Galactic evolution scale

The importance of this frequency for the resonance problem is that any force that is periodic on this time scale will produce a resonant interaction. For the more general problem of galactic structure, the points at which the perturbation frequency is the same as the epicyclic frequency are called the Lindblad resonances. These play a central role in density wave theory, which has been applied to a wide variety of astrophysical problems, including planetary ring systems, accretion disks, and the structure and evolution of spiral disk galaxies. 

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