I "Violation of the Equivalence Principle in Disk Galaxies" newly interpreted

[timmdeeg]

https://www.mdpi.com/2571-712X/8/3/65

In short, if the dynamics of spiral galaxies are dependent on galactic morphology in the way predicted by GR-SI, and the morphology of galaxies are correlated with their number of satellite galaxies (and more generally, their environment), then we expect to see a correlation between the dynamics of galaxies and their external fields. This expected correlation is in direct contrast to the explanation given in [11] wherein external fields directly affect the dynamics of spiral galaxies via the suppression of the MOND effect.

According to Alexandre Deur GR-SI (GR Self-interaction) could replace the assumption of Dark Matter. Here it seems that GR-SI could replace MOND in a very certain context.
The article is much beyond my understanding. Do the calculations in this article strengthen GR-SI?

 

[mitchell porter]

This is a response to an article from five years ago (Chae et al 2020) claiming to detect an effect of MOND (the "external field effect") in a particular population of galaxies. The authors of the present paper say, if true this would be the first violation of general relativity detected, so it's important to try to explain it within general relativity, and they say they can explain it within general relativity via Deur's calculations.

The problem is that most people who have looked at those calculations, don't believe their validity! More broadly, there is a consensus that the nonlinearities in general relativity are not capable of accounting for galactic rotation curves, they are too weak by a factor of one million. Deur's calculations may describe how things work in some kind of modified gravity, but they are not valid for general relativity - that has been the response on this forum, and in the research literature.

If anyone intends to go over this again, I guess they should look at the appendices of this paper, and try to identify how the calculations there differ from those in reference 15, and ask whether there are any novel features.

 

[PeterDonis]

According to Alexandre Deur GR-SI (GR Self-interaction) could replace the assumption of Dark Matter.

We've had a number of previous threads on this where there has been extensive discussion. I would suggest looking them up.

 

[timmdeeg]

More broadly, there is a consensus that the nonlinearities in general relativity are not capable of accounting for galactic rotation curves, they are too weak by a factor of one million.

The more it is astonishing that these physicists seem to overlook this factor.

Thanks for answering.

 

[Sambuco]

The problem is that most people who have looked at those calculations, don't believe their validity! More broadly, there is a consensus that the nonlinearities in general relativity are not capable of accounting for galactic rotation curves, they are too weak by a factor of one million.

On this point, there was a brief but interesting debate a couple of years ago. This paper (https://arxiv.org/abs/2303.11094) pointed out that Deur's results might not be able to explain the effects associated with dark matter. Shortly after, Deur responded (https://arxiv.org/abs/2306.00992) that the approximations used in the first paper avoid the self-interaction effects he proposes, and thus do not invalidate his results. I'm not aware of any subsequent paper that continues this discussion.

Lucas.

 

[kodama]

As I mentioned to ohwilleke
General relativity has been researched via computers since the 1960s when computers became available One term to describe this is numerical relativity.
https://en.wikipedia.org/wiki/Numerical_relativity

in the 60 plus years of studying general relativity using supercomputers and both general relativists mathematicians and computer experts, there has been no computer result that matches Deur's predictions in the literature, that general relativity nonlinear effects not present in Newtonian gravity can replace dark matter.

supercomputers are extensively used to model dark matter models in galaxies rotation curves, dark matter and general relativity in the weak field

I would think a supercomputer by relevant experts would identify such nonlinear effects if they exist as required by Deur. using newtonian gravity and then general relativity into a supercomputer to model galaxies may result in a very small difference, but too weak to replace dark matter by a factor of a million

if Deur is right how do you explain the 60 years of failure by both GR experts and super computers to find such a effect in modeling galaxies

G. O. Ludwig, in a 2021 paper published in The European Physical Journal C, proposed a model suggesting that the gravitomagnetic field could account for the observed rotation curves of galaxies without requiring the existence of dark matter.
gravitomagnetic field does exist but it is too weak by a factor of a million to account for the observed rotation curves of galaxies without requiring the existence of dark matter.

I suspect Deur ideas will fall to the same fate

 

[PeterDonis]

I would think a supercomputer by relevant experts would identify such nonlinear effects

That depends on how the simulations are done. Generally speaking, numerical simulations of any set of equations in physics use approximations that amount to expanding the equation out in powers of some quantity that gets smaller at each order, and then carrying the expansion out to enough orders to get the accuracy you want. But making those decisions requires that you already know, at least to an extent, what effects are significant and what effects aren't, since that governs how you do the expansion and what terms you include. You can't just read off every possible effect in the basic nonlinear equation from a simulation. You have to build the simulation to include whatever effects you believe are relevant.

My understanding is that up to now, numerical simulations of, say, galaxy rotation curves have simply not included the sorts of terms that would capture the kind of effect Deur claims exists, because nobody thought those terms were relevant. I don't know if anyone has done a simulation with the express purpose of including those terms, along with all the others that are already included, just to test whether they make a significant contribution. But I'm not very familiar with the literature on numerical relativity. Possibly someone here is and can give more information.

 

[timmdeeg]

As an aside here is another attempt to get rid of Dark Matter in Galaxies.

"Exact solutions for differentially rotating galaxies in general relativity" / https://arxiv.org/pdf/2406.14157

abstract: "Whether or not abundant collisionless dark matter exists, the new solutions very strongly support suggestions that the phenomenology of galactic rotation curves be fundamentally reconsidered, for consistency with general relativity."


Page 2: "Indeed, we note that differential rotation is a necessary condition for self-gravitating disc systems to achieve dynamical stability in Newtonian dynamics [7–11]. Remarkably, we will find that the mathematical pathologies can be sidestepped within this wider class of models, by refining the consistent treatment of taking Newtonian limits. In particular, conventionally researchers have naïvely applied non relativistic limiting procedures to the gravitational metric before attempting to include nonlinear terms. By contrast, we will include all essential nonlinearities for stationary axisymmetric dust spacetimes before considering the low–energy limit."