
Battle of the bulges: Have astronomers been wrong about spiral galaxies? (2019) - breadbox
https://www.syfy.com/syfywire/battle-of-the-bulges-have-astronomers-been-wrong-about-spiral-galaxies-all-this-time
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astro123
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).

[1]
[https://arxiv.org/pdf/1904.11436.pdf](https://arxiv.org/pdf/1904.11436.pdf)

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acqq
> I think that [1] is the paper

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...](https://ras.ac.uk/news-and-press/research-
highlights/citizen-scientists-re-tune-hubbles-galaxy-classification)

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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.

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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](https://en.wikipedia.org/wiki/Density_wave_theory)

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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.

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hannasanarion
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.

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garmaine
According to the article, it's a classification scheme that doesn't work. It
describes categories that real galaxies don't fit in.

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s1artibartfast
>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.

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speedplane
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.

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DCKing
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].

[1]:
[https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existe...](https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence)
[2]:
[https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...](https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence)

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._

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mellosouls
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.

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antonioni
the site is such a crap on mobile, low contrast letters, and that header that
pops up and takes 30% of the screen every time you scroll up a pixel

