Galactic Rotation and Energy-Momentum Tensor

[Christian Thom]

TL;DR

Is the momentum part of the energy/momentum tensor taken into account in the modelling of the movements of the stars in galaxies ?

Galaxies are very large rotating bodies, so it seems that, as with the Kerr model for black holes, there could be an effect of this global rotation on the energy momentum tensor in the more dense regions of the galaxy that could in turn affect the space-time in the vicinity of the object and so the movements of the stars, with maybe an impact on the evaluation of the dark matter quantity.

 

[PeterDonis]

as with the Kerr model for black holes, there could be an effect of this global rotation on the energy momentum tensor

Kerr black holes are vacuum solutions, so the stress-energy tensor is zero everywhere.

In modeling galaxies, GR effects are typically negligible so a Newtonian model of gravity is used.

 

[Christian Thom]

Thank you.

 

[richardgobeli]

Does the angular momentum of the internal mass need to be accounted for so the frame dragging is included?

 

[Ibix]

Does the angular momentum of the internal mass need to be accounted for so the frame dragging is included?

Depends how precise you want to be. The usual modelling of galaxies is Newtonian because GR effects are negligible and adding them in simply increases the complexity of the model without making any measurable change to the output. But if you wanted to make the effort, you would need to include the rotation of the galactic disc if you wanted to see frame dragging effects.

 

[richardgobeli]

GR can be used to make the rotating galaxy not need dark matter to make the velocities constant in the outer rim of the galaxy. Newtonian gravity does not account for the local frame dragging of the local stars to the test star. Here is a paper that shows that dark matter is not needed when GR is used and the angular momentum of the galactic disk is used. https://arxiv.org/abs/2207.04279 titled

General Relativity versus Dark Matter for rotating galaxies​

Even when the stress-energy tensor is zero, the metric of the rotating galaxy contains the angular momentum of the rotating galaxy which does pull the rim stars along at a higher velocity rate than Newtonian gravity allows.

 

[PeterDonis]

Here is a paper that shows that dark matter is not needed when GR is used and the angular momentum of the galactic disk is used. https://arxiv.org/abs/2207.04279 titled

General Relativity versus Dark Matter for rotating galaxies​

We have had a previous thread on this paper:

https://www.physicsforums.com/threa...ng-gr-explains-dark-matter-phenomena.1016800/

The short version is that it is not clear that the claim of this paper, that the GR frame dragging effect due to rotation is sufficient to account for galaxy rotation curves without dark matter, is correct; it's still an open question. It's a very interesting proposal but it can't be taken as mainstream accepted science, at least not at this point.