Covariant 3+1 correspondence of the spatially covariant gravity and the degeneracy conditions

TL;DR

This paper introduces a covariant 3+1 correspondence method to map spatially covariant gravity to scalar-tensor theories without fixing coordinates, simplifying the identification of ghostfree conditions and recovering Horndeski theory.

Contribution

It develops a covariant 3+1 correspondence approach to analyze degeneracy conditions, enabling the construction of more general ghostfree scalar-tensor theories.

Findings

The method maps spatially covariant gravity to scalar-tensor theories without coordinate fixing.

It simplifies the process of identifying degeneracy conditions for ghost freedom.

Horndeski theory is recovered as an example using this approach.

Abstract

A necessary condition for a generally covariant scalar-tensor theory to be ghostfree is that it contains no extra degrees of freedom in the unitary gauge, in which the Lagrangian corresponds to the spatially covariant gravity. Comparing with analysing the scalar-tensor theory directly, it is simpler to map the spatially covariant gravity to the generally covariant scalar-tensor theory using the gauge recovering procedures. In order to ensure the resulting scalar-tensor theory to be ghostfree absolutely, i.e., no matter if the unitary gauge is accessible, a further covariant degeneracy/constraint analysis is required. We develop a method of covariant 3+1 correspondence, which map the spatially covariant gravity to the scalar-tensor theory in 3+1 decomposed form without fixing any coordinates. Then the degeneracy conditions to remove the extra degrees of freedom can be found easily. As an illustration of this approach, we show how the Horndeski theory is recovered from the spatially covariant gravity. This approach can be used to find more general ghostfree scalar-tensor theory.