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> Hopefully no one is saying that we cannot possibly ever The OP article might as well. The thrust of the article's argument is, "heaven forbid we add third p

> Hopefully no one is saying that we cannot possibly ever The OP article might as well. The thrust of the article's argument is, "heaven forbid we add third parameter to MOND" meanwhile, draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there.

pdonis 4y ago
> draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there. No, it isn't. The distribution of stress-energy is a single free parameter in GR; there is no law in the theory that says the density of any particular kind of stress-energy has to be the same in different galaxies. So adjusting the distribution of stress-energy to match the observed behavior in different galaxies does not require adding a new parameter for each galaxy; it only requires adjusting the single parameter that already exists, in different regions of spacetime.
- deleted 4y ago
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- throwawaymaths 4y ago
  > 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
- AnimalMuppet 4y ago
  The distribution of stress-energy is a single parameter per point in space. That is, I don't think saying "it's only a single parameter" and "it's different in different galaxies" are compatible.
  - pdonis 4y ago
    If you want to say it's a "parameter distribution" or something like that, that's fine. It doesn't change the fact that this thingie, whatever you want to call it, is already present in our existing theory of gravity, GR, and is already adjustable in that theory. So we're not adding anything new to the theory if we adjust this thingie to take into account observed motions for which we can't see the gravity source that causes the motion.
    - throwawaymaths 4y ago
      Do you understand the concept of overfitting?
      - pdonis 4y ago
        See the response by DiogenesKynikos. The "fitting" is not being done the way you appear to think it is.
        throwawaymaths 4y ago
        I don't think you understand the concept of explanatory power. Because dark matter cannot be observed we must manually tune the dark matter distribution for each galaxy. This creates new parameters that don't exist in the theory that are necessary to properly explain phenomena. Whether or not they exist in the theory is irrelevant to the fact that they are necessary.
  - deleted 4y ago
    [deleted]
  - DiogenesKynikos 4y ago

pdonis 4y ago

> draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there. No, it isn't. The distribution of stress-energy is a single free parameter in GR; there is no law in the theory that says the density of any particular kind of stress-energy has to be the same in different galaxies. So adjusting the distribution of stress-energy to match the observed behavior in different galaxies does not require adding a new parameter for each galaxy; it only requires adjusting the single parameter that already exists, in different regions of spacetime.

deleted 4y ago
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throwawaymaths 4y ago
> 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
AnimalMuppet 4y ago
The distribution of stress-energy is a single parameter per point in space. That is, I don't think saying "it's only a single parameter" and "it's different in different galaxies" are compatible.
- pdonis 4y ago
  If you want to say it's a "parameter distribution" or something like that, that's fine. It doesn't change the fact that this thingie, whatever you want to call it, is already present in our existing theory of gravity, GR, and is already adjustable in that theory. So we're not adding anything new to the theory if we adjust this thingie to take into account observed motions for which we can't see the gravity source that causes the motion.
  - throwawaymaths 4y ago
    Do you understand the concept of overfitting?
    - pdonis 4y ago
      See the response by DiogenesKynikos. The "fitting" is not being done the way you appear to think it is.
      - throwawaymaths 4y ago
        I don't think you understand the concept of explanatory power. Because dark matter cannot be observed we must manually tune the dark matter distribution for each galaxy. This creates new parameters that don't exist in the theory that are necessary to properly explain phenomena. Whether or not they exist in the theory is irrelevant to the fact that they are necessary.
- deleted 4y ago
  [deleted]
- DiogenesKynikos 4y ago