[xr4ti]

Possibly naive question from a non-physicist... does this not follow from relativity as a natural a consequence of the effect gravity/mass has on spacetime?

By that I mean that the edges of galaxies experience a "reference" spacetime (e.g. constant period), while the interiors experience a stretched out version spacetime (i.e. inverse time dilation and length constriction)?

[dnautics]

no, the point is that it follows from basic algebra, and high-school physics defitions of velocity.

[zaarn]

Highschool definitions of velocity don't really apply once the distances involved grow beyond a couple light years. Atleast not without corrections for stuff like relativity (which has been measured and confirmed to exist by simply observing curvature of light over the sun.

Or did the entire community of astrophysics simply ignore high school physics in favor or something vastly more complicated because reasons?

[dnautics]

Even at galactic scales the correction is not that great. I don't thing the OP's claim is that they are exact to say 3 sig figs but more like exact to within an order of magnitude

[zaarn]

But how does the proposed model compare to the accuracy of the dark matter model?

In physics, how well your model can predict past and future states from the current state is rather important. Though future states more than past state but any model should be able to also somewhat agree with it's the past it generates.

[dnautics]

dark matter is irrelevant. The speed law is empirical (whether or not you explain it with dark matter) and the first poster points out that the constant edge rotational rate falls out from that, and geometry.

[zaarn]

It is relevant when you compare models. All I asked is how accurate both models are. If Dark Matter is more accurate in predicting how galaxies move and shape then it is obviously the better model even if a simpler one also exists but it's predictions are worse.

You can't simply say "this model also predicts that" without giving on how accurate it does predict and comparing to existing models. Otherwise we'd be using flat earth models for building bridges and planning ship routes.

[trhway]

Also not a physicist. At the galactic scale gravity forces are too weak to take into account any non-Newtonian considerations. At least for myself, most things observed so far at galactic scale, like flat disk star orbital speed for example, can be easily explained by straight 6th grade Newtonian physics.

[titzer]

sigh I wouldn't have replied to this if you hadn't posted half a dozen comments. When I was 16 I thought special relativity was a hoax and I tried to demonstrate this with triangular diagrams scribbled in pencil in my notebooks. I ended up just rediscovering the Doppler effect. I never showed it to anyone, but when I think back, I still cringe a little. Relativity and time dilation are real things with real theory and real experiments.

You'd do well to do a little more research on the topic before posting so many comments on HN.

There is a ton of evidence for dark matter. We still don't know exactly what it is, but we do know lots about it (e.g. it is not baryonic matter because observation differs from theoretical models of how it would interact with ordinary baryonic matter). As for all that other evidence, the first hit on Google is for laymen: https://medium.com/starts-with-a-bang/five-reasons-we-think-...

TLDR: gravitational lensing, missing mass, and three other really good reasons that dark matter is a serious subject of study.

Disclaimer: I am not a physicist. And I'm trying not to represent myself as one on HN.

Talking about this stuff is fun, sure. It's great to learn! But please don't spread ignorance with such high confidence. As for "just" Newtonian physics, there's a whole research field called "Modified Newtonian Dynamics" (MOND). https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

TLDR; there are lots of problems with MOND, and it doesn't explain gravitational lensing and other effects of dark matter that we see.

[phkahler]

Another fun experiment is to write script to cover a disk with points, then do a full calculation of all N gravitational forces at a point along the radius. Calculate the acceleration of a mass at that point and back-calculate the velocity that would orbit with that inward force. Repeat this for many points along the radius and plot the rotation curve without doing any dynamic simulation. There can be some interesting numerical issues with this but a random set of points to "measure" the curve will still show it.