We investigate the renormalization group approach to nonequilibrium field theory. We show that it is possible to derive nontrivial renormalization group flow from iterative coarse graining of a closed-time-path action. This renormalization group is different from the usual in quantum field theory textbooks, in that it describes nontrivial noise and dissipation. We work out a specific example where the variation of the closed-time-path action leads to the so-called Kardar-Parisi-Zhang equation, and show that the renormalization group obtained by coarse graining this action, agrees with the dynamical renormalization group derived by directly coarse graining the equations of motion.
We study de spectrum of primordial fluctuations and the scale dependence of the inflaton spectral index due to self-interactions of the field. We compute the spectrum of fluctuations by applying nonequilibrium renormalization group techniques.
The bilateral simultaneous generation of sound in some oscine songbirds leads to complex sounds that cannot be described in terms of a superposition of the isolated sources alone. In this work, we study the appearance of complex solutions in a model for the acoustic interaction between the two sound sources in birdsong. The origin of these complex oscillations can be traced to the nonlinear mode-mode interaction arising when both sources are active. As an example, we analyze a remarkable sound produced by an oscine songbird and show that the proposed dynamical scenario is compatible with the observed behavior.
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