Large-N expansion and string theory out of equilibrium

We analyze the large-N expansion of general nonequilibrium systems with fluctuating matrix degrees of freedom and SUoNTHORN symmetry, using the Schwinger-Keldysh formalism and its closed real-time contour with a forward and backward component. In equilibrium, the large-N expansion of such systems leads to a sum over topologies of two-dimensional surfaces of increasing topological complexity, predicting the possibility of a dual description in terms of string theory. We extend this argument away from equilibrium and study the universal features of the topological expansion in the dual string theory. We conclude that in nonequilibrium string perturbation theory, the sum over world sheet topologies is further refined: Each world sheet surface Sigma undergoes a triple decomposition into the part Sigma(+) corresponding to the forward branch of the time contour, the part Sigma(-) on the backward branch, and the part Sigma(boolean AND). that corresponds to the instant in the far future where the two branches of the time contour meet. The sum over topologies becomes a sum over the triple decompositions. We generalize our findings to the Kadanoff-Baym time contour relevant for systems at finite temperature and to the case of closed and open, oriented or unoriented strings. Our results are universal and follow solely from the features of the large-N expansion without any assumptions about the world sheet dynamics.