Bovy and I spent the morning discussing places chaos (as in nonlinear dynamics) enters into astrophysics, since he is interested in both subjects. I identified several places: In the Solar System, the Lyapunov time is much shorter than the time over which there has been and will be macroscopic stability. There are also many issues in planetary system formation. In numerical models of globular clusters and similar N-body systems, some of the relaxation times seem to disagree with analytic/scaling expectations. In the disk of the Milky Way, there is very complicated velocity-space structure, possibly caused by caustics and other nonlinear structures in phase space. In the halo of the Milky Way, phase-space structure is expected to be extremely rich because the accretion history is extremely rich. In cosmology, there is my insane idea of constrained realizations of the entire observable Universe, which has a nonlinear dynamics or control theory aspect to it.
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