3 ms·

Appreciate people "going back to the fundamentals" - without sarcasm, I lived for ages without even ever understanding where the idea of dark matter comes from.

Appreciate people "going back to the fundamentals" - without sarcasm, I lived for ages without even ever understanding where the idea of dark matter comes from. To be honest, I still don't understand how "cosmic microwave background fluctuations" is an argument for dark matter, but #1 and #2 at least help me intuitively understand where the theory comes from.

sigmoid10 3y ago
Cosmic microwave background fluctuations are actually some of the strongest, most precisely measured evidence we have today for DM. The CMB was measured with incredibly high resolution thanks to the Planck satellite [1], and when you look at the distribution of angular sizes of the density fluctuations, you get a very peculiar plot [2]. Through some rather complicated math, the position and height of the peaks in this plot directly correspond (among other things) to "normal" matter that interacts electromagnetically and "dark matter" which does not interact electromagnetically. Since at the time of the CMB the whole universe was still very hot and therefore ionized, the data tells us there was a large amount of matter that only interacted gravitationally, but not electromagnetically. Ex post facto, the amount of "dark" matter calculated this way directly corresponds to the amount you would need to also explain galaxy rotation curves and gravitational lensing observations in today's universe. [1] https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2013/03/planck_wmap_comparison/12584155-7-eng-GB/Planck_WMAP_comparison_pillars.jpg https://www.esa.int/var/esa/storage/images/esa_multimedia/im... [2] https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/figure5.jpg https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/fi...
- naasking 3y ago
  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
- exe34 3y ago
  > Ex post facto, the amount of "dark" matter calculated this way directly corresponds to the amount you would need to also explain galaxy rotation curves and gravitational lensing observations in today's universe. Could you comment on this please? Or suggest some further reading? I was under the impression that the amount of dark matter in each galaxy is adjusted to match the rotation curve.
  - mr_mitm 3y ago
    They were talking about the total relative amount. All evidence independently points to DM making up around 85% of all matter.

sigmoid10 3y ago

Cosmic microwave background fluctuations are actually some of the strongest, most precisely measured evidence we have today for DM. The CMB was measured with incredibly high resolution thanks to the Planck satellite [1], and when you look at the distribution of angular sizes of the density fluctuations, you get a very peculiar plot [2]. Through some rather complicated math, the position and height of the peaks in this plot directly correspond (among other things) to "normal" matter that interacts electromagnetically and "dark matter" which does not interact electromagnetically. Since at the time of the CMB the whole universe was still very hot and therefore ionized, the data tells us there was a large amount of matter that only interacted gravitationally, but not electromagnetically. Ex post facto, the amount of "dark" matter calculated this way directly corresponds to the amount you would need to also explain galaxy rotation curves and gravitational lensing observations in today's universe. [1] https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2013/03/planck_wmap_comparison/12584155-7-eng-GB/Planck_WMAP_comparison_pillars.jpg https://www.esa.int/var/esa/storage/images/esa_multimedia/im... [2] https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/figure5.jpg https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/fi...

naasking 3y ago
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
exe34 3y ago
> Ex post facto, the amount of "dark" matter calculated this way directly corresponds to the amount you would need to also explain galaxy rotation curves and gravitational lensing observations in today's universe. Could you comment on this please? Or suggest some further reading? I was under the impression that the amount of dark matter in each galaxy is adjusted to match the rotation curve.
- mr_mitm 3y ago
  They were talking about the total relative amount. All evidence independently points to DM making up around 85% of all matter.