Cosmic sign-reversal: non-parametric reconstruction of interacting dark energy with DESI DR2

Abstract

A direct interaction between dark energy and dark matter provides a natural and important extension to the standard $\Lambda$CDM cosmology. We perform a non-parametric reconstruction of the vacuum energy ($w=-1$) interacting with cold dark matter using the cosmological data from DESI DR2, Planck CMB, and three SNIa samples (PP, DESY5, and Union3). By discretizing the coupling function $\beta(z)$ into 20 redshift bins and assuming a Gaussian smoothness prior, we reconstruct $\beta(z)$ without assuming any specific parameterization. The mean reconstructed $\beta(z)$ changes sign during cosmic evolution, indicating an energy transfer from cold dark matter to dark energy at early times and a reverse flow at late times. At high redshifts, $\beta(z)$ shows a $\sim 2\sigma$ deviation from $\Lambda$CDM. At low redshifts, the results depend on the SNIa sample: CMB+DESI and CMB+DESI+PP yield $\beta(z)$ consistent with zero within $2\sigma$, while CMB+DESI+DESY5 and CMB+DESI+Union3 prefer negative $\beta$ at $\sim2\sigma$. Both $\chi^2$ tests and Bayesian analyses favor the $\beta(z)$ model, with CMB+DESI DR2+DESY5 showing the most significant support through the largest improvement in goodness of fit ($\Delta\chi^2_{\rm MAP}=-17.76$) and strongest Bayesian evidence ($\ln\mathcal{B} = 5.98 \pm 0.69$). Principal component analysis reveals that the data effectively constrain three additional degrees of freedom in the $\beta(z)$ model, accounting for most of the improvement in goodness of fit. Our results demonstrate that the dynamical dark energy preference in current data can be equally well explained by such a sign-reversal interacting dark energy, highlighting the need for future observations to break this degeneracy.