chaos in the Milky Way

Now that Price-Whelan's paper is done (will appear on arXiv next week), we met with Johnston and Sarah Pearson (Columbia) to discuss next projects. Price-Whelan has been looking at regular and chaotic orbits in the Milky Way. Even in the standard models people use to model the Sagittarius stream, many of the possible orbits are chaotic. We discussed how to establish chaos given finite integrations with finite computation time. The nice thing that Price-Whelan is doing is that he is integrating not just the orbits but also a (literally) infinitesimal tensor bundle around the orbit to see the evolution of the bundle with time. This permits measurement of Lyapunov times. From my perspective, the chaos question is not "is the Lyapunov time infinity?" but rather "is it longer than a Hubble time?" I think that's the key question when we are thinking about modeling a real dynamical system.