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Holographic Consistency and the Sign of the Gauss-Bonnet Parameter

If Einstein-Gauss-Bonnet gravity is obtained as a low energy limit of string theory, then the Gauss-Bonnet parameter $α$ is essentially the inverse string tension and thus necessarily positive. If one treats Einstein-Gauss-Bonnet gravity as a modified theory of gravity in the anti-de Sitter bulk in the bottom-up approach of holography, then there is no obvious restriction on the sign of the parameter a priori, though various studies involving boundary causality have restricted the possible range of $α$. In this short note, we argue that if holographic descriptions are to be consistent, then the Gauss-Bonnet parameter has to be positive. This follows from a geometric consistency condition in the Euclidean picture. From the Lorentzian signature perspective, black holes with a negative $α$ lead to uncontrolled brane nucleation in the bulk and so the supposedly static geometry is untenable. In fact, even the ground state without a black hole is problematic. In other words, the bottom-up approach agrees with the top-down approach on the sign of the parameter. Some possible loopholes of the conclusion are discussed.