[mirimir]

Huh.

I thought that there was a solid theoretical basis for the density wave hypothesis. But TFA implies that the main constraint was consistency with the Hubble tuning fork diagram.

That would be funny, because I've never really understood the density wave hypothesis.

[mih]

I don't claim to understand it either, but the animations on the Wiki page at least give a feel for it - https://en.wikipedia.org/wiki/Density_wave_theory

[mirimir]

Thanks :)

I get the idea of standing waves.

But TFA at least implies that galaxy structure might not be stable. So there's perhaps a cycle of waves forming, wrapping up, and then reforming. So at any given time, we'd see whatever mix we see.

I mean, I doubt that we've observed any particular galaxy long enough to see whether its structure is stable or not. At least, if the cycle is longer than some hundreds of years, which seems likely.

But then, we do see what galaxies looked like in the past, at increasing distance. And I vaguely recall something about more distant ones not so much being spiral. Even so, that doesn't necessarily constrain ongoing structural stability.

There's also gravitational lensing, which sometimes lets us see events through different paths, and so at different times. However, I recall that the path/time differences were on the order of years at most. But even so, maybe we wouldn't have noticed cases where path/time differences were great enough to see galaxy structure changing. Or maybe such path/time differences are just implausible.

It really is hard when all you can do is look at stuff.