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Register a new userViscosity Bound, Causality Violation and Instability with Stringy Correction and Charge
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Recently, it has been shown that if we consider the higher derivative correction, the viscosity bound conjectured to be $eta/s=1/4pi$ is violated and so is the causality. In this paper, we consider medium effect and the higher derivative correction simultaneously by adding charge and Gauss-Bonnet terms. We find that the viscosity bound violation is not changed by the charge. However, we find that two effects together create another instability for large momentum regime. We argue the presence of tachyonic modes and show it numerically. The stability of the black brane requires the Gauss-Bonnet coupling constant $lambda$($=2alpha/l^2$) to be smaller than 1/24. We draw a phase diagram relevant to the instability in charge-coupling space.
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In recent work we showed that, for a class of conformal field theories (CFT) with Gauss-Bonnet gravity dual, the shear viscosity to entropy density ratio, $eta/s$, could violate the conjectured Kovtun-Starinets-Son viscosity bound, $eta/sgeq1/4pi$. In this paper we argue, in the context of the same model, that tuning $eta/s$ below $(16/25)(1/4pi)$ induces microcausality violation in the CFT, rendering the theory inconsistent. This is a concrete example in which inconsistency of a theory and a lower bound on viscosity are correlated, supporting the idea of a possible universal lower bound on $eta/s$ for all consistent theories.
Motivated by the vast string landscape, we consider the shear viscosity to entropy density ratio in conformal field theories dual to Einstein gravity with curvature square corrections. After field redefinitions these theories reduce to Gauss-Bonnet gravity, which has special properties that allow us to compute the shear viscosity nonperturbatively in the Gauss-Bonnet coupling. By tuning of the coupling, the value of the shear viscosity to entropy density ratio can be adjusted to any positive value from infinity down to zero, thus violating the conjectured viscosity bound. At linear order in the coupling, we also check consistency of four different methods to calculate the shear viscosity, and we find that all of them agree. We search for possible pathologies associated with this class of theories violating the viscosity bound.
We show how uncertainty in the causal structure of field theory is essentially inevitable when one includes quantum gravity. This includes the fact that lightcones are ill-defined in such a theory - independent of the UV completion of the theory. We include details of the causality uncertainty which arises in theories of quadratic gravity.
We study the spectral function of fermions in a holographic set up with bulk Dirac mass in the regime beyond the conformal unitarity bound, and find that spectral function has the dispersion relation with tachyonic behavior, indicating an instability. Based on linearity between the density and the position of the tip of the k-gap, we suggest that this instability is toward the charge density wave(CDW) and the position of the tip can be identified as the wave vector of CDW. For the physical origin, we point out the similarity of unitarity violation in our non-Fermi Liquid theory and nesting phenomena in the Fermi liquid theory as the mechanism of CDW instability.
Causality in quantum field theory is defined by the vanishing of field commutators for space-like separations. However, this does not imply a direction for causal effects. Hidden in our conventions for quantization is a connection to the definition of an arrow of causality, i.e. what is the past and what is the future. If we mix quantization conventions within the same theory, we get a violation of microcausality. In such a theory with mixed conventions the dominant definition of the arrow of causality is determined by the stable states. In some quantum gravity theories, such as quadratic gravity and possibly asymptotic safety, such a mixed causality condition occurs. We discuss some of the implications.
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