On static solutions of the Einstein - Scalar Field equations

In this article we study self-gravitating static solutions of the Einstein-ScalarField system in arbitrary dimensions. We discuss the existence and the non-existence of geodesically complete solutions depending on the form of the scalar field potential $V(ϕ)$, and provide full global geometric estimates when the solutions exist. Our main results are summarised as follows. For the Klein-Gordon field, namely when $V(ϕ)=m^{2}|ϕ|^{2}$, it is proved that geodesically complete solutions have Ricci-flat spatial metric, have constant lapse and are vacuum, (that is $ϕ$ is constant and equal to zero if $m\neq 0$). In particular, when the spatial dimension is three, the only such solutions are either Minkowski or a quotient thereof (no nontrivial solutions exist). When $V(ϕ)=m^{2}|ϕ|^{2}+2Λ$, that is, when a vacuum energy or a cosmological constant is included, it is proved that no geodesically complete solution exists when $Λ>0$, whereas when $Λ<0$ it is proved that no non-vacuum geodesically complete solution exists unless $m^{2}<-2Λ/(n-1)$, ($n$ is the spatial dimension) and the spatial manifold is non-compact. The proofs are based on techniques in comparison geometry á la Backry-Emery.

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