Higher-derivative operators and effective field theory for general scalar-tensor theories

Understand

We discuss the extent to which it is necessary to include higher-derivative operators in the effective field theory of general scalar-tensor theories.

• We explore the circumstances under which it is correct to restrict to second-order operators only, and demonstrate this using several different techniques, such as reduction of order and explicit field redefinitions.
• These methods are applied, in particular, to the much-studied Horndeski theories.
• The goal is to clarify the application of effective field theory techniques in the context of popular cosmological models, and to explicitly demonstrate how and when higher-derivative operators can be cast into lower-derivative forms suitable for numerical solution techniques.