Degenerate higher order scalar-tensor theories beyond Horndeski up to cubic order
Degenerate higher order scalar-tensor theories beyond Horndeski up to cubic order
We present all scalar-tensor Lagrangians that are cubic in second derivatives of a scalar field, and that are degenerate, hence avoiding Ostrogradsky instabilities. Thanks to the existence of constraints, they propagate no more than three degrees of freedom, despite having higher order equations of motion. We also determine the viable …