According to widely accepted ideas, the visible matter in galaxies exists inside clumps of dark matter. Without dark matter's gravity to stabilise them, galaxies like the Milky Way would tear themselves apart as they spin.
Is this the tired old "galactic rotation curves don't fit kepler's law" still kicking around?
Seems like there are two things which could follow from that:
1) We don't completely understand gravity at scale
2) Dark matter in some form or another exists
It just so happens that the huge majority of evidence points to #2.
I'd be hesitant to accuse a huge body of research collected hundreds of people over decades of making some sort of trivial error like that.
I've seen reports of "fixing" the problem by assuming a galaxy is enclosed in spherical ball of dark matter. But of course no explanation of why it would maintain that shape. In other words, the model of dark matter isn't even complete. In some cases I've seen people misapply the divergence theorem as well.
It would not be the first time people used a hack to "fix" what ultimately boils down to numerical problems or inadequate integration methods.
Somewhere down my bucket list is to do galaxy simulation. So far all I've done is plot rotation curves that don't look like keplers laws, but no dynamic simulation yet.
No, not enough. This is a consequence of its interacting only gravitationally or via the weak force. Much of the distribution of visible matter is down to electromagnetic interactions that effectively allow it to collide and slow down, whereas dark matter naturally wouldn't do that, or at least would take a drastically longer time to do so.
(I don't know what's wrong with people that they think that, somehow, the entire astrophysics/cosmology community has been too stupid to think these fairly simple things through. If there was an obvious flaw in these models, a researcher would have a large incentive to point these out and get the citations!)
Because they are: http://www.csmonitor.com/Science/2015/0412/Universe-s-expans...
So the whole dark energy thing hinges on measurements of distances to galaxies, which in turn depend on this notion that supernovas are all the same. The entire community would rather embrace an exotic hard to prove concept than go back and look at older assumptions.
And I kid you not about Keplers Laws being used too often when they don't apply. But that's my usual story.
Because there is no reason to expect stars going around a galaxy to follow keplers laws for two body systems. I have yet to see the measured rotation curves compared to an "expected" curve where the author doesn't use Kepler as part of the expectation.
Just a year or two ago, I saw a paper in Nature where they were looking at a gas cloud forming a star, and they compared its rotation to something based on Kepler. I just had to shake my head - the correlation was there but not a great fit of course. The curve is interesting, the comparison not so much.
The 'Kepler' law for galaxies are modified to accommodate for a mass distribution. See e.g.: https://en.wikipedia.org/wiki/Galaxy_rotation_curve#Halo_den...
This profile closely matches the expectations of a singular isothermal sphere profile where....
Why should the dark matter form a spherical blob? Right, because that's what makes Kepler behave better. But how the heck does the dark matter behave? Why should it take this distribution? It seems to be all made up to only explain observation, but not itself.