We show that an exact solution of two-dimensional dilaton gravity with matter discovered previously exhibits an irreversible temporal flow towards flat space with a vanishing cosmological constant. This time flow is induced by the back reaction of matter on the space-time geometry. We demonstrate that the system is not in equilibrium if the cosmological constant is non-zero, whereas the solution with zero cosmological constant is stable. The flow of the system towards this stable end-point is derived from the renormalization-group flow of the Zamolodchikov function. This behaviour is interpreted in terms of non-critical Liouville string, with the Liouville field identified as the target time.
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