I think that [1] is the paper that this is based on. I don't work on dynamics and I found it pretty readable. Most of what is in the article is in sections 3.2, section 4 (discussion) and section 5 (conclusions).
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.
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.
The Hubble fork isn't "wrong". It's just a classification scheme. The fork has nothing to do with how spiral structures form or how long they last, it's just a way to quickly describe them.
>Hubble's tuning fork has been more or less the standard for a century now. It's done a decent job, but there have been some galaxies that don't fit the pattern. For example, one might have a small bulge but tight arms, or another with a large bulge and fairly open arms. If there's a galaxy that didn't quite fit, astronomers usually used the arm structure to place it in the diagram.
More specificly, it suggests that there are additional categories that should be added.
The fork used 2 variables to come up with 6 combinations: bulge size( with 3 states) and bar presence (with 2 states). A third variable, arm angle, was assumed to be linked with bulge size.
the data suggests that arm angle does not have the relation to bulge size that was assumed.
Given that most cosmology relies on a huge percentage of dark matter, which cannot be detected, and a huge amount of dark energy, with enormous conflicting calculations, I think it's fair to say that astronomers are wrong about a lot of things.
You seem to be having a weird definition of 'detect'. The only reason the concepts dark matter and dark energy are on the table in the first place is that we seem to be detecting a lot of both. You might not like that scientists can't really explain what it is that's being detected, but for sure it's being detected. There's pretty good summaries of how dark energy and dark matter are being detected on Wikipedia [1][2].
People really seem to be having trouble with the fact that dark matter and dark energy are 'placeholder terms' to account for stuff we're actually seeing. With future discoveries in science the terms will be replaced, superseded or perhaps disappear. But observation or 'detection' of both dark matter and dark energy is in fact there.
Not "stuff we're actually seeing" technically, more effects we can't explain using our current theories.
Placeholder is the correct term but people tend to leap from "matter" and "energy" to "stuff" (I'm assuming you didn't intend the implication) without understanding those terms (dark matter and dark energy) are preliminary - if well informed - guesses.
They are "placeholders" because our models are wrong. Which is fine: all models are wrong, but some models are useful.
And astronomy is particularly vulnerable to this - its models are being applied to make predications at extremes of both time and scale.
But I PROMISE you, we're wrong. We know we're wrong, and we don't know why (and recently there's even been some fair criticism of our models that we use to gauge distance, so no, I'd argue we have not reasonably "detected" anything other than that our models are wrong - STILL USEFUL but wrong)
Of course dark matter can be detected. If its gravitational attraction on visible matter weren't so easy to detect, there wouldn't have been a reason to come up with explanations for it. That explanation may be wrong, but that's a different issue.
Well, most of the other theories back then would seem ridiculous and obviously wrong nowadays. That's the nature of theorizing when you have little information.
Obviously we mostly remember those theories that turned out to be right.
We're not sure it's matter, but let's just say 'dark matter' is a useful abstract label (like 'teapot'). We know that SOMETHING is perturbing our models that are useful (to simplify the analogy, let's just posit that newtonian orbits are casually affected, for example) by measurement. As much as an orbiting 'teapot' is ridiculous, it's also provable that something is unaccounted for, 'teapot' or 'dark matter', whatever the label.
This does not mean that it's matter specifically (just a label), but is a phenomena.
[1] https://arxiv.org/pdf/1904.11436.pdf