3 ms·

I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty st

I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree). Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics?

Filligree 2y ago
The Bullet galaxy.
XorNot 2y ago
For one thing MOND doesn't actually appear to eliminate the need for dark matter[1]. It gets rid of ~80% of the missing mass requirement, but not all of it. Whereas pure DM can just eliminate MOND. 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, but it's pretty trivial for DM: electromagnetically interacting matter is "sticky" where as gravity only matter isn't. The Bullet Cluster shows a galaxy shaped blob of gravitational lensing exactly where you'd expect it to be if a bunch of non-interacting matter had flown through each other, whereas the electromagnetic matter has interacted and re-shaped. NGC 1052-DF2[3] and NGC 1052-DF4 are both ultra-diffuse galaxies which have no, or very little dark matter. That is, they appear to have normal galactic rotation curves fully explained by their observed visible mass. This works totally fine for DM existing (it's a problem for lambda-CDM though because it's not clear how they could've formed without dark matter, but I mean - we also don't yet know how black holes actually manage to ever merge either yet we do observe them too). This one always seems like a problem to me: MOND proposes a new universal principle of matter, then suddenly we have some matter where it's not doing that. The theoretical problems[4] are somewhat beyond me, but they get well into issues with violating relativity and that's a big one: relativity is stupidly, reliably accurate under every single test we put it through, to absurd levels of precision. Build a better instrument, you can just dial in your precision and get the answer out ahead of time before you launch the satellite which is testing it. Also without careful adjustment you get violations of conservation of momentum (conversely, if MOND is real this would be handy because maybe it means we can reactionless spacedrives). It's worth noting that none of this is implicitly fatal. lambda-CDM could be wrong, a MOND variant could be right. But a list of convenient things MOND explains easily doesn't escape the need to also include the things it can't - and appeals to the idea that DM is being "tweaked" to match observations unnaturally ignores the fact that MOND has to have the same thing done to it to fix within cosmology. [1] https://arxiv.org/abs/1404.7525 https://arxiv.org/abs/1404.7525 [2] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster [3] https://en.wikipedia.org/wiki/NGC_1052-DF2 https://en.wikipedia.org/wiki/NGC_1052-DF2 [4] https://arxiv.org/pdf/astro-ph/0104435 https://arxiv.org/pdf/astro-ph/0104435
- naasking 2y ago
  > 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.
burnte 2y ago
> I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree). > Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics? Pick any MOND theory you like, it all fails when it hits relativity, which is a theory with an embarrassment of riches of evidence in its favor, and can't be reconciled with relativity either. So while it explains galactic rotational speeds, it then fails to explain lots of other things, so it's a huge step backwards. It's the equivalent of saying Newtonian physics is wrong because it can't explain Mercury's precession, so let's go back to epicycles. To favor MOND we give up tremendously more than we gain. MOND CAN explain things, if I have been interpreted to say it never predicts anything, that was never my intent. The problem is that MOND can't explain much else, so rather than making our theories simpler (which usually means we're in the right direction) it complicates things. To make relativity work for the things MOND looks at, we only add 1 thing, WIMPs, particles we theorize but haven't seen. We've predicted lots of particle and found them, so this isn't a problem. chances we have missed a particle that turns out to be highly non-interactive? High. It took us ages to really solve the missing neutrino problem by discovering them, and we predicted the Higgs boson with high accuracy too. So this is a road we've been down before. To make MOND work, we throw out a lot of theory, and we have nothing to replace it with under MOND. Chances all those other theories are wrong even though they work great? Low. Dr Becky is amazing with her MOND analyses. Here's one of the latest: https://www.youtube.com/watch?v=HlNSvrYygRc https://www.youtube.com/watch?v=HlNSvrYygRc
- shiandow 2y ago
  Do tell me if I'm completely wrong but isn't part of the problem there that you're simply expecting too much from MOND? I mean a theory that is a modification of classical dynamics is not going to work well in a relativistic regime. That just means that somehow the classical limit ends up being slightly different from what we thought it was for reasons we don't know yet. I've always viewed MOND as more of an empirical law that we don't have a good explanation for yet. To me at least it seems obvious it will be deficient when you extrapolate too far and it can't replace more fundamental models. That MOND can't handle relativity is not a direct reason to dismiss it. It would be a problem if relativity can't handle MOND (i.e. relativity cannot result in a MOND like theory) but I can't tell if that is the case. We could also dismiss MOND if it didn't explain anything, but as you said that's not quite the case either. So yeah, dark matter seems more reasonable, but it's hard to completely ignore a theory seems to have more predictive power (even if we know it is flawed).

Filligree 2y ago

The Bullet galaxy.

XorNot 2y ago

For one thing MOND doesn't actually appear to eliminate the need for dark matter[1]. It gets rid of ~80% of the missing mass requirement, but not all of it. Whereas pure DM can just eliminate MOND. 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, but it's pretty trivial for DM: electromagnetically interacting matter is "sticky" where as gravity only matter isn't. The Bullet Cluster shows a galaxy shaped blob of gravitational lensing exactly where you'd expect it to be if a bunch of non-interacting matter had flown through each other, whereas the electromagnetic matter has interacted and re-shaped. NGC 1052-DF2[3] and NGC 1052-DF4 are both ultra-diffuse galaxies which have no, or very little dark matter. That is, they appear to have normal galactic rotation curves fully explained by their observed visible mass. This works totally fine for DM existing (it's a problem for lambda-CDM though because it's not clear how they could've formed without dark matter, but I mean - we also don't yet know how black holes actually manage to ever merge either yet we do observe them too). This one always seems like a problem to me: MOND proposes a new universal principle of matter, then suddenly we have some matter where it's not doing that. The theoretical problems[4] are somewhat beyond me, but they get well into issues with violating relativity and that's a big one: relativity is stupidly, reliably accurate under every single test we put it through, to absurd levels of precision. Build a better instrument, you can just dial in your precision and get the answer out ahead of time before you launch the satellite which is testing it. Also without careful adjustment you get violations of conservation of momentum (conversely, if MOND is real this would be handy because maybe it means we can reactionless spacedrives). It's worth noting that none of this is implicitly fatal. lambda-CDM could be wrong, a MOND variant could be right. But a list of convenient things MOND explains easily doesn't escape the need to also include the things it can't - and appeals to the idea that DM is being "tweaked" to match observations unnaturally ignores the fact that MOND has to have the same thing done to it to fix within cosmology. [1] https://arxiv.org/abs/1404.7525 https://arxiv.org/abs/1404.7525 [2] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster [3] https://en.wikipedia.org/wiki/NGC_1052-DF2 https://en.wikipedia.org/wiki/NGC_1052-DF2 [4] https://arxiv.org/pdf/astro-ph/0104435 https://arxiv.org/pdf/astro-ph/0104435

naasking 2y ago
> 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.

burnte 2y ago

> I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree). > Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics? Pick any MOND theory you like, it all fails when it hits relativity, which is a theory with an embarrassment of riches of evidence in its favor, and can't be reconciled with relativity either. So while it explains galactic rotational speeds, it then fails to explain lots of other things, so it's a huge step backwards. It's the equivalent of saying Newtonian physics is wrong because it can't explain Mercury's precession, so let's go back to epicycles. To favor MOND we give up tremendously more than we gain. MOND CAN explain things, if I have been interpreted to say it never predicts anything, that was never my intent. The problem is that MOND can't explain much else, so rather than making our theories simpler (which usually means we're in the right direction) it complicates things. To make relativity work for the things MOND looks at, we only add 1 thing, WIMPs, particles we theorize but haven't seen. We've predicted lots of particle and found them, so this isn't a problem. chances we have missed a particle that turns out to be highly non-interactive? High. It took us ages to really solve the missing neutrino problem by discovering them, and we predicted the Higgs boson with high accuracy too. So this is a road we've been down before. To make MOND work, we throw out a lot of theory, and we have nothing to replace it with under MOND. Chances all those other theories are wrong even though they work great? Low. Dr Becky is amazing with her MOND analyses. Here's one of the latest: https://www.youtube.com/watch?v=HlNSvrYygRc https://www.youtube.com/watch?v=HlNSvrYygRc

shiandow 2y ago
Do tell me if I'm completely wrong but isn't part of the problem there that you're simply expecting too much from MOND? I mean a theory that is a modification of classical dynamics is not going to work well in a relativistic regime. That just means that somehow the classical limit ends up being slightly different from what we thought it was for reasons we don't know yet. I've always viewed MOND as more of an empirical law that we don't have a good explanation for yet. To me at least it seems obvious it will be deficient when you extrapolate too far and it can't replace more fundamental models. That MOND can't handle relativity is not a direct reason to dismiss it. It would be a problem if relativity can't handle MOND (i.e. relativity cannot result in a MOND like theory) but I can't tell if that is the case. We could also dismiss MOND if it didn't explain anything, but as you said that's not quite the case either. So yeah, dark matter seems more reasonable, but it's hard to completely ignore a theory seems to have more predictive power (even if we know it is flawed).