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We cannot simulate evolution of galaxies using equations of General Relativity. So at best the simulations are done using Newton mechanics with minimal correct

by _0w8t 4y ago

We cannot simulate evolution of galaxies using equations of General Relativity. So at best the simulations are done using Newton mechanics with minimal corrections like accounting for finite speed of light. And often it is not even done using Newton mechanics per see, but numerical models use artificial constrains like a minimal distance between stars to avoid dealing with numerical instabilities. Yet there is no proof that such simplifications or constrains are valid on the scale of a typical galaxy. Moreover, when people managed to use full equations of General Relativity, then it was possible to account the effects previously thought to require the dark matter hypothesis. So until it is mathematically proven that the current simulations are valid approximations, I will remain very skeptical about the dark matter.

rich_sasha 4y ago
Like the GP, I'm not in love with DM hypothesis and wouldn't mind something else. I also used to think the same as you. But DM explains a lot of disparate effects, in ways that MOND does not. The best-well known effect is the "missing mass" in rotating galaxies. Well recently we found galaxies that look like they have no dark matter in them - i.e. they behave as you would expect from classical gravity/centripetal force arguments [1]. So while you can tweak General Relativity to match some of the effects in typical galaxies, you would then expect the results to hold in all galaxies. The "outliers" are easily explained by Dark Matter - they just don't have it for whatever reason. The simulations of Universe formation, sure have their limitations, but look much worse without DM. Galaxy clusters look like they have unexplained mass in them. Crucially, this isn't only from observing their rotational speeds (so like galaxy rotations) but also from looking at the strength of gravitational lensing they induce. This is as compared to fluid-dynamics-y calculations of gas temperature, pressure etc. Bullet cluster is another interesting case [2]. It is the result of a collision of two galaxies. In such cases, you expect (a) the stars to "fly through" each other, as they are basically point masses and only interact gravitationally, but (b) the gasses and dust to collide. In this case, most of the mass is in fact gas and dust, so it should be distributed in the middle of the cluster (having just had an inelastic collision). But in fact observed mass is where the stars are, further out of what looks like center of mass. [1] https://www.nature.com/articles/d41586-022-01410-x https://www.nature.com/articles/d41586-022-01410-x [2] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster
- _0w8t 4y ago
  The point is that there is no need to tweak GR. For example, [1] shows how properly accounting for GR and using rather detailed data about Milky Way allows to explain the rotational curves. But GR is complex enough to allow very different behavior depending on the initial conditions of galaxy formations. So it would not be surprising to see different scale of deviations from Newtonian mechanics depending on Galaxy history or age. [1] - https://arxiv.org/abs/1810.04445 https://arxiv.org/abs/1810.04445
  - _0w8t 4y ago
    One more thing. It is not true that stars simply fly through. Already in the second-order approximation in GR there are forces that behave like Lorentz force of electromagnetism perpendicular to the line of movement. Granted this a small effect, but on the scale of millions of years it matters. That alone can explain a lot of subtle behavior.

by _0w8t 4y ago

rich_sasha 4y ago

Like the GP, I'm not in love with DM hypothesis and wouldn't mind something else. I also used to think the same as you. But DM explains a lot of disparate effects, in ways that MOND does not. The best-well known effect is the "missing mass" in rotating galaxies. Well recently we found galaxies that look like they have no dark matter in them - i.e. they behave as you would expect from classical gravity/centripetal force arguments [1]. So while you can tweak General Relativity to match some of the effects in typical galaxies, you would then expect the results to hold in all galaxies. The "outliers" are easily explained by Dark Matter - they just don't have it for whatever reason. The simulations of Universe formation, sure have their limitations, but look much worse without DM. Galaxy clusters look like they have unexplained mass in them. Crucially, this isn't only from observing their rotational speeds (so like galaxy rotations) but also from looking at the strength of gravitational lensing they induce. This is as compared to fluid-dynamics-y calculations of gas temperature, pressure etc. Bullet cluster is another interesting case [2]. It is the result of a collision of two galaxies. In such cases, you expect (a) the stars to "fly through" each other, as they are basically point masses and only interact gravitationally, but (b) the gasses and dust to collide. In this case, most of the mass is in fact gas and dust, so it should be distributed in the middle of the cluster (having just had an inelastic collision). But in fact observed mass is where the stars are, further out of what looks like center of mass. [1] https://www.nature.com/articles/d41586-022-01410-x https://www.nature.com/articles/d41586-022-01410-x [2] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster

_0w8t 4y ago
The point is that there is no need to tweak GR. For example, [1] shows how properly accounting for GR and using rather detailed data about Milky Way allows to explain the rotational curves. But GR is complex enough to allow very different behavior depending on the initial conditions of galaxy formations. So it would not be surprising to see different scale of deviations from Newtonian mechanics depending on Galaxy history or age. [1] - https://arxiv.org/abs/1810.04445 https://arxiv.org/abs/1810.04445
- _0w8t 4y ago
  One more thing. It is not true that stars simply fly through. Already in the second-order approximation in GR there are forces that behave like Lorentz force of electromagnetism perpendicular to the line of movement. Granted this a small effect, but on the scale of millions of years it matters. That alone can explain a lot of subtle behavior.