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My very casual understanding is that Mond really has been mostly discarded. The article says it's 'in trouble'. We see some galaxies whose gravity is near the

by Gunax 2y ago

My very casual understanding is that Mond really has been mostly discarded. The article says it's 'in trouble'. We see some galaxies whose gravity is near the baryonic prediction and others where it is not. - explaining this with dark matter is easy: some galaxies have a lot of dark matter and some have little. - explaining this with Mond seems impossible. If there is some gravity law we don't understand, how can it have different effects on different galaxies? - how do we explain the famous bullet cluster finding?

User23 2y ago
> - explaining this with dark matter is easy Well, attributing it to dark matter is easy. It's not really explaining anything though, until you can say what dark matter is and why it's there. At this point it's basically just an abstract parameter that we assume, without any direct evidence, is in fact some kind of matter and not, well, something else. Maybe spacetime just have different geometry in different places? Maybe the assumption that it would be flat in the absence of matter is wrong? It could very well be that the neutral shape of spacetime in some places is more or less curved than we currently assume. Then there are fun ideas like large masses permanently deforming spacetime. Going with the famous rubber mat analogy, what if the mat gets permanently stretched and even once the mass that stretched it is dispersed the sag remains? That's of course assuming there even is such a thing as spacetime. Which, btw, sounds like a reasonable assumption. However, just because the math works doesn't mean it's a physical reality, it merely points to it.
- ffhhj 2y ago
  Nice theory! That would help explain the Great Attractor. The Universe could actually be a deformed blob of spacetime and the Big Bang and its development just happens to be the result of this blob's movements. There are rogue blackholes, maybe they leave spacetime deformations behind them that could be observed?
  - User23 2y ago
    I wouldn’t expect to find any “empty” spacetime deformations, since by their very nature they would attract matter. I would however expect more gravity than the presently observable matter could explain. I wonder if that can explain some of the large scale filament type structures we see.
- wruza 2y ago
  Isn't all that the same idea? If you ignore nomenclature and interpretations, DM basically says there's "a thing", opposed to "a law". Like actual trees vs aerodynamics. "There's trees" doesn't explain anything either, but if there's no trees and spacetime just remembers something, that is a thing as well, not a law. Just dark. A thing is a memory of spacetime.
- XorNot 2y ago
  The analogies literally don't matter though. If it behaves "matter-like" enough, then it's matter. We generally regard this to mean entities which can have an independent momentum and position. Hence: dark matter - because the effect we observe seems to behave that way. MOND is distinct because it declares what we're seeing is not a bunch of entities with that character. It's worth noting we also talk about a lot of things like this even though strictly speaking they're not: i.e. electron-holes in semiconductors can be perfectly well modeled as their own form of matter - they can't just leave the semiconductor environment. But they do have momentum and position. Dark matter could literally be the same type of effect, but if you're a wrinkle in space-time then it doesn't matter unless that description has some sort of experimental reality, and we currently do not observe it to do so. It is not like physicists would not rush to test any experimental formulation of dark matter as such that was accessible with current or near-future instruments.
- hi-v-rocknroll 2y ago
  > It's not really explaining anything though, That's the problem. It lumps unexplained phenomena into a luminiferous aether 2.0. The most likely explanation is our model of gravity isn't yet good enough and so perhaps RelMOND is worth a look.
mkl 2y ago
I (not a physicist) had the impression that it was mostly discarded too. This article from 2022 seems to contradict that: https://theconversation.com/dark-matter-our-review-suggests-its-time-to-ditch-it-in-favour-of-a-new-theory-of-gravity-186344 https://theconversation.com/dark-matter-our-review-suggests-... It doesn't mention the Bullet Cluster though.

by Gunax 2y ago

User23 2y ago

> - explaining this with dark matter is easy Well, attributing it to dark matter is easy. It's not really explaining anything though, until you can say what dark matter is and why it's there. At this point it's basically just an abstract parameter that we assume, without any direct evidence, is in fact some kind of matter and not, well, something else. Maybe spacetime just have different geometry in different places? Maybe the assumption that it would be flat in the absence of matter is wrong? It could very well be that the neutral shape of spacetime in some places is more or less curved than we currently assume. Then there are fun ideas like large masses permanently deforming spacetime. Going with the famous rubber mat analogy, what if the mat gets permanently stretched and even once the mass that stretched it is dispersed the sag remains? That's of course assuming there even is such a thing as spacetime. Which, btw, sounds like a reasonable assumption. However, just because the math works doesn't mean it's a physical reality, it merely points to it.

ffhhj 2y ago
Nice theory! That would help explain the Great Attractor. The Universe could actually be a deformed blob of spacetime and the Big Bang and its development just happens to be the result of this blob's movements. There are rogue blackholes, maybe they leave spacetime deformations behind them that could be observed?
- User23 2y ago
  I wouldn’t expect to find any “empty” spacetime deformations, since by their very nature they would attract matter. I would however expect more gravity than the presently observable matter could explain. I wonder if that can explain some of the large scale filament type structures we see.
wruza 2y ago
Isn't all that the same idea? If you ignore nomenclature and interpretations, DM basically says there's "a thing", opposed to "a law". Like actual trees vs aerodynamics. "There's trees" doesn't explain anything either, but if there's no trees and spacetime just remembers something, that is a thing as well, not a law. Just dark. A thing is a memory of spacetime.
XorNot 2y ago
The analogies literally don't matter though. If it behaves "matter-like" enough, then it's matter. We generally regard this to mean entities which can have an independent momentum and position. Hence: dark matter - because the effect we observe seems to behave that way. MOND is distinct because it declares what we're seeing is not a bunch of entities with that character. It's worth noting we also talk about a lot of things like this even though strictly speaking they're not: i.e. electron-holes in semiconductors can be perfectly well modeled as their own form of matter - they can't just leave the semiconductor environment. But they do have momentum and position. Dark matter could literally be the same type of effect, but if you're a wrinkle in space-time then it doesn't matter unless that description has some sort of experimental reality, and we currently do not observe it to do so. It is not like physicists would not rush to test any experimental formulation of dark matter as such that was accessible with current or near-future instruments.
hi-v-rocknroll 2y ago
> It's not really explaining anything though, That's the problem. It lumps unexplained phenomena into a luminiferous aether 2.0. The most likely explanation is our model of gravity isn't yet good enough and so perhaps RelMOND is worth a look.

mkl 2y ago

I (not a physicist) had the impression that it was mostly discarded too. This article from 2022 seems to contradict that: https://theconversation.com/dark-matter-our-review-suggests-its-time-to-ditch-it-in-favour-of-a-new-theory-of-gravity-186344 https://theconversation.com/dark-matter-our-review-suggests-... It doesn't mention the Bullet Cluster though.