Besides galactic disks, where was the mathematical description of density waves successfully applied?
Answer
The rings of Saturn, specifically the A Ring
The physical principles underlying Density Wave Theory are mathematically robust enough to apply to other disk structures governed by differential rotation. Specifically, the same mathematical description used for spiral density waves in galaxies has been successfully applied to analyze the ring system of Saturn. In Saturn's A Ring, spiral density waves are observed, which are excited by orbital resonances caused by the gravitational influence of the system’s moons. Although the scale is vastly smaller, analyzing these ring waves provides a tangible, closer laboratory setting to test and confirm the physics that explains the beautiful, large-scale structure observed in distant spiral galaxies.
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