Dark Matter & Quantum Gravity: Is Unknown Property at Play?

[joelr]

Regarding dark matter, is it possible that at the quantum level there is some unknown property to gravity that is making it appear that there is this dark matter attracting normal mass but it's actually just that on a galactic scale gravity acts different than GR predicts?

Or is that completely ruled out at this point?

 

[Nugatory]

It is not completely ruled out, but unless and until someone can provide a peer-reviewed theory that makes a quantitative prediction that such a thing might happen, it's speculation not science. The Physics Forums rules do not allow such speculation, so this thread is closed.

 

[Nugatory]

@pervect has reminded me that although there is no theory based on "at the quantum level there is some unknown property", there is a class of theories that otherwise behave as the OP suggests. The thread is reopened to allow discussion of these.

 

[Vanadium 50]

at the quantum level there is some unknown property to gravity

That part doesn't add anything but confusion, so I am going to ignore it.

Yes, gravity can be changed to better explain dark matter. There are two problems with it: the first is that it describes the dynamics better at the scale of galaxies better but worse at all other scales. The second is that there seems not to be a unique relativistic extension that does better.

 

[pervect]

On a somewhat different note, is introducing a scalar field (and I think the relativistic MOND needed both a scalar field and a vector field too) really that much more economical than just introducing whatever sort of field (probably a fermion or boson field?) needed for the dark matter?

 

[Orodruin]

more economical than just introducing whatever sort of field (probably a fermion or boson field?) needed for the dark matter?

Why? A scalar field is a bosonic field. Why would a fermion field be less economical than a scalar field and a vector field combined?

 

[pervect]

I think of most boson fields as spin-1, though your response made me think a bit, and I suppose the Higgs field is usually thought of as a spin-0 field. I'm not aware of any other fundamental spin-0 fields, though. For the most part spin-0 fields seem very artifical to me - this may be a personal prejudice. With the possible exception of the Higgs, but I wouldn't be too terribly surprised if the Higgs mechanism turned out to be different than a spin-0 field.

Regardless, MOND needs an additional field besides the scalar field from what I read, a vector field, making it more complex.

I suppose the real question is which fits experiment best. I would say that that is almost certainly general relativity, though I admit to a lack of familiarity with MOND.

 

[Orodruin]

For the most part spin-0 fields seem very artifical to me - this may be a personal prejudice.

Why? It is the simplest type of field imaginable, a scalar field. That the potential becomes a mess when you start introducing several scalars is another matter.

I suppose the real question is which fits experiment best. I would say that that is almost certainly general relativity, though I admit to a lack of familiarity with MOND.

The latest blow to MOND and other modified gravity theories was the recent discovery of a galaxy with no dark matter. A priori, it would be possible to separate matter from its DM halo, but you cannot change how gravity behaves to accommodate it.

 

[joelr]

@pervect has reminded me that although there is no theory based on "at the quantum level there is some unknown property", there is a class of theories that otherwise behave as the OP suggests. The thread is reopened to allow discussion of these.

This may not help but when I say "at the quantum level" I mean is it possible that if we came up with a quantum theory of gravity that worked and we were able to merge gravity with quantum mechanics, is it possible that this quantum theory of gravity could explain that gravity acts differently on galactic scales?

Mordehai Milgrom an Israeli physicist and professor had an article on his MOND in SciAm 2002:
https://www.astro.umd.edu/~ssm/mond/sad0802Milg6p.pdf

but proposed a modification to Newton’s second law that changed the relation between force and acceleration when the acceleration is low. My question was rather than modify Newton's law could a quantum description of gravity possibly reveal some large scale property of gravity that would account for dark matter?