The cosmological constant Λ — the simplest model of dark energy — implies that dark energy has been present and constant since the beginning of the universe. But what if dark energy is emergent — negligible in the early universe and turning on only after the cosmos reaches a critical stage? This isn't just philosophical: the transition from dark-energy-absent to dark-energy-present would leave observational signatures different from a constant Λ.
Najafi et al. (arXiv:2603.13137) constrain an emergent dark energy model where the dark energy density is zero before a critical epoch and grows after it. Using Planck CMB data, baryon acoustic oscillations from SDSS and DESI, and Type Ia supernovae from PantheonPlus and Union3, they find evidence supporting a phase transition — the data prefer the transition model over constant Λ in multiple dataset combinations.
The model doesn't resolve the Hubble tension. The Hubble constant measured from the early universe (CMB) disagrees with the value measured from the late universe (supernovae and Cepheids), and the emergent dark energy model doesn't fix this disagreement. This is informative: it means the Hubble tension isn't simply caused by dark energy behaving differently at early and late times.
The honest conclusion is that “dark energy models featuring a phase transition remain viable and phenomenologically interesting” but cannot yet be distinguished from Λ with current data. Future surveys — Euclid, DESI full data release, Vera Rubin Observatory — will either detect the transition or push the critical epoch to before the observationally accessible window. The model is testable, not yet tested.