Phantom crossing or dark interaction?

Abstract Recent results from DESI BAO measurements, together with Planck CMB and Pantheon+ data, suggest that there may be a 'phantom' phase ( w de < -1) in the expansion of the Universe. This inference follows when the w 0 , w a parametrization for the dark energy equation of state w de is used to fit the data. Since phantom dark energy in general relativity is unphysical, we investigate the possibility that the phantom behaviour is not intrinsic, but effective — due to a non-gravitational interaction between dark matter and non-phantom dark energy. To this end, we assume a physically motivated thawing quintessence-like form of the intrinsic dark energy equation of state w de . Then we use a w 0 , w a model for the effective equation of state of dark energy. We find that the data favours a phantom crossing for the effective dark energy, but only at low significance. The intrinsic equation of state of dark energy is non-phantom, without imposing any non-phantom priors. A nonzero interaction is favoured at more than 3 σ at z ∼ 0.3. The energy flows from dark matter to dark energy at early times and reverses at later times.