4 ms·

> MOND has more points in favour for it as a preferred theory than DM models. Isn't MOND completely falsified by the discovery of galaxies without dark matter?

sigmoid10 4y ago
It was already falsified by well established limits on self interaction of DM from galaxy collisions. And from the already mentioned temperature fluctuations in the CMB. Every now and then some MOND supporter comes out if they found a way to tune their model to one specific observation, but particle DM remains the only approach that can fit all these observations at the same time. Ofc that doesn't guarantee that particle DM is the only explanation. After all, it could be a combination of DM+MOND. But MOND alone is just not possible, whereas DM alone might be.
- naasking 4y ago
  By that argument, DM was already falsified by the observations pointed out in this article and the paper I linked. Your claim that DM could "fit all of these observations at the same time" is just wrong. DM has been tuned to correct its parameter-free predictions, so if you're going to compare like against like, then MOND should be permitted to be tuned to fit as well, and we can judge in the end which theory requires the least fine-tuning. As for galaxies without DM, those observations are still being debated, but even if true, that doesn't entail MOND can't explain those results.
  - eterevsky 4y ago
    I don't think it's fair to say that it's "tuned" to fit observation. It's not like several different types of DM are introduced to fit the observation. Rather the observations add some constraints to the DM properties. That's how it's supposed to be.
    - naasking 4y ago
      If DM has to have a very specific initial distribution to reproduce what we see, for example, that too is a constraint but it's also fine-tuning. Another theory that is free of such parameters should be preferred. Some types of constraints really should decrease a theory's plausibility.
      - sigmoid10 4y ago
        MOND needs some pretty crazy arguing to explain decoupling of gravity from baryonic matter distributions. And it also needs a varying length scale parameter to account for different galaxy types. Particle DM can explain all of these things at the same time without re-tuning the model every time.
        naasking 4y ago
        > MOND needs some pretty crazy arguing to explain decoupling of gravity from baryonic matter distributions. And it also needs a varying length scale parameter to account for different galaxy types. You mean the theory that has received 2-3 orders of magnitude less attention than DM has some conceptual holes to patch up? Colour me shocked. The fact that DM still fails despite all of that investment should worry you more. > Particle DM can explain all of these things at the same time without re-tuning the model every time. DM has already been tweaked and fine-tuned to patch up its own endless list of holes, many of which were recently reviewed by McGaugh [1]. Many more remain despite that, like the dynamical friction discussed in this article. [1] https://tritonstation.com/2022/04/08/two-hypotheses/ https://tritonstation.com/2022/04/08/two-hypotheses/
        sigmoid10 4y ago
        Here's an explanation why McGaugh's work is highly dubious: https://www.preposterousuniverse.com/blog/2011/02/26/dark-matter-just-fine-thanks/ https://www.preposterousuniverse.com/blog/2011/02/26/dark-ma... It also talks about the general problems with alternatives to particle DM.
  - sigmoid10 4y ago
    The best thing this article can provide is that some galaxies work without DM. Only particle DM can explain the early universe observations from the CMB (which this article completely glosses over), decoupling from baryonic matter (which also isn't even mentioned) and galaxy rotation curves at the same time. The arguments in favour of DM are far more general and rightly accepted for what they are, whereas the sporadic argument for MOND that works in one domain is rightly shunned as long as its proponents blissfully keep ignoring the overwhelming incompatibilities in the remaining domains.
    - naasking 4y ago
      > Only particle DM can explain the early universe observations from the CMB Are you actually arguing that DM is the only conceivable theory that can possibly explain those observations? Be serious. This obsession with DM when it's been clearly falsified numerous times is restricting imagination and attention on novel approaches.
      - sigmoid10 4y ago
        I explicitly said it might very well not be the only one. But the data clearly shows that MOND is definitely not the only one. Since Occam's Razor tells us to go with the simplest solution, science goes for DM, instead of DM+MOND. If we ever discover DM (eg in the form of WIMPs) and it can't account for all the extra mass in the universe, then people will be able to look closer at modified gravity again. But right now these models are just not at a point where they could explain more than noise. If you're interested in how difficult it actually is to come up with a robust alternative theory of gravity that can fully explain all aspects of DM we see in the universe, I suggest this lecture here: https://www.youtube.com/watch?v=WECVq2YBduY https://www.youtube.com/watch?v=WECVq2YBduY tl;dr: it's hard. Very hard. Noone has done it yet.
        naasking 4y ago
        > I explicitly said it might very well not be the only one. Actually you didn't. Perhaps you claimed that in another thread, but I quoted exactly what you wrote in this thread, which is that only DM can explain those results. > Since Occam's Razor tells us to go with the simplest solution, science goes for DM, instead of DM+MOND. That's not a correct formulation of Occam's razor. Occam's razor tells us not to posit the existence of more things than are necessary to explain observations. Observations already tell us that DM either does not exist, or doesn't exist in the amounts most people think exists, ie. most DM models have been refuted. Therefore, even if DM exists, something else is still needed to explain our observations.

sigmoid10 4y ago

It was already falsified by well established limits on self interaction of DM from galaxy collisions. And from the already mentioned temperature fluctuations in the CMB. Every now and then some MOND supporter comes out if they found a way to tune their model to one specific observation, but particle DM remains the only approach that can fit all these observations at the same time. Ofc that doesn't guarantee that particle DM is the only explanation. After all, it could be a combination of DM+MOND. But MOND alone is just not possible, whereas DM alone might be.

naasking 4y ago
By that argument, DM was already falsified by the observations pointed out in this article and the paper I linked. Your claim that DM could "fit all of these observations at the same time" is just wrong. DM has been tuned to correct its parameter-free predictions, so if you're going to compare like against like, then MOND should be permitted to be tuned to fit as well, and we can judge in the end which theory requires the least fine-tuning. As for galaxies without DM, those observations are still being debated, but even if true, that doesn't entail MOND can't explain those results.
- eterevsky 4y ago
  I don't think it's fair to say that it's "tuned" to fit observation. It's not like several different types of DM are introduced to fit the observation. Rather the observations add some constraints to the DM properties. That's how it's supposed to be.
  - naasking 4y ago
    If DM has to have a very specific initial distribution to reproduce what we see, for example, that too is a constraint but it's also fine-tuning. Another theory that is free of such parameters should be preferred. Some types of constraints really should decrease a theory's plausibility.
    - sigmoid10 4y ago
      MOND needs some pretty crazy arguing to explain decoupling of gravity from baryonic matter distributions. And it also needs a varying length scale parameter to account for different galaxy types. Particle DM can explain all of these things at the same time without re-tuning the model every time.
      - naasking 4y ago
        > MOND needs some pretty crazy arguing to explain decoupling of gravity from baryonic matter distributions. And it also needs a varying length scale parameter to account for different galaxy types. You mean the theory that has received 2-3 orders of magnitude less attention than DM has some conceptual holes to patch up? Colour me shocked. The fact that DM still fails despite all of that investment should worry you more. > Particle DM can explain all of these things at the same time without re-tuning the model every time. DM has already been tweaked and fine-tuned to patch up its own endless list of holes, many of which were recently reviewed by McGaugh [1]. Many more remain despite that, like the dynamical friction discussed in this article. [1] https://tritonstation.com/2022/04/08/two-hypotheses/ https://tritonstation.com/2022/04/08/two-hypotheses/
        sigmoid10 4y ago
        Here's an explanation why McGaugh's work is highly dubious: https://www.preposterousuniverse.com/blog/2011/02/26/dark-matter-just-fine-thanks/ https://www.preposterousuniverse.com/blog/2011/02/26/dark-ma... It also talks about the general problems with alternatives to particle DM.
- sigmoid10 4y ago
  The best thing this article can provide is that some galaxies work without DM. Only particle DM can explain the early universe observations from the CMB (which this article completely glosses over), decoupling from baryonic matter (which also isn't even mentioned) and galaxy rotation curves at the same time. The arguments in favour of DM are far more general and rightly accepted for what they are, whereas the sporadic argument for MOND that works in one domain is rightly shunned as long as its proponents blissfully keep ignoring the overwhelming incompatibilities in the remaining domains.
  - naasking 4y ago
    > Only particle DM can explain the early universe observations from the CMB Are you actually arguing that DM is the only conceivable theory that can possibly explain those observations? Be serious. This obsession with DM when it's been clearly falsified numerous times is restricting imagination and attention on novel approaches.
    - sigmoid10 4y ago
      I explicitly said it might very well not be the only one. But the data clearly shows that MOND is definitely not the only one. Since Occam's Razor tells us to go with the simplest solution, science goes for DM, instead of DM+MOND. If we ever discover DM (eg in the form of WIMPs) and it can't account for all the extra mass in the universe, then people will be able to look closer at modified gravity again. But right now these models are just not at a point where they could explain more than noise. If you're interested in how difficult it actually is to come up with a robust alternative theory of gravity that can fully explain all aspects of DM we see in the universe, I suggest this lecture here: https://www.youtube.com/watch?v=WECVq2YBduY https://www.youtube.com/watch?v=WECVq2YBduY tl;dr: it's hard. Very hard. Noone has done it yet.
      - naasking 4y ago
        > I explicitly said it might very well not be the only one. Actually you didn't. Perhaps you claimed that in another thread, but I quoted exactly what you wrote in this thread, which is that only DM can explain those results. > Since Occam's Razor tells us to go with the simplest solution, science goes for DM, instead of DM+MOND. That's not a correct formulation of Occam's razor. Occam's razor tells us not to posit the existence of more things than are necessary to explain observations. Observations already tell us that DM either does not exist, or doesn't exist in the amounts most people think exists, ie. most DM models have been refuted. Therefore, even if DM exists, something else is still needed to explain our observations.