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I dont know much about it, but that may not necessarily be the case. Lets assume dark matter does not exist and the people talking about it believe it does. I

I dont know much about it, but that may not necessarily be the case. Lets assume dark matter does not exist and the people talking about it believe it does. It could be that the people describing it are only describing it through the lens of what they already believe. As in, I believe there are invisible blobs that keep cars close together. I observe some cars far apart and i say "hey look at all that missing blobs"

anyfoo 9y ago
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.
- autokad 9y ago
  > "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?
- wuliwong 9y ago
  I still don't think it's good logic. There is no mechanism explaining why dark matter exists. So not seeing it in this case doesn't shake that theory because it gives no explanation in the first place. MOND does give some explanation so this becomes problematic. But what about a possible new 'dark force' being a possible explanation? Maybe this 'dark force' only radiates under certain circumstances. 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.
  - millstone 9y ago
    There are indeed many reasonable dark matter candidates: sterile neutrinos, lightest supersymmetric partner, axions... These are particles which are theoretically motivated, but by their nature would be difficult to detect because they don't interact electromagnetically. If we exclude the existence of such particles it is more difficult to construct a coherent theory. 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.
- noetic_techy 9y ago
  This "galaxy" is so sparse, what if the key to dark matter is some unknown property of the central black hole. Maybe an unknown space-time interaction we have yet to conceive?
  - cowboysauce 9y ago
    Well, then you encounter problems with things like the triangulum Galaxy. Which has dark matter, but no supermassive black hole.
    - noetic_techy 9y ago
      Does not appear to have one: https://en.wikipedia.org/wiki/Triangulum_Galaxy https://en.wikipedia.org/wiki/Triangulum_Galaxy Nice shot!
      - dwringer 9y ago
        In fact that article states that the upper limit for the size of a central black hole in the Triangulum galaxy is estimated at 3000 solar masses, so it simply wouldn't be "supermassive" - but there is not a claim that no black hole exists there. 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.
    - zamalek 9y ago
      According to my [limited] reading of MOND, it revolves around an unknown function μ(x). That means it could be adjusted to account for this. Which is a handwavium thesis, but that's what I like about it: how long have we been doing science for? Are we so arrogant that we can claim to know nearly everything? What if we simply started from the beginning? That is the essence that MOND captures. 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.
thisrod 9y ago
In this case, the theory also predicts that airplanes will need extra thrust when they fly over roads, to pull away from the blobs. The new discovery is a road where that doesn't happen, but the cars still stick together as expected.

anyfoo 9y ago

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.

autokad 9y ago
> "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?
wuliwong 9y ago
I still don't think it's good logic. There is no mechanism explaining why dark matter exists. So not seeing it in this case doesn't shake that theory because it gives no explanation in the first place. MOND does give some explanation so this becomes problematic. But what about a possible new 'dark force' being a possible explanation? Maybe this 'dark force' only radiates under certain circumstances. 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.
- millstone 9y ago
  There are indeed many reasonable dark matter candidates: sterile neutrinos, lightest supersymmetric partner, axions... These are particles which are theoretically motivated, but by their nature would be difficult to detect because they don't interact electromagnetically. If we exclude the existence of such particles it is more difficult to construct a coherent theory. 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.
noetic_techy 9y ago
This "galaxy" is so sparse, what if the key to dark matter is some unknown property of the central black hole. Maybe an unknown space-time interaction we have yet to conceive?
- cowboysauce 9y ago
  Well, then you encounter problems with things like the triangulum Galaxy. Which has dark matter, but no supermassive black hole.
  - noetic_techy 9y ago
    Does not appear to have one: https://en.wikipedia.org/wiki/Triangulum_Galaxy https://en.wikipedia.org/wiki/Triangulum_Galaxy Nice shot!
    - dwringer 9y ago
      In fact that article states that the upper limit for the size of a central black hole in the Triangulum galaxy is estimated at 3000 solar masses, so it simply wouldn't be "supermassive" - but there is not a claim that no black hole exists there. 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.
  - zamalek 9y ago
    According to my [limited] reading of MOND, it revolves around an unknown function μ(x). That means it could be adjusted to account for this. Which is a handwavium thesis, but that's what I like about it: how long have we been doing science for? Are we so arrogant that we can claim to know nearly everything? What if we simply started from the beginning? That is the essence that MOND captures. 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.

thisrod 9y ago

In this case, the theory also predicts that airplanes will need extra thrust when they fly over roads, to pull away from the blobs. The new discovery is a road where that doesn't happen, but the cars still stick together as expected.