Unified dark fluid with null sound speed as an alternative to phantom dark energy

Recent BAO measurements from DESI, when combined with CMB and supernovae data, suggest evolving dark energy and in particular point to a possible phantom regime, with an equation of state parameter w<-1. We explore an alternative phenomenological way to model dark matter and dark energy based on a unified dark fluid (UDF). By construction, our model reproduces the same background expansion history as DESI's best-fit using the CPL parametrization, but assumes a vanishing rest-frame sound speed and no anisotropic stress. This simple prescription ensures a consistent and physical treatment of perturbations, and the use of a unified dark sector avoids phantom behaviour by construction. We model CMB, LSS, and redshift-space distortion observables, and find differences with CPL that are typically at the few-percent level. As a result, we forecast that even stage IV CMB and galaxy surveys will have limited power to discriminate between our UDF and CPL at the linear level. At the non-linear level, we study spherical collapse in the UDF and show that within this framework, structure formation proceeds very similarly to standard scenarios. Using Planck, DESI BAO DR2, and DESY5 supernovae data, we demonstrate that this simple UDF model fits current observations nearly as well as CPL, while treating perturbations consistently. Because most cosmological observations are not sensitive to how the dark sector is split, the unified framework can also approximate the phenomenology of interacting dark energy-dark matter scenarios or evolving dark matter, making it a general way to model the data, at least as long as the dark components have a vanishing sound speed, which is the most distinctive feature of our analysis. This highlights that a unified dark fluid with evolving equation of state and null sound speed is sufficient to pass current constraints without a phantom component.