4 ms·

"MOND predictions keep being corroborated, yet the community persists in ignoring its implications, even in terms of dark matter. It’s gotta be telling us somet

by burnte 2y ago

"MOND predictions keep being corroborated, yet the community persists in ignoring its implications, even in terms of dark matter. It’s gotta be telling us something." No, it doesn't. MOND falls apart in every single theory they put forth. The fact that this happens without fail should lead one to understand the answer probably lies elsewhere than MOND.

kurthr 2y ago
It's not like MOND (or even dark matter) are even single theories with a fixed set of predictions. They are groups of competing ever evolving theories. I don't understand the need to believe one over the other as a predictive tool without some conclusive evidence. If you're not the astronomer who makes that measurement (or explains it), it just feels like galactic sports betting.
MattPalmer1086 2y ago
MOND successfully predicts many things we observe that DM fails to, or that requires special tuning for DM to fit observations (which is not prediction). DM actually has quite a poor predictive record, even if it can be made to fit observation. MOND as a theory is probably not correct. It's not relativistic for a start, although I believe there are other modified gravity theories that are.
throwawaymaths 2y ago
MOND explains (discovered before Mond, but the Mond equations unexpectedly fit): Tully fisher relation Renzos rule MOND predicted (these were observed after mond predicted it): External field effect Keplerian descent in milky way Early galaxies after the big bang Consistently "No dark matter found" in elliptical and lenticular galaxies
shiandow 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?
- 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
firebot 2y ago
The latest is, afaik, is called RelMOND, short for relativistic MOND. It describes the cosmos very similarly to the standard model, but with MOND as well. So slightly better matching observations than lambda CDM, but still imperfect. They plan to incorporate electromagnetism in future models.

by burnte 2y ago

kurthr 2y ago

It's not like MOND (or even dark matter) are even single theories with a fixed set of predictions. They are groups of competing ever evolving theories. I don't understand the need to believe one over the other as a predictive tool without some conclusive evidence. If you're not the astronomer who makes that measurement (or explains it), it just feels like galactic sports betting.

MattPalmer1086 2y ago

MOND successfully predicts many things we observe that DM fails to, or that requires special tuning for DM to fit observations (which is not prediction). DM actually has quite a poor predictive record, even if it can be made to fit observation. MOND as a theory is probably not correct. It's not relativistic for a start, although I believe there are other modified gravity theories that are.

throwawaymaths 2y ago

MOND explains (discovered before Mond, but the Mond equations unexpectedly fit): Tully fisher relation Renzos rule MOND predicted (these were observed after mond predicted it): External field effect Keplerian descent in milky way Early galaxies after the big bang Consistently "No dark matter found" in elliptical and lenticular galaxies

shiandow 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?

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

firebot 2y ago

The latest is, afaik, is called RelMOND, short for relativistic MOND. It describes the cosmos very similarly to the standard model, but with MOND as well. So slightly better matching observations than lambda CDM, but still imperfect. They plan to incorporate electromagnetism in future models.