On boundary terms and conformal transformations in curved space-times

TL;DR

This paper examines how boundary terms in scalar-tensor gravity theories transform under conformal changes, clarifying their role in different frames and implications for total energy in asymptotically flat space-times.

Contribution

It provides a detailed analysis of boundary term transformations in scalar-tensor theories during conformal mappings between frames.

Findings

Boundary terms in 5D gravity yield correct 4D Brans-Dicke boundary terms after compactification.

Conformal transformations alter boundary terms, affecting the total energy in asymptotically flat space-times.

The study clarifies the role of boundary terms in different conformal frames of scalar-tensor gravity.

Abstract

We intend to clarify the interplay between boundary terms and conformal transformations in scalar-tensor theories of gravity. We first consider the action for pure gravity in five dimensions and show that, on compactifing a la Kaluza-Klein to four dimensions, one obtains the correct boundary terms in the Jordan (or String) Frame form of the Brans-Dicke action. Further, we analyze how the boundary terms change under the conformal transformations which lead to the Pauli (or Einstein) frame and to the non-minimally coupled massless scalar field. In particular, we study the behaviour of the total energy in asymptotically flat space-times as it results from surface terms in the Hamiltonian formalism.