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Not an astrophysicist but something like MOND makes sense to me. It seems like it's much more probable that our model is slightly broken vs. there being a massi

Not an astrophysicist but something like MOND makes sense to me. It seems like it's much more probable that our model is slightly broken vs. there being a massive amount of... well, mass that we can't find no matter how hard we look. Is there something about the idea that's really damning and nonsensical that a lay person may not notice?

ninkendo 3y ago
The Bullet Cluster is typically touted as the biggest evidence that dark matter is right and MOND is wrong: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to_dark_matter https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to... Basically, the center of mass in the cluster, and the center of the matter we can see, are way far apart from one another, in a way that ends up being super clear to observe. We can infer the center of mass by the surveying the gravitational lensing we observe, and we can observe the center of the baryonic matter by looking at the x-ray sources. The offset in distance here ends up being pretty clear evidence that most of the mass exists as stuff we can't see (in any wavelength, only via the lensing.)
ianburrell 3y ago
There are two problems with MOND. One is that it exchanges fine arbitrary parameters of dark matter with the arbitrary parameters of MOND. There isn’t a theory the explains MOND or the parameters. It is just as much of bodge as dark matter. Second is that MOND doesn’t explain all the observations. There are galaxies that have too much or too little gravity. Either MOND varies exactly as we neeed it, or need MOND and dark matter. That isn’t any simpler. It might work if there was theory that explained smaller amount of dark matter and MOND could handle the rest.
ddellacosta 3y ago
This is a bit of an indirect answer, but I watched this not too long ago and it made it seem clear that MOND doesn't work so well, if not in general at least in the case discussed, which seems pretty damning regardless: https://youtu.be/HlNSvrYygRc?si=6on0pD9_jsed9Dlm https://youtu.be/HlNSvrYygRc?si=6on0pD9_jsed9Dlm I've also heard the bullet cluster pointed out as a strong counterpoint to MOND--I'll let you read the wikipedia page rather than attempting to explain this, poorly: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster As a layman who watches tons of astronomy and physics videos, my general sense is that dark matter is more or less simply a placeholder, such that everything else around it is pretty consistent, but it just represents this unknown matter that we can clearly observe interacting _gravitationally_ with galaxies and other objects in the universe, but otherwise doesn't seem to interact. Whereas my impression of MOND is that it's a set of heuristics that requires a lot of tweaking to work, whereas GR is quite simple and very strongly experimentally verified. But, again, layman
badosu 3y ago
This is a dense video, I highly recommend it though: https://www.youtube.com/watch?v=PbmJkMhmrVI https://www.youtube.com/watch?v=PbmJkMhmrVI TLDW: there's a lot of evidence on why the phenomena we watch is mostly favored to be described as 'something' that is conveniently called dark matter as opposed to a revision of gravity.

ninkendo 3y ago

The Bullet Cluster is typically touted as the biggest evidence that dark matter is right and MOND is wrong: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to_dark_matter https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to... Basically, the center of mass in the cluster, and the center of the matter we can see, are way far apart from one another, in a way that ends up being super clear to observe. We can infer the center of mass by the surveying the gravitational lensing we observe, and we can observe the center of the baryonic matter by looking at the x-ray sources. The offset in distance here ends up being pretty clear evidence that most of the mass exists as stuff we can't see (in any wavelength, only via the lensing.)

ianburrell 3y ago

There are two problems with MOND. One is that it exchanges fine arbitrary parameters of dark matter with the arbitrary parameters of MOND. There isn’t a theory the explains MOND or the parameters. It is just as much of bodge as dark matter. Second is that MOND doesn’t explain all the observations. There are galaxies that have too much or too little gravity. Either MOND varies exactly as we neeed it, or need MOND and dark matter. That isn’t any simpler. It might work if there was theory that explained smaller amount of dark matter and MOND could handle the rest.

ddellacosta 3y ago

This is a bit of an indirect answer, but I watched this not too long ago and it made it seem clear that MOND doesn't work so well, if not in general at least in the case discussed, which seems pretty damning regardless: https://youtu.be/HlNSvrYygRc?si=6on0pD9_jsed9Dlm https://youtu.be/HlNSvrYygRc?si=6on0pD9_jsed9Dlm I've also heard the bullet cluster pointed out as a strong counterpoint to MOND--I'll let you read the wikipedia page rather than attempting to explain this, poorly: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster As a layman who watches tons of astronomy and physics videos, my general sense is that dark matter is more or less simply a placeholder, such that everything else around it is pretty consistent, but it just represents this unknown matter that we can clearly observe interacting _gravitationally_ with galaxies and other objects in the universe, but otherwise doesn't seem to interact. Whereas my impression of MOND is that it's a set of heuristics that requires a lot of tweaking to work, whereas GR is quite simple and very strongly experimentally verified. But, again, layman

badosu 3y ago

This is a dense video, I highly recommend it though: https://www.youtube.com/watch?v=PbmJkMhmrVI https://www.youtube.com/watch?v=PbmJkMhmrVI TLDW: there's a lot of evidence on why the phenomena we watch is mostly favored to be described as 'something' that is conveniently called dark matter as opposed to a revision of gravity.