Sure it is, it is linked in this sentence of the article:
“Now, though, new observations have thrown a monkey in the wrench with all this. Those galaxies aren't the exception. They're the rule.”
The other relevant link is:
https://ras.ac.uk/news-and-press/research-highlights/citizen...
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.
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.
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.
[1]: https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existe... [2]: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
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 With future discoveries in science the terms will be replaced, superseded or perhaps disappear.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.
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)
If you cannot detect something, it's can be equally ridiculous to say it exists.
It's a limitation of all purely observational sciences.
