LQG Legend Writes Paper Claiming GR Explains Dark Matter Phenomena

[PeterDonis]

You don't need to be a GR expert to linearize the EFE

Linearizing the EFE wouldn't be sufficient to resolve the issue, since the claims in question are that nonlinear effects are much more significant (as in, orders of magnitude more significant) in galaxies than standard cosmology assumes.

 

[PAllen]

Numerical relativity is just a fast way to avoid the paper back and forth -- faithfully simulate it and see what happens. Would clear up the mystery pretty quickly.

If numerical relativity for galactic rotation were simple, someone would have done it.

You don't need to be a GR expert to linearize the EFE, it's a standard GR intro exercise. The papers are quite readable to anyone familiar with PDEs and perturbation theory, no expertise needed. It's really more about lack of rigor in doing the perturbation analysis and its consequences being negligible (or not).

Obviously this characterization is not reasonable. All authors are top notch GR experts, this debate has been ongoing for at least 15 years, and is still not resolved to the level of clear consensus. However, among GR experts I know personally, the view of Ciotti is the one most believe.

 

[wumbo]

Linearizing the EFE wouldn't be sufficient to resolve the issue, since the claims in question are that nonlinear effects are much more significant (as in, orders of magnitude more significant) in galaxies than standard cosmology assumes.

Not entirely. Linearized EFE includes the gravitomagnetism bit that also has a pretty decent influence. See this explanation. In short, it's suppressed by a factor of 1/c^2 and would otherwise be negligible, but there's a wrench in the works since it changes the characteristics of the PDEs critically (hyperbolic to elliptic). Hence the ask for numerical relativity simulations of the whole EFE as a tiebreaker.

If numerical relativity for galactic rotation were simple, someone would have done it.

Obviously this characterization is not reasonable. All authors are top notch GR experts, this debate has been ongoing for at least 15 years, and is still not resolved to the level of clear consensus. However, among GR experts I know personally, the view of Ciotti is the one most believe.

I mean, yeah, that's the point of scientific debate right? The difference here is that the math is easy and the argument are comprehensible to anyone with PDE and perturbation theory experience. You don't need to be an expert on general relativity to work through the argument, which is unusual.

FWIW, the standard derivation of linearized gravity is very cavalier about declaring terms negligible and needs more rigor.

Ciotti's paper is correct nevertheless, see the November 2022 response that agrees but makes the point that he didn't consider relevant homogeneous solutions which matter.

 

[PeterDonis]

Provides a pretty thorough counter-argument to it IMO.

I'm not so sure. Fig. 1 in the paper you cite does show numerically different velocity profiles vs. the Newtonian ones as a function of the parameter $\lambda$, which measures the "strength" of the GEM effects, and the corrections, as the authors state, are around 10% to 15%, so not negligible. But all of those profiles have the same general shape as the Newtonian one. None of the profiles are flatter than the Newtonian one once the "peak" is reached, which is what would be required to help reduce the disconnect between the visible matter and observed rotation curves without adding dark matter to the model. Indeed, if anything they are less flat, meaning that these corrections make the problem worse, not better.

 

[PeterDonis]

Ciotti's paper is correct nevertheless, see the November 2022 response that agrees but makes the point that he didn't consider relevant homogeneous solutions which matter.

Per my post #144 just now, if these homogeneous solutions do indeed matter, it is by showing that with the GEM corrections in these solutions, the disconnect between visible matter and observed rotation curves is worse than in the Newtonian approximation, not better.

 

[PeterDonis]

https://arxiv.org/abs/2205.03091

I don't understand equation 76 in this paper, which appears to be a crucial one. This equation says that in what is called the "strong gravitomagnetic limit", the quantity $\chi$ is of order $c^0$, the same as $\gamma^2 ( - H )$. But in the general equation for the low energy expansion of $\chi$, equation 55, the leading term is of order $c^{-1}$. So I don't understand where an order $c^0$ term would come from.

 

[PeterDonis]

See this explanation. In short, it's suppressed by a factor of 1/c^2 and would otherwise be negligible, but there's a wrench in the works since it changes the characteristics of the PDEs critically (hyperbolic to elliptic).

Where is this discussed in the paper you reference here?

 

[wumbo]

Where is this discussed in the paper you reference here?

That paper is just a link to background on GEM and linearized gravity, which has qualitatively different behavior to Newtonian gravity despite being linear. It's a response to this:

since the claims in question are that nonlinear effects are much more significant (as in, orders of magnitude more significant

which is wrong. The effects are from linear equations (take a look yourself!) and need not be more significant, just big enough to violate the underlying assumptions of the perturbation expansion you do in the Newtonian limit.

The following sentence is my interpretation of the critical difference between what is normally done and what linearized GR keeps around. Poisson's equation is elliptic. The linearized GR equations are hyperbolic, which is required to preserve causality. That's something an intro PDE course covers, and an intro perturbation theory course covers the issues with doing the usual c -> infinity rule.

If you demand a PDF that goes into detail read this

 

[PeterDonis]

The effects are from linear equations

The GEM effects are. But some of the effects that Deur is claiming (and a paper by Deur was what started this thread) are not.

(take a look yourself!)

Take a look at the entire thread before snarking.

The following sentence is my interpretation

Ok. I'm not sure I agree with it, but discussion of personal interpretations is off topic. At least I'm clear now that I don't need to look in the paper itself for those claims.

If you demand a PDF that goes into detail read this

I'm not sure how this paper is relevant to what we're discussing.

 

[timmdeeg]

It is noticeable that several authors take reference to Gravitomagnetism in order to explain the observed flat galactic rotation curves, e.g.

On the gravitomagnetic origins of the anomalous flat rotation curves of spiral galaxies​

https://www.sciencedirect.com/science/article/abs/pii/S1384107618301970?via=ihub

Galactic rotation curve and dark matter according to gravitomagnetism​

https://link.springer.com/article/10.1140/epjc/s10052-021-08967-3

Galactic Dynamics in General Relativity: the Role of Gravitomagnetism
https://arxiv.org/pdf/2112.08290.pdf

On the rotation curve of disk galaxies in General Relativity​

https://arxiv.org/abs/2207.09736

while Alexandre Deur seems to be quite alone arguing the gravitational field has an energy and hence gravitates too leading to field self-interaction:

Relativistic corrections to the rotation curves of disk galaxies
https://arxiv.org/pdf/2004.05905.pdf

The reason could be that GR experts do trust Gravitomagnetism but don't trust Deur's field self-interaction which based on the heuristic that "GR and QCD have similar Lagrangians".

Now let me come to my questions: e.g. G.O.Ludwig says:

Near the origin, where the gravitational field did not build up yet, the rotation curve shows a linear rise. Farther away from the origin the rotation speed shows a transition to a nearly constant value. At large distances the gravitomagnetic field is sufficiently intense to balance the decaying gravitational and centrifugal forces. Although the relativistic effects are weak (with a beta ratio of the order of 1/2000), the nonlinear coupling provides the mechanism that drives the transition in the rotation profile.

Is the mentioned "nonlinear coupling" thing without controversy or rather an interpretation of the author?

According to Deur the field self-interaction decreases with increasing sphericity of elliptic galaxies:

An important point for the present article is that the morphology of the massive structures in which gravity may be trapped determines how effective the trapping is: the less isotropic and homogeneous a system is, the larger the trapping is. For example, this implies a correlation between the missing mass of elliptical galaxies and their ellipticities. The correlation was predicted in [4] and subsequently verified in [9].

Do the Gravitomagnetism paper predict something similar or something else which could find support by observation?

​

 

[wumbo]

The GEM effects are. But some of the effects that Deur is claiming (and a paper by Deur was what started this thread) are not.Take a look at the entire thread before snarking.Ok. I'm not sure I agree with it, but discussion of personal interpretations is off topic. At least I'm clear now that I don't need to look in the paper itself for those claims.I'm not sure how this paper is relevant to what we're discussing.

Don't move the goal posts -- we were discussing ciotti's paper and it's relevance to GEM effects being significant, the topic moving beyond just Duer's nonlinear claims, which I have no real opinion on.

My point is that the common interpretation of GEM's post Newton _linear_ effects being negligible is untrue, for reasons covered in perturbation theory 101.

I'm not so sure. Fig. 1 in the paper you cite does show numerically different velocity profiles vs. the Newtonian ones as a function of the parameter $\lambda$, which measures the "strength" of the GEM effects, and the corrections, as the authors state, are around 10% to 15%, so not negligible. But all of those profiles have the same general shape as the Newtonian one. None of the profiles are flatter than the Newtonian one once the "peak" is reached, which is what would be required to help reduce the disconnect between the visible matter and observed rotation curves without adding dark matter to the model. Indeed, if anything they are less flat, meaning that these corrections make the problem worse, not better.

And if personal opinions are off topic, I'll point out that in fig 1, the purple curve is flatter than the blue Newtonian curve unless your opinion of flat is very different from the common definition. You can quite clearly see the curvature of the purple curve starts to decrease (I.e. It flattens) more than the blue curve as the normalized radius increases. The Kuzmin-Toomre disk shows it especially.

Regardless, as soon as the negligible effects become non negligible the entire approximation needs to be thrown out and redone because those effects must be taken into account from the get-go. So any model of galactic rotation velocity is invalid unless it includes the basic linear extensions--let alone any nonlinear ones as Duer claims.

 

[PeterDonis]

Don't move the goal posts -- we were discussing ciotti's paper

This thread is not discussing Ciotti's paper. It started by citing a claim by Deur. It then expanded to a general discussion of possible alternate explanations for the phenomena that are attributed to dark matter. Ciotti's paper and the discussion of GEM effects is one piece of that, but not the only piece. It might be the only piece you want to discuss, but this is not your thread and it is not limited to your preferred topic.

in fig 1, the purple curve is flatter than the blue Newtonian curve

Possibly (I would want to look at the actual numbers rather than try to eyeball a graph), but it still doesn't look like an actual galaxy rotation curve. The speed falls off significantly from the peak, as it does for all the curves in the paper. In an actual galaxy rotation curve (at least in galaxies where significant amounts of dark matter are hypothesized to be present), it doesn't fall off at all; it rises to a "peak" value and then stays at that peak. None of the curves in the GEM papers look like that.

 

[wumbo]

This thread is not discussing Ciotti's paper. It started by citing a claim by Deur. It then expanded to a general discussion of possible alternate explanations for the phenomena that are attributed to dark matter. Ciotti's paper and the discussion of GEM effects is one piece of that, but not the only piece. It might be the only piece you want to discuss, but this is not your thread and it is not limited to your preferred topic.

Sure, but it _is_ on-topic and the response was to when it _did_ discuss Ciotti's paper in the context of critiquing Duer. Regardless, I think we agree that this topic is broader than just Duer's paper.

Possibly (I would want to look at the actual numbers rather than try to eyeball a graph), but it still doesn't look like an actual galaxy rotation curve. The speed falls off significantly from the peak, as it does for all the curves in the paper. In an actual galaxy rotation curve (at least in galaxies where significant amounts of dark matter are hypothesized to be present), it doesn't fall off at all; it rises to a "peak" value and then stays at that peak. None of the curves in the GEM papers look like that.

Given that Fig 1 is limited to 5 curves I can't speculate on whether there's a value of lambda that matches exactly, but I don't think you need numbers to verify that the purple curve flattens out. Would need to infer the required lambda from the observed data (if it exists). And you'd need to specify the initial mass distribution for the GEM solution which will significantly change the shape of the curve. I think Ludwig's paper chooses a different distribution, spheroidal Miyamoto-Nagai, than thin disks.

Even if it's not a perfect match, any dark matter that can be explained by "doing the math correctly" really should be eliminated from the standard models. It's kind of troubling that it hasn't been done already, especially since it's so unsophisticated mathematically.

It is noticeable that several authors take reference to Gravitomagnetism in order to explain the observed flat galactic rotation curves, e.g. while Alexandre Deur seems to be quite alone arguing the gravitational field has an energy and hence gravitates too leading to field self-interaction:

Relativistic corrections to the rotation curves of disk galaxies
https://arxiv.org/pdf/2004.05905.pdf

The reason could be that GR experts do trust Gravitomagnetism but don't trust Deur's field self-interaction which based on the heuristic that "GR and QCD have similar Lagrangians".

Duer isn't alone in saying the field itself has energy, that's the crux of the sticky bead argument. GEM is linearized so it ignores self interaction entirely. No idea if that's a valid assumption to make. Regardless, even without self-interaction you see significant contributions.

 

[PeterDonis]

Duer isn't alone in saying the field itself has energy, that's the crux of the sticky bead argument.

This is due to the fact that, in GR, two different meanings of "energy" that we are used to having go together, don't. The sticky bead argument shows that gravitational waves can do work, so they "carry energy" in that sense. But it is also true that spacetime curvature in itself has no stress-energy, and gravitational waves are spacetime curvature; so gravitational waves, and the "gravitational field" in general, do not "carry energy" in that sense.

 

[PeterDonis]

The reason could be that GR experts do trust Gravitomagnetism but don't trust Deur's field self-interaction which based on the heuristic that "GR and QCD have similar Lagrangians".

I would put it that gravitomagnetism and the GEM framework in general are already well known to be derivable from the Einstein Field Equation, whereas Deur's model is not. One thing I have not seen in any of Deur's papers, which surprises me somewhat, is an actual derivation of his proposed interactions (things like gravitational "flux tubes") from the GR Lagrangian. Deriving them from the QCD Lagrangian is, AFAIK, straightforward, so if his heuristic is correct, it should also be straightforward to show that such effects exist in GR.

 

[timmdeeg]

Duer isn't alone in saying the field itself has energy, that's the crux of the sticky bead argument. GEM is linearized so it ignores self interaction entirely. No idea if that's a valid assumption to make. Regardless, even without self-interaction you see significant contributions.

Ok, but it seems no one else claims that field self-interaction leads to increased gravity within matter distributions and decreased gravity outside matter distributions. And thus to a completely different universe as his Friedmann Equations are showing.

 

[timmdeeg]

I would put it that gravitomagnetism and the GEM framework in general are already well known to be derivable from the Einstein Field Equation, whereas Deur's model is not. One thing I have not seen in any of Deur's papers, which surprises me somewhat, is an actual derivation of his proposed interactions (things like gravitational "flux tubes") from the GR Lagrangian. Deriving them from the QCD Lagrangian is, AFAIK, straightforward, so if his heuristic is correct, it should also be straightforward to show that such effects exist in GR.

That he didn't do it "Deriving them from the QCD Lagrangian" could be a sign that it's not possible for principal reasons, e.g. lack of consistency with Gr. If it were possible though that derivation would support his claim enormously.

 

[ohwilleke]

I would put it that gravitomagnetism and the GEM framework in general are already well known to be derivable from the Einstein Field Equation, whereas Deur's model is not. One thing I have not seen in any of Deur's papers, which surprises me somewhat, is an actual derivation of his proposed interactions (things like gravitational "flux tubes") from the GR Lagrangian. Deriving them from the QCD Lagrangian is, AFAIK, straightforward, so if his heuristic is correct, it should also be straightforward to show that such effects exist in GR.

The first published paper is working from the gravitational Lagrangian.

 

[ohwilleke]

Another paper in the self-interaction paradigm rather than the GEM paradigm is this one: W.M. Stuckey, Timothy McDevitt, A.K. Sten, Michael Silberstein, "The Missing Mass Problem as a Manifestation of GR Contextuality" 27(14) International Journal of Modern Physics D 1847018 (2018). DOI: 10.1142/S0218271818470181.

 

[wumbo]

Ok, but it seems no one else claims that field self-interaction leads to increased gravity within matter distributions and decreased gravity outside matter distributions. And thus to a completely different universe as his Friedmann Equations are showing.

My understanding of Deur's papers over the years:

He's a QCD physicist, so he approaches the GR Einstein-Hilbert Lagrangian via a QFT-like Dyson series expansion to 1-loop level about flat spacetime as opposed to a "traditional" method of simplifying the PDEs down first. They are both valid but GR isn't usually handled this way, so I'm not surprised it's difficult to connect it to the literature and that he's alone in claiming this.

In doing so, he can make the analogy between QCD and GR: both are non-abelian gauge theories, with self interacting gauge Bosons, so what qualitative behavior transfers between the two? I don't believe he's using the QCD as anything more than a much more well-studied example of a self-interacting gauge theory, even if GR can't be a YM theory.

So I think he then posits that gravitational fields form something similar to flux tubes, with a non-trivial amount of mass-energy in the binding of gravitational bound systems. My guess is that he connected the 95% dark matter/energy to 5% non-dark matter/energy ratio as something analogous to the 99%-QCD binding energy to 1% quark intrinsic mass ratio. I'm not too sure about this aspect.

It's interesting, for sure, and it's nice to have an expert in perturbation theory examine GR in the same manner. That said, the notion of energy in GR is wack, so ¯\_(ツ)_/¯
edit: apparently Cooperstock still argues against the stick bead

 

[PeterDonis]

The first published paper is working from the gravitational Lagrangian.

As I recall, he doesn't actually derive the claims like flux tubes from the Lagrangian, he just writes down the GR Lagrangian and then argues by analogy with the QCD Lagrangian. It's been some time since I looked at his papers, though.

 

[timmdeeg]

My understanding of Deur's papers over the years:

...
So I think he then posits that gravitational fields form something similar to flux tubes, with a non-trivial amount of mass-energy in the binding of gravitational bound systems. My guess is that he connected the 95% dark matter/energy to 5% non-dark matter/energy ratio as something analogous to the 99%-QCD binding energy to 1% quark intrinsic mass ratio. I'm not too sure about this aspect.

Thanks for your answer, you are much deeper in the details that I (not a physicist) am able to get.

In his paper Relativistic corrections to the rotation curves of disk galaxies Deur computes numerically the distortion of the field lines which is shown in Fig. 3. whereby the bending of the field lines "increases gravity’s strength". But if so why isn't this increase a source of gravity in the "sense" of the stress-energy tensor?

If I understand the meaning of Feynman's "stick bead" correctly it is more than just a heuristic.

 

[ohwilleke]

But if so why isn't this increase a source of gravity in the "sense" of the stress-energy tensor?

In Einstein's Field Equations which he is relying upon in that paper, the self-interaction of gravity effects appear on the left hand side, not on the right hand side that include the stress-energy tensor.

 

[ohwilleke]

Deur's analysis of the self-interaction of gravitational fields addresses the Hubble tension.

One of the most important problems vexing the ΛCDM cosmological model is the Hubble tension. It arises from the fact that measurements of the present value of the Hubble parameter performed with low-redshift quantities, e.g., the Type IA supernova, tend to yield larger values than measurements from quantities originating at high-redshift, e.g., fits of cosmic microwave background radiation. It is becoming likely that the discrepancy, currently standing at 5σ, is not due to systematic errors in the measurements.

Here we explore whether the self-interaction of gravitational fields in General Relativity, which are traditionally neglected when studying the evolution of the universe, can explain the tension. We find that with field self-interaction accounted for, both low- and high-redshift data are simultaneously well-fitted, thereby showing that gravitational self-interaction could explain the Hubble tension. Crucially, this is achieved without introducing additional parameters.

Corey Sargent, Alexandre Deur, Balsa Terzic, "Hubble Tension and Gravitational Self-Interaction" arXiv:2301.10861 (January 25, 2023).

 

[mitchell porter]

How it works, according to Deur et al:

Gravitational self-interaction causes tighter binding of localized massive systems. This also leads to depletion of the gravitational field at large distances. The magnitude of gravitational depletion changes over the course of cosmic time, in a way depending on the number and type of gravitationally bound systems that have formed.

The Hubble tension can be resolved because the early universe was relatively homogeneous, so there was no gravitational depletion at high redshift; but once structure formation began, so did gravitational depletion.

A specific depletion function is proposed (equation 2, based on a 2017 paper mentioned in this thread at #136), depending on the rate and frequency of galactic mergers (parameters b and A in equation 2). The value of these parameters is inferred by fitting the depletion function to various cosmological data.

 

[Madeleine Birchfield]

Gravitational self-interaction as the source of the Hubble tension discrepancy is much more convincing than any of the early dark energy cosmological models proposed to resolve the Hubble Tension. It is also favored by Occam's razor over the FLRW Lambda CDM cosmological model since it doesn't assume homogeneity and isotropy for the universe. Furthermore, Deur also explains in the article why gravitational self-interaction yields a non-FLRW metric for the universe.

 

[PeterDonis]

Gravitational self-interaction causes tighter binding of localized massive systems. This also leads to depletion of the gravitational field at large distances.

So basically, the gravitational self-interaction is an additional negative contribution (same sign as the ordinary gravitational binding energy) to the mass of the system?

If this is the purported explanation for galaxy rotation curves instead of dark matter, it doesn't seem like it could work: the issue with rotation curves is that there is more mass present, based on the rotation curves, than the visible matter can account for, not less.

 

[ohwilleke]

So basically, the gravitational self-interaction is an additional negative contribution (same sign as the ordinary gravitational binding energy) to the mass of the system?

If this is the purported explanation for galaxy rotation curves instead of dark matter, it doesn't seem like it could work: the issue with rotation curves is that there is more mass present, based on the rotation curves, than the visible matter can account for, not less.

No. The concept is driven by conservation of energy.

It is easiest to explain in terms of gravitons, although the graviton mechanism isn't actually necessary and it could work equally well in terms of fields or space-time curvature that are less intuitive (to me, anyway, your mileage may vary).

In a graviton heuristic, mass-energy spews out gravitons at a fixed rate per gram or GeV/c² or whatever other unit you want to use.

Gravitational self-interaction causes more gravitons to stay within a galaxy pulling it things in the galaxy towards each other more strongly than in Newtonian gravity.

But as a result, fewer gravitons escape the galaxy to causes galaxies to be attracted to each other. Hence, the gravitational field between galaxies is weaker than in Newtonian gravity, i.e. it is depleted.

Over cosmological time scales, the average amount of gravitational depletion between galaxies represented by the depletion function looks like this (from the paper):

The more non-spherical galaxy and galaxy cluster formation takes place in the universe (which gives rise to dark matter phenomena through gravitational self-interaction), the more the gravitational pull between galaxies is depleted.

This also, incidentally, explains the cosmic coincidence, i.e. why the aggregate amount of apparent dark matter is on the same order of magnitude as the aggregate amount of apparent dark energy, in the current era.

To be clear, the dynamical effects of gravitational depletion are not exactly equivalent to dark energy or a cosmological constant on cosmological time scales, although the direction of the effect (i.e. the tendency of galaxies to move apart from each other more strongly than they would in Newtonian gravity) is the same. For example:

* Unlike the cosmological constant in LambdaCDM, the depletion of the gravitational field between galaxies is not constant and the "Hubble constant" is likewise not constant.

* Unlike dark energy, depletion of gravitational pull due to gravitational self-interaction has an asymptotic upper limit. It can't get below the point at which there is no gravitational attraction between galaxies.

 

[mitchell porter]

Actually I think the simplest intuition for the proposed mechanism is in terms of lines of gravitational flux (this is in one of Deur's papers). The galaxy is a source of gravitational flux. Because of self-interaction, the gravitational flux lines cluster together (similar to gluons forming strings in QCD). So masses up to the edge of the galaxy are absorbing more gravitational flux than there would be without self-interaction; but at a large enough distance (far beyond the edge of the galaxy, and ultimately at cosmological distances), there is less gravitational flux than otherwise, because of the extra absorption within galaxies.

So "dark energy" - a less tighly bound cosmos - is a long-distance consequence of "dark matter" - more tighly bound local systems.

@PeterDonis is that making sense?

 

[PeterDonis]

The concept is driven by conservation of energy.

So what energy is involved? That's what I'm trying to understand. Ordinary gravitational binding energy is negative: the total rest mass of a gravitationally bound system is less than the sum of the rest masses of its constituents.

What you and @mitchell porter are describing sounds like an additional effect that makes the system more tightly bound: that means its total rest mass will be less than the "standard" GR gravitational binding energy calculation would indicate, because the effect you're describing makes an additional negative contribution, i.e., it's an additional form of gravitational binding energy.

But that doesn't seem to be what galaxy rotation curves are telling us; they're telling us the masses of the galaxies are larger than a simple calculation based on the visible matter would indicate. The effect you're describing seems to be saying it should be smaller.

So "dark energy" - a less tighly bound cosmos - is a long-distance consequence of "dark matter" - more tighly bound local systems.

@PeterDonis is that making sense?

No, because more tightly bound local systems means their masses are smaller than a simple calculation based on the visible matter would indicate. But galaxy rotation curves seem to be telling us the masses of galaxies are larger than a simple calculation based on the visible matter would indicate. So the effect you're describing seems to be in the opposite direction from what the evidence indicates.

 

[PeterDonis]

"dark energy" - a less tighly bound cosmos

That's not what dark energy is. A less tightly bound cosmos--less density of ordinary matter and energy--would mean deceleration, but of a smaller magnitude. But dark energy is shown by acceleration; its equation of state is completely different from that of ordinary matter and energy.

 

[mitchell porter]

If you want to see Deur et al's own words, see page 2 of the paper in #164, starting with "One consequence of SI in QCD" (SI stands for self-interaction; the authors talk about "GR-SI", GR with self-interaction).

They are definitely saying there's no repulsive force, only an increasing screening of gravitation. It's a little like those attempts to explain the accelerating expansion as due to us being near the center of a giant void (underdensity).

 

[PeterDonis]

They are definitely saying there's no repulsive force, only an increasing screening of gravitation. It's a little like those attempts to explain the accelerating expansion as due to us being near the center of a giant void (underdensity).

Ok, so I'm understanding the basic idea correctly. I'm just not sure it's actually consistent with the data (I have the same feeling about the underdensity void arguments). But it's an open area of research; hopefully as we get more and better data and more work is done on these various models, we will be better able to distinguish between them using observations.

 

[ohwilleke]

In terms of the magnitude of the effects involved, it is also worth observing that the self-interaction effects hypothesized are second order effects that are swamped by the first order direct gravitational effects except in very weak fields.

For example, in a spiral galaxy, up to the radius where the acceleration due to the gravitational field is more than the MOND constant a₀ the self-interaction effect is just noise that is swamped by the first order, approximately Newtonian gravitational field. So, you have an enhancement of the primary approximately Newtonian gravitational field that is material only at the fringes of the galaxy where the fields are weakest. The depletion effect, likewise, is a depletion from gravitational fields experienced by observers well outside that spiral galaxy in places where that gravitational field is already extreme weak (and the depletion at the level of granularity of a single galaxy is directional with the least depletion in the plane of the spiral galaxy and the most above and below that plane - the depletion function is averaging out that directionality).

This is a much more parsimonious way to get the same effects than having a uniform scalar field that pervasively fills all of space time to generate a "dark energy" effect, or a dark matter halo of particles enveloping the entire galaxy and basically cancelling out in the central parts of the galaxy to produce the "dark matter" effect.

I also wonder if the seeming acceleration may be due to changes in the amount of depletion that is there over time.

Part of the issue is also that the astronomy data is currently interpreted in a moderately model dependent manner, and it isn't obvious that trying to quantify and explain that data in this very different model wouldn't lead to a different top level interpretation of what the raw data implies.

For example, none of the omega fractions in LambdaCDM cosmology are numbers that correspond to an actual physical reality in this model.

 

[timmdeeg]

On page 5 in https://arxiv.org/pdf/2301.10861.pdf Deur mentiones zL = 1728, whereby "zL is the redshift at the time of last rescattering". How does this make sense remembering the temperatures 3000 K at last scattering and ~ 2.7 K today?