Dark matter and energy can cross the phantom divide without breaking physics

Dark energy's equation of state appears to have recently crossed the phantom divide—a boundary where acceleration flips sign—according to DESI observations. The authors propose a field theory where fermionic dark matter couples to a tachyonic scalar field, naturally producing this double crossing at the effective level while keeping the underlying dynamics stable and well-behaved. The model fits DESI, Planck, and supernova data and predicts fermionic dark matter at 0.002 eV, opening a new window on the dark sector.