by isolating geometric information in weak lensing measurements from the DES Year 3 survey and combining it with different datasets. We introduce a hyperparameter, $Ω_{\rm m}^{\rm growth}$, to decouple the growth contribution from the lensing 2-point correlation and thus bypass the possible effect of the $σ_8$ tension in our analysis. We then combine with the late-time geometric probes provided by BAO and SN, along with CMB primary data. The preference for evolving dark energy is consistent with the DESI-DR2 findings: when combining BAO, primary CMB, and weak lensing data, the $w_0w_a$CDM is preferred at about the $3σ$ significance. However, when we add SN, the result is sensitive to the choice of data: if we leave out $z<0.1$ SN data in the analysis, as a test of the effect of inhomogeneous calibration, we obtain a statistical significance below $2σ$ for time evolving dark energy. Indeed, the high-z only SN data \textbf{lowers} the evidence for evolving dark energy in all the data combinations we have examined. This underscores the importance of improved SN samples at low redshift and of alternative data combinations. We show that cosmic shear measurements with LSST Year 1 data will provide comparable power to current SN data. We discuss other low-redshift probes provided by lensing and galaxy clustering to test for evolving dark energy.

Figures (7)

• Figure 1: The effect of $\Omega_{\rm m}^{\rm growth}$ on the power spectrum. The effect of the growth parameter is much weaker than the geometry parameter, and it primarily changes the amplitude of the power spectrum. Thus marginalizing over $\Omega_{\rm m}^{\rm growth}$ leaves cosmic shear insensitive to the amplitude of matter fluctuations. The oscillatory features at high $k$ are due to non-linear matter evolution. This figure is reproduced from Ref. Zhong:2023how
• Figure 2: The log posterior as a function of time steps, in the tempered MCMC approach. At each stage, the temperature and scale factor are decreased, forcing the walkers to search optimal values within a confined spaced.
• Figure 3: 1D marginalized posterior for $\Omega_{\rm m}$ in the $\Lambda$CDM model. The main source of preference for evolving dark energy is the tension between BAO and CMB. The tension is even higher between BAO and DES-Y5 SN. However, if low-z SN are excluded, the tension gets much weaker (compare the purple and orange posteriors). Indeed, we find that $z>0.1$ SN lower the evidence for evolving dark energy in all data combinations we have studied. Cosmic shear also prefers a low $\Omega_{\rm m}$, although the uncertainty is relatively large. We have shown two choices of the CMB posterior: the one with higher $\ell_{max}$ is shown for reference -- see discussion in Sec. \ref{['sec:other_diff']}.
• Figure 4: Left Panel: Posterior in the $w_0-w_a$ plane for the combination of BAO and cosmic shear, with and without the growth–geometry split ($\xi_\pm^{\rm geo}$ and $\xi_\pm^{\rm all}$). Introducing the additional hyperparameter $\Omega_{\rm m}^{\rm growth}$ leads to an unconstrained posterior on $\sigma_8$, as expected (it is prior dominated). However, the constraints on $w_0$ and $w_a$ remain almost unchanged, which shows that the geometric part of cosmic shear dominates information on dark energy. Right Panel: Posterior distributions for the combination of BAO, primary CMB (CMB-p), SN, and cosmic shear. The geometry isolation only slightly degrades the constraining power.
• Figure 5: Left Panel: Posterior in the $w_0-w_a$ plane for the combination BAO, SN, CMB-p and $\xi_{\pm}^{\rm geo}$. The addition of cosmic shear in this case improves constraints on $w_0$ and $w_a$ and increases the statistical significance in favor of evolving dark energy by $0.3\sigma$. Right Panel: Posterior with different choices of SN data: no SN (orange, filled contours); full DES SN sample (blue filled contours) and $z>0.1$ SN sample (dashed contours). The latter case shows consistency with LCDM at the $2-\sigma$ level, showing the strong dependence of the evolving dark energy result on the low $z$ SN.