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Comment by gamegoblin | Hacker News Reader

Even in pretty accurate simulations using just the law of gravity as we know it, if you simulate the galaxy, it'll fall apart. If you add in some mysterious dark matter, it stays together.

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.

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But if you throw is dark matter and have it influence the other matter... Shouldn't the regular matter influence the dark matter too? At that point doesn't it end up in the same distribution as the regular matter? And isn't it then just unaccounted for? And wouldn't it have zero effect on the shape of the rotation curve?

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.

The evolution of a galaxy is affected by collisions -- the interactions of regular matter. Dark matter doesn't interact with itself in the same way. That's exactly what this article is about!

(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!)

>> 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.

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.

> Shouldn't the regular matter influence the dark matter too? At that point doesn't it end up in the same distribution as the regular matter?

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.

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