Crossing the phantom divide in scalar-tensor and vector-tensor theories

Abstract: DESI observations of baryon acoustic oscillations (BAOs), combined with cosmic microwave background (CMB) and type-Ia supernova (SN Ia) data, suggest that the dark energy equation of state $w_{\rm DE}$ crosses the phantom divide from $w_{\rm DE} < -1$ to $w_{\rm DE} > -1$ at low redshifts. In shift-symmetric Horndeski and generalized Proca theories with luminal gravitational-wave speed and no direct couplings to dark matter, we show that such a phantom-divide crossing is generically difficult without introducing theoretical pathologies. In Horndeski theories, however, breaking the shift symmetry allows this transition. We present an explicit model with broken shift symmetry, in which the scalar field has a potential in addition to a Galileon self-interaction and a higher-order derivative term. This model realizes the desired phantom-divide crossing at low redshifts while avoiding ghosts and Laplacian instabilities.