I find density waves, used as an explanation for the spiral arms in a Galaxy a curious phenomenon:
http://en.wikipedia.org/wiki/Density_wave_theory
" ..so, the gravitational attraction between stars can only maintain the spiral structure if the frequency at which a star passes through the arms is less than the epicyclic frequency, κ(R), of the star. This means that a long-lived spiral structure will only exist between the inner and outer Linblad resonance (ILR, OLR, respectively), which are defined as the radii such that: Ω(R) = Ωgp + κ / m and Ω(R) = Ωgp − κ / m, respectively. Past the OLR and within the ILR, the extra density in the spiral arms pulls more often than the epicyclic rate of the stars, and the stars are thus unable to react and move in such a way as to "reinforce the spiral density enhancement".
What's more, there is observational evidence (& predictions) supporting this theory. (Eg: Saturn's moon-on-ring interactions).
So in general, might one view the arms as the end result of the 'twisting' in spacetime caused by the stars' gravitational fields interacting with their inertial/centrifugal forces ?
Yet another 'spacetime' warping phenomenon ? ... I wonder ...
Cheers