The most general second-order field equations of bi-scalar-tensor theory in four dimensions

Understand

The Horndeski theory is known as the most general scalar-tensor theory with second-order field equations.

• In this paper, we explore the bi-scalar extension of the Horndeski theory.
• Following Horndeski's approach, we determine all the possible terms appearing in the second-order field equations of the bi-scalar-tensor theory.
• We compare the field equations with those of the generalized multi-Galileons, and confirm that our theory contains new terms that are not included in the latter theory.