Gravitomagnetic explanation of Dark Matter

[DaveC426913]

TL;DR

There was a long thread around here (which I can no longer find) about DM being explained by gravitomagnetic effects. Is there a 25 words or less explanation?

I was reading a thread on my phone that was reviewing a paper about DM being explainable by gravitomagnetic effects. Now I can't find it in any search. It was on its fourth page - so at least 80 posts over at least two years.

Anyway, what I wanted to ask was for a description that a layperson might be able to give another layperson about DM.

My tentative conclusion was that:

DM is (so this paper claims) essentially a faulty assumption in how gravity in a spatially-extended object such as a galaxy ought to behave in a Newtonian model. i.e. the 1/r^2 formula is a good approximation in other cases but, here, it just does not apply. When the gravity curve is calculated using GR, it does not work out so simply as 1/r^2, - it tends to level off to a plateau near the edges of such an object. That's just The Way Gravity Works in GR. I also got the impression that an exact solution using GR formulae is impractical, which may be why this solution has been elusive.

Am I close enough for horseshoes and hand grenades?

 

[anorlunda]

I think you are asking about Modified Newton Dynamics, or MOND for short.

https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

By the way, I think the consensus here on PF was that evidence does not support MOND.

 

[PeterDonis]

I was reading a thread on my phone that was reviewing a paper about DM being explainable by gravitomagnetic effects

Are you thinking of this thread?

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

There have been a number of threads on this general topic, I think this is the most recent.

 

[PeterDonis]

DM is (so this paper claims) essentially a faulty assumption in how gravity in a spatially-extended object such as a galaxy ought to behave in a Newtonian model. i.e. the 1/r^2 formula is a good approximation in other cases but, here, it just does not apply. When the gravity curve is calculated using GR, it does not work out so simply as 1/r^2, - it tends to level off to a plateau near the edges of such an object. That's just The Way Gravity Works in GR. I also got the impression that an exact solution using GR formulae is impractical, which may be why this solution has been elusive.

This is a reasonable quick summary of the general idea behind the paper discussed in the thread I linked to in post #3, and similar ones we have had in the past on PF. The work of Deur, in particular (which is mentioned in that thread), has been the subject of quite a bit of PF discussion. The only correction I would make is that the models used are not using straight Newtonian gravity: Newtonian gravity using the visible matter in galaxies is known and agreed by all participants in the debate to not give the right answer for galaxy rotation curves. The models used are GR models in the weak field limit, which is often referred to as the "Newtonian" limit but is known to include corrections not present in Newtonian gravity (for example, the extra precession of the perihelion of planets).

The major caveat, of course, is that claiming that the GR corrections are large enough even in the weak field limit to explain the actual galaxy rotation curves using just the visible matter in some cases (basically, cases like spiral galaxies which are very far from spherical symmetry) is one thing: actually demonstrating it is another. It is true that exact solutions are not known for such cases, so any investigation has to use approximations and numerical methods. It is also true, however, that GR has well tested approximation schemes and numerical methods that work well in, for example, the solar system, and these schemes do not appear to support the kinds of claims being made in papers like the one in the above thread.

 

[PeterDonis]

I think you are asking about Modified Newton Dynamics, or MOND for short.

I don't think so, because the OP specified that just standard GR would be used, whereas MOND is a modified theory of gravity, not standard GR. (In fact there is no well accepted relativistic version of MOND at all.)

 

[DaveC426913]

Are you thinking of this thread?

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

That's the one!

 

[mitchell porter]

Gravitomagnetism is the preferred explanation of G.O. Ludwig. One of his papers got a lot of press last year.

I don't quite get what Deur's mechanism is, but I don't think it's gravitomagnetism.

The mainstream view (exemplified by Ciotti in post #44 in the thread) is that in general relativity, the gravitational field produced by a galaxy's baryonic matter is far too weak at the galactic edge for any such effects to be produced, so there has to be a mistake in papers like these.

 

[jedishrfu]

Since it appears we already have a thread on the topic it would be a good time to close and consolidate our posts into one thread:

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

Thank you all for your contributions here.

Jedi