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> we just haven't been taking into account the proper relativistic corrections at galactic scales Dark Matter was first postulated because galaxy rotation curv

> we just haven't been taking into account the proper relativistic corrections at galactic scales Dark Matter was first postulated because galaxy rotation curves strongly suggested if there was not unseen matter, galaxies would fly apart. But these observations of galaxies were each of a galaxy in isolation. So in essence, Dark Matter is a fudge to explain the observation that galaxies are not flying apart. Since, other observations that can't be explained have been lumped into Dark Matter. But it turns out, Dark Matter is unnecessary to explain galaxy rotation curves.[0] Galaxies are never isolated. They come in clusters and superclusters. [0] https://www.youtube.com/watch?v=PL0ewiwqoTw https://www.youtube.com/watch?v=PL0ewiwqoTw

ravi-delia 6y ago
The scope of observations goes a little further than rotational curves. The Bullet Cluster, for instance, clearly shows mass where there is no visible matter. Plus, there are lots of smaller galaxies that behave just as our normal theories prescribe, which dark matter (or the lack thereof) can explain and other theories have trouble with.
- bjornsing 6y ago
  So? The relativistic effects of mass currents (that the parent is talking about) could put “apparent mass” where there is none, no?
  - ravi-delia 6y ago
    In principle, yes. In practice, these proposals match the spin curve very well and the bullet cluster not so much. That isn't a knock down argument; the bullet cluster offers oddities for dark matter too. However OP implied that because galactic rotation works without dark matter, and dark matter was first posited to deal with galactic rotation, we don't need dark matter. That's not right.
    - bjornsing 6y ago
      I find it a bit hard to believe that the mass current calculations for the bullet cluster have even been made. How can I convince myself that they have?
      - ravi-delia 6y ago
        It really depends on your choice of modified gravity. Normal MOND isn't extended to relativity, and your choice of how to do that is going to make some big differences. I found [this](https://arxiv.org/abs/astro-ph/0212293 https://arxiv.org/abs/astro-ph/0212293) paper from a pro-mond researcher that found a non-relavistic model couldn't account for the behavior of dense galactic clusters like the bullet cluster, and there are plenty of papers by less sympathetic researchers that agree. I couldn't find any non proof of concept papers that tested relativistic extensions of MOND for bullet-cluster-esq situations. Those proof of concepts tended to disagree greatly with reality, but thanks to the weird effects of (for instance) mass current models you'd expect to need a pretty accurate simulation to avoid compounding error. It certainly hasn't been ruled out. That being said, while we haven't ruled out that some super weird interacting effect doesn't cause the apparent mass, we know that under a broad range of DM models the bullet cluster is normal. If you assume that apparent mass shows where actual mass is, pretty much all of the weird behavior goes away. It does suggest that so-called "hot" dark matter is unlikely, but that was indicated by cosmological surveys as well.
Keysh 6y ago
I kind of assumed that a YouTube video on that subject would be a waste of time, and boy was it. That was amazingly ignorant. Galaxies are often isolated. They come in clusters, and also in groups, and in isolation (including isolated galaxies inside cosmic voids). The isolated galaxies have the same "dark matter" phenomena as galaxies in clusters (which, by the way, do not have perfectly circular orbits, just to start with).
- raattgift 6y ago
  Also, of course, asymptotic flatness.

ravi-delia 6y ago

The scope of observations goes a little further than rotational curves. The Bullet Cluster, for instance, clearly shows mass where there is no visible matter. Plus, there are lots of smaller galaxies that behave just as our normal theories prescribe, which dark matter (or the lack thereof) can explain and other theories have trouble with.

bjornsing 6y ago
So? The relativistic effects of mass currents (that the parent is talking about) could put “apparent mass” where there is none, no?
- ravi-delia 6y ago
  In principle, yes. In practice, these proposals match the spin curve very well and the bullet cluster not so much. That isn't a knock down argument; the bullet cluster offers oddities for dark matter too. However OP implied that because galactic rotation works without dark matter, and dark matter was first posited to deal with galactic rotation, we don't need dark matter. That's not right.
  - bjornsing 6y ago
    I find it a bit hard to believe that the mass current calculations for the bullet cluster have even been made. How can I convince myself that they have?
    - ravi-delia 6y ago
      It really depends on your choice of modified gravity. Normal MOND isn't extended to relativity, and your choice of how to do that is going to make some big differences. I found [this](https://arxiv.org/abs/astro-ph/0212293 https://arxiv.org/abs/astro-ph/0212293) paper from a pro-mond researcher that found a non-relavistic model couldn't account for the behavior of dense galactic clusters like the bullet cluster, and there are plenty of papers by less sympathetic researchers that agree. I couldn't find any non proof of concept papers that tested relativistic extensions of MOND for bullet-cluster-esq situations. Those proof of concepts tended to disagree greatly with reality, but thanks to the weird effects of (for instance) mass current models you'd expect to need a pretty accurate simulation to avoid compounding error. It certainly hasn't been ruled out. That being said, while we haven't ruled out that some super weird interacting effect doesn't cause the apparent mass, we know that under a broad range of DM models the bullet cluster is normal. If you assume that apparent mass shows where actual mass is, pretty much all of the weird behavior goes away. It does suggest that so-called "hot" dark matter is unlikely, but that was indicated by cosmological surveys as well.

Keysh 6y ago

I kind of assumed that a YouTube video on that subject would be a waste of time, and boy was it. That was amazingly ignorant. Galaxies are often isolated. They come in clusters, and also in groups, and in isolation (including isolated galaxies inside cosmic voids). The isolated galaxies have the same "dark matter" phenomena as galaxies in clusters (which, by the way, do not have perfectly circular orbits, just to start with).

raattgift 6y ago
Also, of course, asymptotic flatness.