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Comment by JKCalhoun | Hacker News Reader

"which led them to conclude that the outer rims of all disk galaxies take roughly a billion years to complete one rotation"

I was curious about "outer rims". I know little about astrophysics, but is the "outer rim" a definable thing? Because my sense is that, like a whirlpool (and planetary physics), material within a galaxy have different rotational rates depending upon their distance from the center of mass. It seems odd to describe "the spin" of a galaxy at all — but since they specify "outer rim" I expect that to have an agreed-upon definition.

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Actually a galaxy's spin curve significantly diverges from a what we'd expect, like a "whirlpool" or say a solar system. It turns out stars on the rim do not complete orbits significantly slower than ones nearer to the center. It acts more like a frisbee. They're moving faster than they should but not getting "flung" out.

It is one of the reasons why dark matter was originally postulated; basically the galaxy system is much larger/massive than what we can see.

Adding a "Halo" of massive particles that are weakly interacting explains the curves fairly well--hence Dark Matter.

In case anyone gets the wrong idea, when dark matter is referred to as weakly interacting in the literature, as in WIMPs, they are specifically discussing particles that interact via the Weak force, not merely particles that don't interact very often. It's a good idea to keep the usage of that phrasing limited to avoid confusion.

Well... Sort of. The 'weakly interacting' means it interacts via gravity and another force no stronger than the Weak force. As per Wikipedia [0]: "There exists no clear definition of a WIMP, but broadly a WIMP is a new elementary particle which interacts via gravity and any other force (or forces), potentially not part of the standard model itself, which is as weak as or weaker than the weak nuclear force, but also non-vanishing in its strength."

[0] https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...

I think the radial scales on which galaxies are frisbee-like (i.e., the tangential velocity is roughly proportional to radius so that rotation period is constant) are much smaller than the scales on which we notice the discrepancy from naive expectations that suggests dark matter. The discrepancy in the velocity curve is at radiuses where both dark-matter and non-dark-matter models predict that velocity increases sub-linearly with radius. (It's true the dark-matter models are closer to linear than the non-dark-matter models, both are clearly distinct for it, and hence show whirlpool-like motion rather than frisbee-like motion). See the third figure here:

https://physics.stackexchange.com/questions/134159/what-is-a...

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