On the classification of consistent boundary conditions for $$ f ( R )$$ f(R) -gravity

Abstract

Abstract Using a completely covariant approach, we discuss the role of boundary conditions (BCs) and the corresponding Gibbons–Hawking–York (GHY) terms in $$ f ( R ) $$ f(R) -gravity in arbitrary dimensions. 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 $$f( R )$$ f(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.