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What often goes conveniently unmentioned is that DM failed to correctly predict the first and second peaks of the power spectrum (which MOND correctly predicted

What often goes conveniently unmentioned is that DM failed to correctly predict the first and second peaks of the power spectrum (which MOND correctly predicted) and only correctly predicted the third peak (which MOND did not correctly predict). Rotation curves are also not compelling evidence for LCDM because it's not predictive, eg. each galaxy needs to be fit post-hoc with a custom distribution of DM to match the rotation curve. Lensing in clusters is about the only evidence that there's some amount of DM, but the amount postulated by DM-only theories may be excessive, ie. under modified gravity, only an amount of dark matter about equivalent to normal matter is needed, not the 5x needed under LCDM. I'd say that's pretty far from a slam dunk for DM theories over other possibilities.

throw1234651234 3y ago
Appreciate both this and sigmoid's comment above. I don't really understand cosmic background radiation enough to understand though. I get it at an absolutely superficial level and understand that it's used as evidence of the big bang, but that's about it. Which is why I fall back to lensing as sort of the only argument for DM which I really understand.
sigmoid10 3y ago
>DM failed to correctly predict the first and second peaks of the power spectrum I don't know what you mean by this. The peaks are completely explainable by standard LCDM, while MOND without DM totally fails to do so - no matter how much you tune it (see black vs red lines in [1]). >pretty far from a slam dunk for DM theories Individually, maybe. But not when you combine everything together. Then MOND is completely hopeless. You will find no paper out there that can reach the same level of predictions as DM. Of course that doesn't mean that there is guaranteed to be no modification to gravity at all. We already know it must be modified in the UV, so it's not impossible that a UV-complete description will bleed down to modified, far IR behaviour. But even if that is the case (and that's a pretty big if right now with the current theory landscape), some form of DM will still be required. And the evidence from the CMB strongly suggests that it will not be a small amount, but pretty much what we expect from original rotation curves and lensing. That's why no serious researcher in the field bothers with modifications of gravity alone, while there are many who focus only on DM. [1] https://www.researchgate.net/profile/Bin-Wang-115/publication/269577584/figure/fig7/AS:667090436493329@1536058162712/The-CMB-temperature-angular-power-spectra-for-the-fiducial-LCDM-modelsolid-black-curve.png https://www.researchgate.net/profile/Bin-Wang-115/publicatio...
- naasking 3y ago
  > I don't know what you mean by this. Sure, after more curve fitting [1]. It's just more inconvenient history that's been forgotten to favour LCDM. > You will find no paper out there that can reach the same level of predictions as DM. That's just wrong. LCDM's curve fitting has become great due to all of the adjustments, it's a priori predictions have not been great: https://arxiv.org/abs/2110.06936 https://arxiv.org/abs/2110.06936 > some form of DM will still be required This was always the case as I already pointed out in my previous reply, going back to at least the 90s [2]. The point is that cold dark matter has seemingly been adding epicycles in order to reduce all explanations to its one underlying cause. [1] see section 5.3, https://arxiv.org/abs/1404.7525 https://arxiv.org/abs/1404.7525 [2] http://astroweb.case.edu/ssm/mond/moti_bullet.html http://astroweb.case.edu/ssm/mond/moti_bullet.html
  - sigmoid10 3y ago
    >after more curve fitting What do you mean "more"? There are no new additional parameters and the curve literally fits, unlike for MOND (which doesn't fit at all unless you also introduce some form of DM). >This was always the case Then why would you trust any paper like this that explicitly doesn't use DM? Even if by mere chance it got a correct fit to one of the observations, you'll know immediately it could only have been a fluke since it didn't account for any DM.
    - naasking 3y ago
      > What do you mean "more"? I literally pointed you to the section that discusses how the a posteriori fit of LCDM to the spectrum was done. > you'll know immediately it could only have been a fluke since it didn't account for any DM. Right, so DM is necessary because if you used any explanation other than DM it must be wrong. Sounds perfectly scientific to me.
      - sigmoid10 3y ago
        LCDM existed before the Planck (or WMAP) data became available. As did MOND (including TeVeS). There is no argument here regarding "a posteriori" either way. The only relevant fact is that pure LCDM can explain the curve, while pure MOND can't. >DM is necessary because if you used any explanation other than DM it must be wrong You completely missed the point. Although at this point I fathom it must be intentional. DM is necessary. End of story. You even said it yourself. By your own statement, any explanation (such as the one linked by OP) that uses no DM at all must be wrong, yes. Of course it is much more complicated to come up with a sound theory of both, especially if you also want to retain some predictability. That's why we regularly see lone researches publish stuff like this. But it is neither the consensus nor really relevant to the field as a whole.

throw1234651234 3y ago

Appreciate both this and sigmoid's comment above. I don't really understand cosmic background radiation enough to understand though. I get it at an absolutely superficial level and understand that it's used as evidence of the big bang, but that's about it. Which is why I fall back to lensing as sort of the only argument for DM which I really understand.

sigmoid10 3y ago

>DM failed to correctly predict the first and second peaks of the power spectrum I don't know what you mean by this. The peaks are completely explainable by standard LCDM, while MOND without DM totally fails to do so - no matter how much you tune it (see black vs red lines in [1]). >pretty far from a slam dunk for DM theories Individually, maybe. But not when you combine everything together. Then MOND is completely hopeless. You will find no paper out there that can reach the same level of predictions as DM. Of course that doesn't mean that there is guaranteed to be no modification to gravity at all. We already know it must be modified in the UV, so it's not impossible that a UV-complete description will bleed down to modified, far IR behaviour. But even if that is the case (and that's a pretty big if right now with the current theory landscape), some form of DM will still be required. And the evidence from the CMB strongly suggests that it will not be a small amount, but pretty much what we expect from original rotation curves and lensing. That's why no serious researcher in the field bothers with modifications of gravity alone, while there are many who focus only on DM. [1] https://www.researchgate.net/profile/Bin-Wang-115/publication/269577584/figure/fig7/AS:667090436493329@1536058162712/The-CMB-temperature-angular-power-spectra-for-the-fiducial-LCDM-modelsolid-black-curve.png https://www.researchgate.net/profile/Bin-Wang-115/publicatio...

naasking 3y ago
> I don't know what you mean by this. Sure, after more curve fitting [1]. It's just more inconvenient history that's been forgotten to favour LCDM. > You will find no paper out there that can reach the same level of predictions as DM. That's just wrong. LCDM's curve fitting has become great due to all of the adjustments, it's a priori predictions have not been great: https://arxiv.org/abs/2110.06936 https://arxiv.org/abs/2110.06936 > some form of DM will still be required This was always the case as I already pointed out in my previous reply, going back to at least the 90s [2]. The point is that cold dark matter has seemingly been adding epicycles in order to reduce all explanations to its one underlying cause. [1] see section 5.3, https://arxiv.org/abs/1404.7525 https://arxiv.org/abs/1404.7525 [2] http://astroweb.case.edu/ssm/mond/moti_bullet.html http://astroweb.case.edu/ssm/mond/moti_bullet.html
- sigmoid10 3y ago
  >after more curve fitting What do you mean "more"? There are no new additional parameters and the curve literally fits, unlike for MOND (which doesn't fit at all unless you also introduce some form of DM). >This was always the case Then why would you trust any paper like this that explicitly doesn't use DM? Even if by mere chance it got a correct fit to one of the observations, you'll know immediately it could only have been a fluke since it didn't account for any DM.
  - naasking 3y ago
    > What do you mean "more"? I literally pointed you to the section that discusses how the a posteriori fit of LCDM to the spectrum was done. > you'll know immediately it could only have been a fluke since it didn't account for any DM. Right, so DM is necessary because if you used any explanation other than DM it must be wrong. Sounds perfectly scientific to me.
    - sigmoid10 3y ago
      LCDM existed before the Planck (or WMAP) data became available. As did MOND (including TeVeS). There is no argument here regarding "a posteriori" either way. The only relevant fact is that pure LCDM can explain the curve, while pure MOND can't. >DM is necessary because if you used any explanation other than DM it must be wrong You completely missed the point. Although at this point I fathom it must be intentional. DM is necessary. End of story. You even said it yourself. By your own statement, any explanation (such as the one linked by OP) that uses no DM at all must be wrong, yes. Of course it is much more complicated to come up with a sound theory of both, especially if you also want to retain some predictability. That's why we regularly see lone researches publish stuff like this. But it is neither the consensus nor really relevant to the field as a whole.