Higher derivative scalar-tensor theory and spatially covariant gravity: The correspondence

We investigate the correspondence between generally covariant higher derivative scalar-tensor theory and spatially covariant gravity theory. The building blocks are the scalar field and spacetime curvature tensor together with their generally covariant derivatives for the former, and the spatially covariant geometric quantities together with their spatially covariant derivatives for the later. In the case of a single scalar degree of freedom, they are transformed to each other by gauge fixing and recovering procedures, of which we give the explicit expressions. We make a systematic classification of all the scalar monomials in the spatially covariant gravity according to the total number of derivatives up to d = 4, and their correspondence to the scalar-tensor monomials. We discuss the possibility of using spatially covariant monomials to generate ghost-free higher derivative scalar-tensor theories. We also derive the covariant 3 + 1 decomposition without fixing any specific coordinate, which will be useful when performing a covariant Hamiltonian analysis.