On the classification of consistent boundary conditions for $ \mathit{f}(\mathit{R})$-Gravity

TL;DR

This paper investigates boundary conditions in $f(R)$-gravity, showing the necessity of scalar field reformulation for consistent BCs and deriving GHY terms for various boundary conditions, revealing compatibility with certain mixed BCs.

Contribution

It introduces a scalar field via Ostrogradsky approach to establish consistent boundary conditions in $f(R)$-gravity and derives GHY terms for multiple BC types.

Findings

$f(R)$-gravity requires scalar field reformulation for consistent BCs.

Certain mixed boundary conditions are compatible without additional GHY terms.

GHY terms are expressed in ADM variables for each boundary condition.

Abstract

Using a completely covariant approach, we discuss the role of boundary conditions (BCs) and the corresponding Gibbons--Hawking--York (GHY) terms in $ \mathit{f}(\mathit{R}) $-gravity in arbitrary dimensions. We show that $ f(\mathit{R}) $-gravity, as a higher derivative theory, is not described by a degenerate Lagrangian, in its original form. Hence, without introducing additional variables, one can not obtain consistent BCs, even by adding the GHY terms (except for $f(\mathit{R})=R$). However, following the Ostrogradsky approach, we can introduce a scalar field in the framework of Brans-Dicke formalism to the system to have consistent BCs by considering appropriate GHY terms. In addition to the Dirichlet BC, the GHY terms for both Neumann and two types of mixed BCs are derived. We show the remarkable result that the $f(\mathit{R})$-gravity is itself compatible with one type of mixed BCs, in $D$ dimension, i.e. it doesn't require any GHY term. For each BC, we rewrite the GHY term in terms of Arnowit-Deser-Misner (ADM) variables.