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A Note On Boundary Conditions In Euclidean Gravity

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arxiv 1805.11559 v3 pith:IQZOBL54 submitted 2018-05-29 hep-th

classification hep-th
keywords boundaryconditionconditionscurvatureellipticeuclideanextrinsicgeneral
verification ladder T0 review T1 audit T2 compute T3 formal
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We review what is known about boundary conditions in General Relativity on a spacetime of Euclidean signature. The obvious Dirichlet boundary condition, in which one specifies the boundary geometry, is actually not elliptic and in general does not lead to a well-defined perturbation theory. It is better-behaved if the extrinsic curvature of the boundary is suitably constrained, for instance if it is positive- or negative-definite. A different boundary condition, in which one specifies the conformal geometry of the boundary and the trace of the extrinsic curvature, is elliptic and always leads formally to a satisfactory perturbation theory. These facts might have interesting implications for semiclassical approaches to quantum gravity. (Submitted to a volume in honor of Roman Jackiw.)

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