Tootfinder

Crossing the phantom divide in scalar-tensor and vector-tensor theories
Shinji Tsujikawa
https://arxiv.org/abs/2508.17231 https://arxiv.org/pdf/2508.17231

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