Constraints on Horndeski Gravity with Phantom Crossing

Abstract Gravity models in which the dark energy equation of state crosses w = −1, also known as the phantom divide, have received extensive interest due to recent analyses favouring this behaviour. We introduce a new subclass of Horndeski scalar-tensor models capable of generating phantom crossing, whilst remaining minimally coupled to matter: the Asymptotic Cubic Galileon (ACG) models. We show that ACG models can jointly fit the expansion history inferred from observations of the Planck cosmic microwave background, baryon acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument, and distance-ladder supernovae measurements from the Dark Energy Survey. We then demonstrate that requiring a positive galaxy-ISW cross-correlation, together with physically viable screening behaviour, substantially restricts the viable ACG parameter space. Model comparison metrics, including χ2 and Bayesian evidence, favour both ACG and w0waCDM models over ΛCDM, with ACG providing a fit of comparable quality to w0waCDM. Crucially, ACG models ground the observationally preferred w0waCDM behaviour in a robust Lagrangian formulation. This enables interpretation beyond mere phenomenological fits, and motivates further tests of these models on nonlinear scales.