Dark matter + gravity is 'right', or our understanding of gravity is wrong?

[Ibix]

Anti-matter has a negative charge, so will repel matter.

No. This is not correct. Antimatter has an opposite electric charge (which may be positive or negative and could form neutral atoms) but it has positive mass so is gravitationally attractive the same as normal matter.

I've never formally studied Astronomy,

Then you don't even understand the details of the problem. Please note that PF is not for discussion of personal theories, as set out in the terms you agreed to when you joined.

 

[DaveC426913]

Anti-matter has a negative charge.

No. Antimatter simply has opposite charge: anti-electrons have a positive charge, whereas antiprotons have a negative charge. Anti hydrogen is neutral.

Antimatter produces the same positive gravity as matter.

 

[cmb]

Anti-matter has a negative charge

Positively not positrons, which I am positive are positive.

Anyway, galactic matter is net neutral, as would be galactic anti-matter.

 

[ohwilleke]

I once read that there was an imbalance in the observed matter and anti-matter in the Universe i.e. not enough anti-matter. Is it possible that galaxies may have a corona of anti-matter, in some form, which we may not be able to see but which "pushes" matter into its well?

No. This isn't possible. Anti-matter interacts vigorously with ordinary matter. If there were anti-matter haloes around galaxies we'd see massive numbers of diffuse flashes everywhere.

 

[ohwilleke]

I'm unconvinced. Why would people stick with "Newtonian Gravity" if there was something better to use?

Surely GR is not 'sooooo' complicated that it is impossible to model it (instead of "Newtonian gravity"). Is that what you are trying to say?

Thanks for your contribution but I can't follow your point. If you have some relevant reference confirming your assertion, that would be helpful.

The basic problem is that there isn't a literature out there of really rigorous analysis of GR minus Newtonian gravity in galaxy scale systems. There is back of napkin rough estimates making lots of spherical cow type assumptions, but there isn't much literature quantifying the magnitude of the non-linear effects of GR in non-spherical systems of galaxy scale. They are clearly not zero and haven't been rigorously bounded.

This isn't entirely a coincidence.

There is a statement in the Misner, Thorne and Archibald Gravitation textbook which implies that no observable effects arise from self-interactions of gravitational fields, which is almost surely an overstatement, and leaves people with an overstated impression of what is being said.

There have also been a number of papers looking at different non-linear GR effect (like gravitomagnetism) that don't work and have been rejected.

There is also the look for you keys under the lamp post effect. It is much, much easier to do Newtonian approximations, and it is much much easier to model GR effects in the spherically symmetric case in which the effect the Deur is interested in cancels out due to symmetry considerations.

Doing hard work to find something with a low Baysean probability of working out isn't the most attractive course of action.

I'd also note that another possibility is that Deur is indeed actually using equations that for subtle reasons are not actually standard Einstein equation GR (as he has asserted that they are). But suppose that is the case.

The equations still reproduce dark matter and dark energy and early structure phenomena. They reproduce MOND in disk galaxies and solve the insufficiency of dark matter phenomena in galactic clusters. They make novel observational predictions that have been confirmed that aren't found in other modified gravity theories. And, they manifestly reduce to the weak field Newtonian approximation (in a manner naturally generalized to be relativistic) on their face below a threshold that corresponds to the MOND constant. These equations do all of these things with a single field, not the scalar-tensor or scalar-vector-tensor fields that many other relativistic modified gravity theories require. These equations do all of these things without particle dark matter and without a cosmological constant of a substance to provide dark energy. This is still a very good day's work.

So, even if he is mistaken and he is not actually doing exactly Einstein's GR, if he has a versatile set of equations that can reproduce the phenomenology of both dark matter and dark energy, not just dark matter as MOND does, over a much wide range of applicability than MOND, in a quite simple and elegant way. Thus, whatever he has done to subtly modify Einstein's GR may be an accurate description of nature, and Einstein's GR may not be quite the right description of how gravity really acts in the infrared in systems that aren't spherically symmetrical. Whether or not it is truly faithful to Einstein's GR is something of an academic question if it works better to describe the universe.

After all, it's a dirty little secret but GR theorists actually have several different slight variations of GR on offer already that differ in subtle ways that aren't currently experimentally or observationally testable, such as teleparallel gravity, Einstein-Gauss-Bonnet, Einstein-Cartan theory, metric-affine gravity, etc. So, it really wouldn't be all that remarkable if the correct theory was a subtle tweak from the original 1917 version.

 

[bhobba]

Maybe because most people who knows technical details of both dark matter models and alternative models think that dark matter models are better and simpler?

That's it, really. GR as we know it is almost inevitable because of Lovelock's Theorem. In layman's language, it says, basically, if our knowledge of the theory of fields is correct (technically, it is assumed it can be derived from a Lagrangian - a generalisation of ordinary mechanics), Einsteins Equations are inevitable. Of course, we know that GR needs modification at small scales - about the Plank Scale - because of issues with QM. But we even have a quantum theory of gravity thought to be correct to the Plank Scale:
https://blogs.umass.edu/donoghue/research/quantum-gravity-and-effective-field-theory/.
It is also thought the Standard Model breaks down at the Plank Scale - possible even before. So that is nothing new, really.

Anyway, the bottom line is if GR were not true, some longstanding physical principles would have to be wrong. That is indeed possible, but so far, nobody has come up with an alternative that is just so damn elegant, requires so few assumptions and stands up to experimental verification. Lev Landau was fond of saying, 'a rapture on first making one's acquaintance with this theory should, in general, be a characteristic of any born theoretical physicist.'

Thanks
Bill