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> "you would not expect to see any galaxies that behave like galaxies with no dark matter" you miss my point entirely, your explanation is around an assumption

> "you would not expect to see any galaxies that behave like galaxies with no dark matter" 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.

rmcpherson 9y ago
This finding certainly weakens the case for MOND. The authors write that in the conclusion of their paper, as quoted in this article: https://arstechnica.com/science/2018/03/galaxy-seems-to-lack-dark-matter-stumping-astronomers/ https://arstechnica.com/science/2018/03/galaxy-seems-to-lack... 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.)
autokad 9y ago
@rmcpherson I am not defending MOND or attacking dark matter. I am stating based on what they have said, it doesnt make sense unless they provide some basis on why its problematic for MOND
- jsweojtj 9y ago
  Consider galaxy 1, from visible matter, we might expect rotation curve A, but we see curve B. 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.
nate_meurer 9y ago
> "You are not saying this galaxy behaves in such a way that MOND cannot explain" 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.
- autokad 9y ago
  Was any of that said above? No.
  - nate_meurer 9y ago
    GP actually said exactly that, just in a slightly different way: > "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?

rmcpherson 9y ago

This finding certainly weakens the case for MOND. The authors write that in the conclusion of their paper, as quoted in this article: https://arstechnica.com/science/2018/03/galaxy-seems-to-lack-dark-matter-stumping-astronomers/ https://arstechnica.com/science/2018/03/galaxy-seems-to-lack... 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.)

autokad 9y ago

@rmcpherson I am not defending MOND or attacking dark matter. I am stating based on what they have said, it doesnt make sense unless they provide some basis on why its problematic for MOND

jsweojtj 9y ago
Consider galaxy 1, from visible matter, we might expect rotation curve A, but we see curve B. 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.

nate_meurer 9y ago

> "You are not saying this galaxy behaves in such a way that MOND cannot explain" 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.

autokad 9y ago
Was any of that said above? No.
- nate_meurer 9y ago
  GP actually said exactly that, just in a slightly different way: > "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?