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> the single parameter that already exists, Ok I concede. It is a single parameter that contains a millions of subparameters, at least one for each galaxy. A

> the single parameter that already exists, Ok I concede. It is a single parameter that contains a millions of subparameters, at least one for each galaxy. Against three parameters, each of which is a single number. But it is three parameters! That's 3x more parameters! Moreover, we had a problem and we had to increase the number of parameters by 50%. So yeah One parameter is much, much better, especially since we have never changed that it's one parameter.

pdonis 4y ago
> It is a single parameter that contains a millions of subparameters, at least one for each galaxy. No, it's a single parameter with a distribution in space and time. It's the same one we already have to vary in space and time anyway based on our observations of light. Dark matter just means adjusting the same distribution further based on observed motions even if we can't see the source of gravity that is producing the motion. If MOND actually gave the choice of adjusting a single parameter in place of having to adjust the distribution of matter and energy, that would be one thing. But, as I noted in another comment upthread, once you take into account that the theory of gravity we are using is actually General Relativity, not Newtonian gravity, there is no such single parameter anyway. Relativistic versions of MOND actually end up looking just like dark matter: they have to invent new fields, new forms of energy, for which we have no other evidence. There is no way to get the desired effect in GR by just tweaking a parameter in the law of gravity.
- throwawaymaths 4y ago
  > distribution You're right. I was wrong. It doesn't have millions of subparameters. It is a single parameter, a vector with countably infinite dimension.
  - andrewflnr 4y ago
    > vector with countably infinite dimension. This is equally true for baryonic matter. You might counter that baryonic matter is easier to observe/constrain. I would point out that LCDM also has ways to observe and constrain DM, and the universe doesn't owe us perfect legibility. What is it you think you're proving?
    - naasking 4y ago
      What doesn't add up is that MOND can reproduce many of those observations using only the visible mass and a tweak to gravitation. This makes all of these tuned distributions seem very suspicious by comparison. It would be a remarkable coincidence if there wasn't something to MOND.
      - pdonis 4y ago
        > MOND can reproduce many of those observations using only the visible mass and a tweak to gravitation No, it can't. It can only reproduce a subset of galaxy rotation curve observations. It cannot reproduce other observations at all (such as the amount of structure in the universe as compared with the amount of baryonic matter that is consistent with Big Bang nucleosynthesis, which is about the same as the amount of matter we can see, but is much too small to account for the amount of structure--gravitational clumping into galaxies and galaxy clusters--that we see).
        naasking 4y ago
        I suggest you review the following, it's not nearly as rosy for DM or dire for MOND as you imply: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm
        pdonis 4y ago
        > it's not nearly as rosy for DM or dire for MOND as you imply According to that one paper, perhaps. There are lots of papers published in this area of research. If you want to say the jury is still out on a final resolution, yes, I'll agree with that.
      - andrewflnr 4y ago
        My guess would be something about the dynamics of dark matter in galaxy evolution pushes the distribution into a shape that produces those mathematically simple rotation curves. That seems Occam's-razor compliant to me; it would be weird if rotation curves with dark matter were really complicated and unpredictable. It's also possible that both MOND and LCDM have some truth to them. But usually people are trying to get rid of dark matter, and that's just not happening.

pdonis 4y ago

> It is a single parameter that contains a millions of subparameters, at least one for each galaxy. No, it's a single parameter with a distribution in space and time. It's the same one we already have to vary in space and time anyway based on our observations of light. Dark matter just means adjusting the same distribution further based on observed motions even if we can't see the source of gravity that is producing the motion. If MOND actually gave the choice of adjusting a single parameter in place of having to adjust the distribution of matter and energy, that would be one thing. But, as I noted in another comment upthread, once you take into account that the theory of gravity we are using is actually General Relativity, not Newtonian gravity, there is no such single parameter anyway. Relativistic versions of MOND actually end up looking just like dark matter: they have to invent new fields, new forms of energy, for which we have no other evidence. There is no way to get the desired effect in GR by just tweaking a parameter in the law of gravity.

throwawaymaths 4y ago
> distribution You're right. I was wrong. It doesn't have millions of subparameters. It is a single parameter, a vector with countably infinite dimension.
- andrewflnr 4y ago
  > vector with countably infinite dimension. This is equally true for baryonic matter. You might counter that baryonic matter is easier to observe/constrain. I would point out that LCDM also has ways to observe and constrain DM, and the universe doesn't owe us perfect legibility. What is it you think you're proving?
  - naasking 4y ago
    What doesn't add up is that MOND can reproduce many of those observations using only the visible mass and a tweak to gravitation. This makes all of these tuned distributions seem very suspicious by comparison. It would be a remarkable coincidence if there wasn't something to MOND.
    - pdonis 4y ago
      > MOND can reproduce many of those observations using only the visible mass and a tweak to gravitation No, it can't. It can only reproduce a subset of galaxy rotation curve observations. It cannot reproduce other observations at all (such as the amount of structure in the universe as compared with the amount of baryonic matter that is consistent with Big Bang nucleosynthesis, which is about the same as the amount of matter we can see, but is much too small to account for the amount of structure--gravitational clumping into galaxies and galaxy clusters--that we see).
      - naasking 4y ago
        I suggest you review the following, it's not nearly as rosy for DM or dire for MOND as you imply: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm
        pdonis 4y ago
        > it's not nearly as rosy for DM or dire for MOND as you imply According to that one paper, perhaps. There are lots of papers published in this area of research. If you want to say the jury is still out on a final resolution, yes, I'll agree with that.
    - andrewflnr 4y ago
      My guess would be something about the dynamics of dark matter in galaxy evolution pushes the distribution into a shape that produces those mathematically simple rotation curves. That seems Occam's-razor compliant to me; it would be weird if rotation curves with dark matter were really complicated and unpredictable. It's also possible that both MOND and LCDM have some truth to them. But usually people are trying to get rid of dark matter, and that's just not happening.