Speaker: Hayato Motohashi (IFIC, University of Valencia)
Scalar-tensor theories serve models for inflation and dark energy. Many efforts have been made recently for constructing most general scalar-tensor theories with higher-order derivatives in their Lagrangian. While the Horndeski theory has the most general second-order Euler-Lagrange equation, there exist more general healthy class of theories beyond Horndeski. Since higher-derivative theories are typically associated with Ostrogradsky ghost which causes unbounded Hamiltonian, it is important to clarify how to evade it. In this talk, I will discuss construction of healthy degenerate theories with higher derivatives which circumvent Ostrogradsky ghost.