极端极限下的体积粘度：从动力学理论到弦
Bulk Viscosity at Extreme Limits: From Kinetic Theory to Strings
摘要：
本文研究了具有大量颜色的热QCD模型在两个极端情况下的体粘滞系数：很弱的和很强的t Hooft耦合。弱耦合的情况下是基于动力学理论，和一个可以去非常强的耦合动力学通过一个中间耦合制度。虽然前者在动力学理论方面有一个明确的描述，中间耦合制度，它使用晶格的结果，遭受通常的技术挑战，使显式确定的体积粘度有点困难。另一方面，很强的t Hooft耦合动力学可以分别用IIB型引力场和M理论在弱弦耦合和强弦耦合下用弦论研究。在类型IIB中，我们提供了精确的波动模式的度量在重力对偶负责体积粘度，计算声速在介质中，并分析体积剪切粘度的比率。在M理论中，我们提升了UV完全IIB型模型的IIA型镜像对偶，通过分析强IIA弦耦合下系统中的准简正模，我们研究和比较了体粘滞系数和声速。通过推导的谱函数，我们表明，我们的结果的一致性，无论是为实际值的参数，以及为约束的体积剪切粘度的比率。
英文摘要：
In this paper we study bulk viscosity in a thermal QCD model with large number of colors at two extreme limits: the very weak and the very strong 't Hooft couplings. The weak coupling scenario is based on kinetic theory, and one may go to the very strong coupling dynamics via an intermediate coupling regime. Although the former has a clear description in terms of kinetic theory, the intermediate coupling regime, which uses lattice results, suffers from usual technical challenges that render an explicit determination of bulk viscosity somewhat difficult. On the other hand, the very strong 't Hooft coupling dynamics may be studied using string theories at both weak and strong string couplings using gravity duals in type IIB as well as M-theory respectively. In type IIB we provide the precise fluctuation modes of the metric in the gravity dual responsible for bulk viscosity, compute the speed of sound in the medium and analyze the ratio of the bulk to shear viscosities. In M-theory, where we uplift the type IIA mirror dual of the UV complete type IIB model, we study and compare both the bulk viscosity and the sound speed by analyzing the quasi-normal modes in the system at strong IIA string coupling. By deriving the spectral function, we show the consistency of our results both for the actual values of the parameters involved as well for the bound on the ratio of bulk to shear viscosities.
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