Metastability in Emergent Dark Energy: A New Framework Confronting Cosmological Observations

We propose the metastable emergent dark energy (MEDE) model, a novel phenomenological extension of the phenomenological and generalized emergent dark energy frameworks, in which dark energy exhibits a transitionary behavior, appearing at late times and vanishing toward the future. This model naturally enables a smooth crossing of the phantom dividing line in the dark energy equation of state, as hinted at by recent observations. The MEDE model is defined by a hyperbolic tangent dark energy equation of state w ( z ) = − 1 − Δ tanh [ log 10 ( ( 1 + z ) / ( 1 + z t ) ) ] , introducing only two free parameters—the transition redshift z t and the variation amplitude Δ—allowing both the emergent and transitionary behavior of dark energy. We constrain the MEDE model using a combined dataset of Planck cosmic microwave background (CMB) data, Dark Energy Spectroscopic Instrument (DESI) DR2 baryon acoustic oscillation measurements, and different compilations of Type Ia supernovae, obtaining z t = 0.42 5 − 0.120 + 0.084 and Δ = 0.8 7 − 0.35 + 0.29 (for CMB+DESI+PantheonPlus), indicating a statistically significant deviation from the cosmological constant. Statistical comparisons show that the MEDE model is preferred over ΛCDM by the combined dataset, with ΔDIC MEDE−ΛCDM = −9.29. The MEDE model performs comparably to the Chevallier–Polarski–Linder (CPL) dynamical dark energy parameterization (ΔDIC MEDE −CPL = 0.74), with no strong statistical distinction from CPL using current data. Notably, MEDE preserves the success of ΛCDM in describing early-universe physics and naturally accommodates the phantom-crossing signature indicated by the latest low-redshift observations. The MEDE scenario provides a compelling dark energy phenomenology that may guide us toward interesting theoretical implications.