Holography and Causality in Einstein-Gauss-Bonnet Gravity

Date

2013-07-23

Authors

Harder, Michael

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Abstract

Field theories with higher derivative gravity duals can violate the viscosity bound. However the extent of the violation is not arbitrary since it depends on the coupling of the higher derivative interactions, which can be constrained by requiring consistency of the boundary field theory. In particular, in Einstein-Gauss-Bonnet (EGB) gravity, the coupling λ can be constrained by requiring that the dual theory respect causality. We investigate the upper bound on λ by computing the quasinormal modes of an EGB black hole in order to explicitly find and interpret the causality violating excitations. We find that in the limit of infinite spatial momentum the imaginary part of these modes approaches 0, while the phase velocity approaches 1 from above. This behaviour at high momentum is confirmed by the existence of a lightlike pole in the stress-energy tensor two-point function. We therefore confirm that the requirements to interpret the poles of the two-point function as causality violating, propagating modes are met in the limit of infinite spatial momentum. The presence of such excitations not only constrains the viscosity bound but also limits the allowed couplings of EGB gravity.

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Keywords

derivative, EGB, viscosity bound

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