That's missing the point entirely, which is that under theories like MOND, you would not expect to see any galaxies that behave like galaxies with no dark matter, as it posits that dark matter does not exist.
https://en.wikipedia.org/wiki/Triangulum_Galaxy
Nice shot!
noetic_techy:
> what if the key to dark matter is some unknown property of the central black hole [...] ?
cowboysauce:
> Well, then you encounter problems with things like the triangulum Galaxy. Which has dark matter, but no supermassive black hole.
The initial comment was about a "central" black hole (which may indeed exist), not a "supermassive" black hole.
The fact that galaxies form and light is being bent is not evidence: it is the problem. We don't actually have any classical scientific evidence of the stuff. Dark matter is handwavium. It may be real, but it not being is just as plausible. When the Earth was flat and the universe revolved around it, we solved problems with our theories by adding escape clauses to them: epicycles. An effect was explained with an effect, and dark matter fits that's profile perfectly.
MOND could be incorrect, but it is valuable as an alternative line of thought. There needs to be more like it. Science needs to get into the routine of failure once again, we are making no progress without it.
Here’s a not-so-bold prediction: sometime this century some team will detect something new that explains some but not all of the effects we currently attribute to dark matter. We will call it the explanation to the problem and the remainder will be given a new catchy name and people on the HN of the future will complain that this thing is simple hand waving.
> people on the HN of the future will complain that this thing is simple hand waving.
So long as we don't fall into a local minimum in the theory space forever, I'd be happy. Progress is progress, no matter how slow.
I seem to have to qualify my statements with this but I am not attacking the idea of dark matter (nor am I defending MOND) I am just questioning the logic of this one point.
----Edit------ I think it could be confusing what i mean by saying there is no mechanism explaining why dark matter exists. I just mean specifically why. It is a theory that postulates the existence of something but does not give a mechanism to how it gets there. This is not my field of research though so I could be terribly wrong about that.
Dark matter is exciting in part because it intersects particle physics with cosmology, and has the potential to solve problems in both (e.g. rotation curves and neutrino masses). It's appealing in part because it's parsimonious.
you miss my point entirely, your explanation is around an assumption of dark matter. You are not saying this galaxy behaves in such a way that MOND cannot explain, you are saying the galaxy behaves in such a way that based on believing in dark matter, this galaxy moves as if it doesnt have it thus MOND is wrong.
MOND can not explain a galaxy without dark matter as it is a proposal for the modification of the laws of gravity at large distances, and can explain the already-observed galaxies that behave as if they have dark matter (or, as MOND posits, the laws of gravity are different than what we have observed on earth.)
Explanation 1: MOND -- modified gravity at that scale, just works differently than we guessed.
Explanation 2: Dark Matter
Now this is galaxy 2, and from visible matter, we expect rotation curve B, and we get rotation curve A!
So no, if it's just visible matter interacting w/ a modified version of gravity, it should predict consistent rotation curves. But we see two types. Thus it is directly problematic for MOND.
No, that's exactly what's being stated. MOND doesn't predict this galaxy. This galaxy rotates slower than others of similar directly observable mass. In other words, it confirms the current model of gravity, which by definition is evidence against any modified model of gravity.
> "This galaxy discovery, though, appears to be strong evidence against MOND or other modified-gravity theories since the dark matter is missing in this case."
Since MOND assumes that dark matter does not exist (or at least does not interact with gravity), then under MOND this galaxy should not exist. MOND stipulates that all galaxies of a given size should rotate at the same speed, and that speed is greater than that predicted by newtonian mechanics. This galaxy defies MOND predictions, and appears to obey Newton's. Does that make sense or am I missing something else?