Brustein, R and Medved, A.J.M (2011) Unitarity constraints on the ratio of shear viscosity to entropy density in higher derivative gravity. Physical Review D, 83 (12). ISSN 15507998

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Official URL: http://link.aps.org/doi/10.1103/PhysRevD.83.126005
Abstract
We discuss corrections to the ratio of shear viscosity to entropy density η/s in higherderivative gravity theories. Generically, these theories contain ghost modes with Planckscale masses. Motivated by general considerations about unitarity, we propose new boundary conditions for the equations of motion of the graviton perturbations that force the amplitude of the ghosts modes to vanish. We analyze explicitly fourderivative perturbative corrections to Einstein gravity which generically lead to fourderivative equations of motion, compare our choice of boundary conditions to previous proposals and show that, with our new prescription, the ratio η/s remains at the Einsteingravity value of 1/4π to leading order in the corrections. It is argued that, when the new boundary conditions are imposed on six and higherderivative equations of motion, η/s can only increase from the Einsteingravity value. We also recall some general arguments that support the validity of our results to all orders in the strength of the corrections to Einstein gravity. We then discuss the particular case of GaussBonnet gravity, for which the equations of motion are only of twoderivative order and the value of η/s can decrease below 1/4π when treated in a nonperturbative way. Our findings provide further evidence for the validity of the KSS bound for theories that can be viewed as perturbative corrections to Einstein gravity.
Item Type:  Article 

Uncontrolled Keywords:  AdSCFT Correspondence, Black Holes in String Theory 
Subjects:  Q Science > QC Physics 
Divisions:  Faculty > Faculty of Science > Physics & Electronics 
ID Code:  1916 
Deposited By:  Mrs Eileen Shepherd 
Deposited On:  09 Jun 2011 14:59 
Last Modified:  06 Jan 2012 16:21 
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