Dark Degeneracy in DESI DR2: Interacting or Evolving Dark Energy?

Abstract

The standard $\Lambda$CDM model, despite its success, is challenged by persistent observational tensions in the Hubble constant ($H_0$) and the matter clustering amplitude ($S_8$), motivating the exploration of alternative cosmological scenarios. We investigate a dark energy model with a phenomenological interaction in the dark sector, constructed to be exactly degenerate at the background level with the Chevallier-Polarski-Linder (CPL) parameterization. This setup allows us to test whether models with identical expansion histories but distinct physical mechanisms can be distinguished by cosmological data. We perform a Bayesian analysis using a combination of recent datasets: DESI DR2 BAO measurements, DESY5 supernovae, and CMB data from Planck and ACT. We find that both the interacting model and the CPL model provide significantly better fits to the data than $\Lambda$CDM. Although indistinguishable in background observables, the interacting model predicts a distinct matter-sector evolution driven by a late-time sign change in the dark sector interaction at $z\approx 0.8$, corresponding to the $w=-1$ crossing in the CPL description. In this sense, the interacting picture may be considered more physical, since it avoids the problematic crossing by construction. The resulting decay of dark energy into dark matter lowers $S_8$, potentially alleviating the weak-lensing $S_8$ tension. At the same time, it predicts a sharp suppression of the growth rate $f{\sigma }_8(z)$ at $z\lesssim 0.8$, which is in tension with current measurements of structure formation. This indicates that the model may not simultaneously reconcile the expansion history and the observed growth of cosmic structure, highlighting the need for a more comprehensive analysis to fully assess its viability.

12 pages, 7 figures

References in corpus (6)

• Large-scale instability in interacting dark energy and dark matter fluids
• Aspects of the cosmological "coincidence problem"
• The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics
• Is Local $H_0$ At Odds With Dark Energy EFT?
• Assessing cosmological evidence for non-minimal coupling
• Dark degeneracy I: Dynamical or interacting dark energy?

Cited by in corpus (5)

• Evidence for evolving dark energy from DESI DR2 BAO and Pantheon${}^{+}$, DES-Dovekie, and Union3
• Measuring neutrino mass in light of ACT DR6 and DESI DR2
• UV-complete and stable Quintom Dark Energy models in the light of DESI DR2
• Probing Interacting Dark Sectors with upcoming Post-Reionization and Galaxy Surveys
• Observational Constraints and Cosmological Dynamics of Interacting Fractional Holographic Dark Energy in Light of DESI DR2