Sign-Switching Dark Energy: Smooth Transitions with Recent \textit{DESI DR2} Observations

TL;DR

Sign-switching dark energy offers a physically economical way to modify the late-time expansion without new fields. The authors constrain three smooth realisations—LΛCDM, SSCDM, and ECDM—against Planck 2018 compressed CMB, DESI DR2 BAO, and Pantheon+ & SH0ES SN using MCMC with a BBN prior on $\Omega_{\rm b0}h^2$. They find potential alleviation of the Hubble tension, with information criteria showing smooth models competitive and abrupt Λ_sCDM often favored for simplicity, though results are sensitive to priors and dataset combinations. Reconstructed histories reveal possible late-time sign-switching and a potential intermediate acceleration phase, particularly in ECDM, highlighting distinctive background signatures that can be tested with future transverse BAO data and a fuller Planck likelihood. Overall, sign-switching DE remains a promising avenue worth tighter priors and richer data to robustly assess its viability against ΛCDM.

Abstract

Sign-switching dark energy provides a novel mechanism for modifying the late-time expansion history of the Universe without invoking additional fields or finely tuned initial conditions. In this work, we investigate a class of background--level cosmological models in which the dark energy contribution changes sign at a transition redshift $z_\dagger$, producing a sharp deviation from standard $Λ$CDM dynamics. We confront these models with a comprehensive set of cosmological observations, including \textit{Planck 18} cosmic microwave background (CMB) measurements, \textit{\textit{DESI DR2}} Baryonic Acoustic Oscillation (BAO) data and the \textit{Pantheon+} $\&$ \textit{SH0ES} Type Ia supernova sample (SN). Using a full Markov Chain Monte Carlo (MCMC) analysis, we find that the sign-switching scenario significantly alleviates the Hubble tension while obtaining better results when statistically comparing with $Λ$CDM, as quantified by the Akaike and Bayesian information Criteria. Although the model is explored only at the background level, the improvement in the inferred Hubble constant demonstrates that sign-switching dark energy offers a promising and physically economical pathway toward resolving late-universe discrepancies.

Figures (4)

• Figure 1: Two-dimensional posterior distributions for the sign-switching models, using the latest DESI DR2 (BAO) release, Planck18 (CMB) and Pantheon+ & SH0ES (SN) datasets. The contours correspond to the $68\%$ and $95\%$ confidence levels (C.L.).
• Figure 2: Two-dimensional posterior distributions for the sign-switching models, using the combinations DESI DR2 (BAO) + Planck18 (CMB) and DESI DR2 (BAO) + Planck18 (CMB) + Pantheon+ $\&$ SH0ES (SN) datasets. The contours correspond to the $68\%$ and $95\%$ confidence levels (C.L.).
• Figure 3: Reconstructed EoS, energy density normalized to the current value, Hubble parameter, deceleration parameter and jerk for the SSCDM models with Planck18 + DESI DR2 + Pantheon+ $\&$ SH0ES (left-most column), and with Planck18 + DESI DR2 (central column) and the Planck18 (right-most column). We also plot the reconstructed Hubble rate and energy density normalized to the Planck PR4 values for $\Lambda$CDM, for which $\Omega_{m0}$ = 0.315 and $H_0$ = 67.26 km/s/Mpc Rosenberg:2022sdy. The solid colored lines represent the most-probable value and the shaded regions show the 68$\%$ and 95$\%$ confidence intervals around it. The grey dashed lines correspond to $\Lambda$CDM values. In the plots of $q(z)$ we also show in black dash-dotted line the border between deceleration ($q>$0) and acceleration ($q<$0) regimes, i.e., $q$=0.
• Figure 4: Reconstructed EoS, energy density normalized to the current value, deceleration parameter and jerk for the ECDM model with Planck18 + DESI DR2 + PantheonPlus $\&$ SH0ES (left-most column), and with Planck18 + DESI DR2 (central column) and the Planck18 (right-most column). We also plot the reconstructed Hubble rate and energy density normalized to the Planck PR4 values for $\Lambda$CDM, for which $\Omega_{m0}$ = 0.315 and $H_0$ = 67.26 km/s/Mpc [114]. The solid colored lines represent the most-probable value and the shaded regions show the 68$\%$ and 95$\%$ confidence intervals around it. The grey dashed lines correspond to $\Lambda$CDM values. In the plots of $q(z)$ we also show in black dash-dotted line the border between deceleration ($q>$0) and acceleration ($q<$0) regimes, i.e., $q$=0.