Evolving Dark Energy Is Vacuum Energy After All

Abstract

We investigate a physically motivated model of dynamical dark energy arising from the non-perturbative topological structure of the Quantum Chromodynamics (QCD) vacuum. Unlike conventional dark-energy scenarios, the model does not introduce any new fundamental field or propagating degree of freedom. Instead, the dark-energy density emerges as a global vacuum effect associated with the response of the QCD vacuum to an expanding spacetime, representing a possible paradigm shift in the interpretation of cosmic acceleration. We develop the first comprehensive cosmological implementation of this QCD-induced dark-energy scenario and confront it with current observations, including the latest combination of Planck, ACT and SPT-3G cosmic microwave background measurements, DESI DR2 baryon acoustic oscillation data, and Type Ia supernova samples from Pantheon+ and DES-Dovekie. We compare the model with both the standard $Λ$CDM cosmology and the widely used CPL ($w_0w_a$CDM) parametrization of evolving dark energy. We find that the model provides an excellent fit to the data and reproduces the late-time dark-energy evolution preferred by DESI observations. The inferred cosmological parameters are robust against different implementations of the dark-energy activation mechanism, indicating that the cosmological predictions are largely insensitive to the specific form of the transition. The model naturally predicts an effective phantom-crossing behaviour at intermediate redshifts while remaining free from the theoretical instabilities commonly associated with phantom scalar-field models. Using a combination of goodness-of-fit statistics and Bayesian model-selection techniques, including Akaike and Deviance Information Criteria and Bayesian evidence estimated from Markov Chain Monte Carlo chains, [abridged]