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> Then you've got the "Bullet Cluster"[2] - where two colliding galaxies have had their observable and dark matter masses apparently separated. MOND can't expla

> Then you've got the "Bullet Cluster"[2] - where two colliding galaxies have had their observable and dark matter masses apparently separated. MOND can't explain this one without a lot of tweaking The bullet cluster is so over-played as a refutation of MOND. "A lot of tweaking" basically reduces to adding sterile neutrinos, as one possible solution. All galactic clusters have issues in both MOND and LCDM, the bullet cluster was nothing new when it was discovered, it was just visually dramatic because they could image the gravitational lensing. > but it's pretty trivial for DM: electromagnetically interacting matter is "sticky" where as gravity only matter isn't. Actually LCDM can't explain the bullet cluster either: https://tritonstation.com/2024/02/06/clusters-of-galaxies-ruin-everything/ https://tritonstation.com/2024/02/06/clusters-of-galaxies-ru... As always, MOND and LCDM appear to just trade off one set of issues for other equally problematic set of issues. Neither is favoured very strongly by the sum of evidence. Physicists have just gotten in the habit of ignoring all of the problems with LCDM and consider even trivial problems with MOND to be fatal. > relativity is stupidly, reliably accurate under every single test we put it through, to absurd levels of precision Those precise tests do not extend to galactic scales, which is exactly where the problems appear. It would be nice if our existing theory worked across all scales, but that doesn't mean it must.

isthatafact 2y ago
> "The bullet cluster is so over-played as a refutation of MOND. "A lot of tweaking" basically reduces to adding sterile neutrinos, as one possible solution." If MOND requires "dark matter" to explain the bullet cluster, then what is the appeal of MOND?
- naasking 2y ago
  Read the article this thread is about. Particle DM is not sufficient to explain all observations without extreme contortions that make MOND seem more reasonable. Also, MOND predictions made decades ago keep being validated. Why does this keep happening if there's nothing to MOND? Finally, as I said, particle DM can't fully explain the Bullet Cluster either. The evidence is screaming in our faces that we need better thinking here.
  - isthatafact 2y ago
    > Read the article this thread is about. It is one brand new paper. I am skeptical. Rotation curves are perfectly flat out to whatever arbitrary distance that they happen to be able to measure? I am very skeptical. > "particle DM can't fully explain the Bullet Cluster either." According to one scientist, who happens to be the same scientist claiming that particle DM cannot explain rotation curves. I will not check every claim, but the bullet cluster collision speed "problem" is readily explained in the reference in the wikipedia article: https://arxiv.org/abs/1410.7438 https://arxiv.org/abs/1410.7438 The broader point is that every time MOND has claimed to refute dark matter so far, the refutation has been refuted, so I will wait to see the outcome of this new claim.
    - naasking 2y ago
      > The broader point is that every time MOND has claimed to refute dark matter so far, the refutation has been refuted, By adding more epicycles to LCDM. I'll leave it to you to decide when to call shenanigans on that, but it's been going on for 30+ years now.
      - isthatafact 2y ago
        I am no expert, but I do not follow. Nothing was added to the LCDM model there, so using the epicycle fallacy does not help the discussion. The research was simply done again with better accuracy.
        naasking 2y ago
        I wasn't referring to the bullet cluster specifically, but this obsession with the bullet cluster is typical of the confirmation bias in this field: hyperfocus on what confirms bias and ignore the countervailing evidence. The past 30+ years have seen many "corrections" to get LCDM to fit observations it did not predict [1]. Clusters in general pose challenges to both MOND and LCDM for different reasons [2,3], but LCDM's typically get ignored and MOND's treated as a fatal blow. As I said, neither theory is fully satisfactory, but it's clear that research on these questions is fairly one-sided. [1] https://www.mdpi.com/2073-8994/14/7/1331 https://www.mdpi.com/2073-8994/14/7/1331 [2] https://iopscience.iop.org/article/10.3847/1538-4357/ace62a https://iopscience.iop.org/article/10.3847/1538-4357/ace62a [3] https://www.sciencedirect.com/science/article/pii/S1387647322000185 https://www.sciencedirect.com/science/article/pii/S138764732...

isthatafact 2y ago

> "The bullet cluster is so over-played as a refutation of MOND. "A lot of tweaking" basically reduces to adding sterile neutrinos, as one possible solution." If MOND requires "dark matter" to explain the bullet cluster, then what is the appeal of MOND?

naasking 2y ago
Read the article this thread is about. Particle DM is not sufficient to explain all observations without extreme contortions that make MOND seem more reasonable. Also, MOND predictions made decades ago keep being validated. Why does this keep happening if there's nothing to MOND? Finally, as I said, particle DM can't fully explain the Bullet Cluster either. The evidence is screaming in our faces that we need better thinking here.
- isthatafact 2y ago
  > Read the article this thread is about. It is one brand new paper. I am skeptical. Rotation curves are perfectly flat out to whatever arbitrary distance that they happen to be able to measure? I am very skeptical. > "particle DM can't fully explain the Bullet Cluster either." According to one scientist, who happens to be the same scientist claiming that particle DM cannot explain rotation curves. I will not check every claim, but the bullet cluster collision speed "problem" is readily explained in the reference in the wikipedia article: https://arxiv.org/abs/1410.7438 https://arxiv.org/abs/1410.7438 The broader point is that every time MOND has claimed to refute dark matter so far, the refutation has been refuted, so I will wait to see the outcome of this new claim.
  - naasking 2y ago
    > The broader point is that every time MOND has claimed to refute dark matter so far, the refutation has been refuted, By adding more epicycles to LCDM. I'll leave it to you to decide when to call shenanigans on that, but it's been going on for 30+ years now.
    - isthatafact 2y ago
      I am no expert, but I do not follow. Nothing was added to the LCDM model there, so using the epicycle fallacy does not help the discussion. The research was simply done again with better accuracy.
      - naasking 2y ago
        I wasn't referring to the bullet cluster specifically, but this obsession with the bullet cluster is typical of the confirmation bias in this field: hyperfocus on what confirms bias and ignore the countervailing evidence. The past 30+ years have seen many "corrections" to get LCDM to fit observations it did not predict [1]. Clusters in general pose challenges to both MOND and LCDM for different reasons [2,3], but LCDM's typically get ignored and MOND's treated as a fatal blow. As I said, neither theory is fully satisfactory, but it's clear that research on these questions is fairly one-sided. [1] https://www.mdpi.com/2073-8994/14/7/1331 https://www.mdpi.com/2073-8994/14/7/1331 [2] https://iopscience.iop.org/article/10.3847/1538-4357/ace62a https://iopscience.iop.org/article/10.3847/1538-4357/ace62a [3] https://www.sciencedirect.com/science/article/pii/S1387647322000185 https://www.sciencedirect.com/science/article/pii/S138764732...