Extended Scalar-Tensor Theories of Gravity

Understand

We study new consistent scalar-tensor theories of gravity recently introduced by Langlois and Noui with potentially interesting cosmological applications.

• We derive the conditions for the existence of a primary constraint that prevents the propagation of an additional dangerous mode associated with higher order equations of motion.
• We then classify the most general, consistent scalar-tensor theories that are at most quadratic in the second derivatives of the scalar field.
• In addition, we investigate the possible connection between these theories and (beyond) Horndeski through conformal and disformal transformations.