A LQG Legend Writes Paper Claiming GR Explains Dark Matter Phenomena

[Bandersnatch]

Why do they use the term 'last rescattering', though? Is that something different than last scattering, or just a mannerism?

Come to think of it, probably recombination and last scattering jumbled together.

 

[timmdeeg]

Why do they use the term 'last rescattering', though? Is that something different than last scattering, or just a mannerism?

Come to think of it, probably recombination and last scattering jumbled together.

I think last scattering, recombination and decoupling have all the same meaning, the short period where the universe changed from opaque to transparent. See also here post #3:

https://www.physicsforums.com/threa...diation-temperature-at-recombination.1049591/

 

[Bandersnatch]

Yes, they do. It's 'rescattering' I've never seen before.

 

[timmdeeg]

Indeed, one "last rescattering", two "last scattering", a bit weird, but seems to mean the same. I don't understand "re" anyway.

 

[PeterDonis]

I don't understand "re" anyway, because at least to my knowledge there wasn't any scattering before that time.

Yes, there was: the term "last scattering" is used precisely because that's when the scattering that had been going on until then, because the matter in the universe was plasma (electrons and ions), stopped, because the matter in the universe became neutral atoms.

 

[timmdeeg]

Yes, there was: the term "last scattering" is used precisely because that's when the scattering that had been going on until then, because the matter in the universe was plasma (electrons and ions), stopped, because the matter in the universe became neutral atoms.

Yes, thanks. I've recognized that and deleted this sentence before I've seen your answer.

 

[timmdeeg]

There is another issue with that new paper Hubble Tension and Gravitational Self-Interaction which is unclear for me.

They mention the current values:

The discrepancy presently reaches a 5σ significance: the combined high-z measurements yield 67.28 ± 0.60 km/s/Mpc while the combined low-z measurements yield H0 = 73.04 ± 1.04 km/s/Mpc [8].

But they don't mention the values which according to their model match well without tension.

How to interpret this statement:

Finally, we verify that with the constrained parameters, the GR-SI fit reproduces better the CMB power spectrum with the low-z value of H0 than with the high-z H0 determination. We also find that if H0 is left a free parameter, its best fit value agrees with the low-z determination rather than the high-z one. This indicates an absence of Hubble tension in the GR-SI model.

Does this mean that their GR-SI fit yields the low-z value ~ 73 of the Hubble constant also for the early (high-z) universe? But they don't mention that explicitly somewhere.

Any ideas?

 

[ohwilleke]

There is another issue with that new paper Hubble Tension and Gravitational Self-Interaction which is unclear for me.

They mention the current values:

The discrepancy presently reaches a 5σ significance: the combined high-z measurements yield 67.28 ± 0.60 km/s/Mpc while the combined low-z measurements yield H0 = 73.04 ± 1.04 km/s/Mpc [8].

But they don't mention the values which according to their model match well without tension.

How to interpret this statement:

Finally, we verify that with the constrained parameters, the GR-SI fit reproduces better the CMB power spectrum with the low-z value of H0 than with the high-z H0 determination. We also find that if H0 is left a free parameter, its best fit value agrees with the low-z determination rather than the high-z one. This indicates an absence of Hubble tension in the GR-SI model.

Does this mean that their GR-SI fit yields the low-z value ~ 73 of the Hubble constant also for the early (high-z) universe? But they don't mention that explicitly somewhere.

Any ideas?

In their model they aren't really predicting a Hubble constant, which, of course, isn't a constant in their model anyway. They are using the Hubble constant measurements as inputs to fit their depletion function, rather than as outputs predicted from some other inputs.

Their depletion function, a bit like the proportions of ordinary matter, dark matter, and dark energy in LCDM, isn't something that one can determine with all parameters set with precision from first principles. It is a summary description of the way a big complex system of the structure of the universe at various points in time evolved that has some free parameters to match to observations. The evolution and impacts of their depletion function, however, once you fix the parameters, can be determined more precisely.

What they are saying is that you can adjust the parameters in their model such that it is consistent with both low-z and high-z values of the Hubble "constant" thereby alleviating the tension. Hence, from their perspective, you can use the Hubble constant measurement to calibrate their deletion function's free parameters.

we verify that with the constrained parameters, the GR-SI fit reproduces better the CMB power spectrum with the low-z value of H0 than with the high-z H0 determination

This is neither here nor there, since the Hubble constant isn't a constant in their model.

It is an observation which is a bit surprising. But no really big conclusions are drawn from it. It is just mentioned.

It is surprising because you would think that the high-z Hubble constant measurement ought to produce the best CMB fit since the CMB arises at high-z.

But, it doesn't necessarily mean much.

The determination of the high-z Hubble constant value in LCDM is an output that is derived (solely) from the input of the CMB fit. This Hubble constant determination from the CMB fit is model dependent. So, it isn't necessarily that crazy that in a different model, the CMB fit would imply a modestly different inferred Hubble constant value.

Comparing Hubble constant values in a GR-SI model isn't truly an apples to apples comparison with the Hubble constant in the LCDM model. The models assign different meaning to what observations of the Hubble constant at a particular point in time mean, even though the observational measurement at a point in time is the same. And, there is no way to directly measure the Hubble constant at the time of the CMB imprint apart from looking at the CMB to determine if the Hubble constant inferred in LCDM from the CMB fit is consistent with other ways of observing it in that era.

 

[timmdeeg]

In their model they aren't really predicting a Hubble constant, which, of course, isn't a constant in their model anyway. They are using the Hubble constant measurements as inputs to fit their depletion function, rather than as outputs predicted from some other inputs.

A Hubble constant can be determined model dependent by observation. I have no clue how their "GR-SI fit" works but if they say it " ... reproduces the CMB power spectrum with the low-z value ..." doesn't seem to include a calculation of the Hubble constant. Perhaps implicitly?

I have been thinking differently. Supposed their fit includes the matter density at certain times then it should be possible the obtain the values of the corresponding Hubble constants.

Focusing at the time of the CMB makes it quite easy. Then in their model the universe is isotropic, so no field self-interaction and thus the depletion function D(z) has the value one, FIG. 1. So neglecting the K-term H² is proportional to the baryonic matter density $\rho$, resulting from the CMB data, whereas in the L-CDM model $\rho$ includes Dark Matter.
It should be possible to calculate the Hubble constant at low-z as $\rho$ goes with 1/a³ using D(z) and their CMB redshift.

That's just my reasoning, no guarantee for correctness.

 

[member 673059]

I'm slightly suspicious of their use of the value of the high redshift around 67.28 ± 0.60 in their model, because the original calculations of the Hubble constant of the Planck data measurements are based upon FLRW and Lambda; if one gets rid of FLRW and Lambda and tries to recalculate the Hubble constant from Planck data with a different model, wouldn't one get a different Hubble constant than 67.28 ± 0.60?

Unless I am reading the article wrong and they are saying that they did recalculate the Hubble constant from Planck data and ended up getting the low redshift around 73.04 ± 1.04 in their model.

 

[timmdeeg]

I'm slightly suspicious of their use of the value of the high redshift around 67.28 ± 0.60 in their model, because the original calculations of the Hubble constant of the Planck data measurements are based upon FLRW and Lambda; if one gets rid of FLRW and Lambda and tries to recalculate the Hubble constant from Planck data with a different model, wouldn't one get a different Hubble constant than 67.28 ± 0.60?

I think this is a good question. In general their model is based on the assumption that the universe is anisotropic with the exception of the early universe as shown in Fig. 1 in Hubble Tension and Gravitational Self-Interaction, where the Depletion function has the value 1 for z > 10. So that's in accordance with the L-CDM model but the matter density $\rho$ isn't. Their matter density is purely baryonic whereas the L-CDM matter density includes Dark Matter in addition.

At that time neglecting $\Lambda$ we obtain

So the deviating values of the matter densities should produce deviating values of the Hubble constant, if I see it correctly.

 

[timmdeeg]

He comes at it by analogy to QCD which is, of course, formulated as a quantum theory. And, the logic of why it should have that effect is a lot more obvious when formulated in quantum form and in a way that can exploit known analogies in QCD.

But, fundamentally, the self-interaction that matters is already present in classical GR. It is just a lot harder to see when you try to work directly with Einstein's field equations, in which, of course, the gravitational field isn't on the right hand side in the stress-energy tensor, but instead appears on the left hand side as the non-linearity in the gravitational field part.

In this paper

On the Invention of Dark Matter and Dark Energy Craig Mackay, Institute of Astronomy, Cambridge, UK.*

the author argues with the Einstein-Hilbert Lagrangian (page 6), whereby the nonlinearities take the self-interaction into account.

The polynomial comes from expanding gµν around the constant metric ηµν , with the gravitational field φµν = gµν - ηµν . The square brackets indicate sums over Lorentz-invariant terms. Setting n = 0 gives L = [δφδφ], the Lagrangian for Newtonian dynamics. This result is consequent on suppressing higher order (n>0) terms in the Lagrangian and assuming v/c <<1. This asserts that the system under consideration is homogeneous and isotropic, effectively spherically symmetric by setting the Page 7 of 15 energy-momentum tensor, Tµν to be diagonal. Nonlinearities are always part of the full Lagrangian but their effects are suppressed where the system is homogeneous and isotropic. Nonlinearities which arise in the Lagrangians for both GR and QCD are consequent on field self-interaction.
...
Most galaxies and clusters of galaxies are clearly inhomogeneous and far from uniform. The additional terms that give rise to the nonlinearity or field self-interaction start to become important when √ (GM/L) >~ 10-3 (in “natural” units, see van Remortel, N. (2016) ),where L is the characteristic scale for the system (Deur et al, 2020).

The paper doesn't seem to be peer-reviewed though.

 

[member 673059]

Barker, Hobson, and Lasenby wrote an article stating that the gravitational flux collapse effect isn't strong enough to explain the MOND/dark matter effects in galaxies:

https://arxiv.org/abs/2303.11094

Does that also weaken the gravitational depletion effect in Deur's model to the point where it isn't able to explain dark energy or the Hubble tension?

 

[PeterDonis]

Does that also weaken the gravitational depletion effect in Deur's model

The paper references some of Deur's paper, and the term "colour-confining chromoelectric flux tube model" in the abstract certainly looks to me like a reference to Deur's proposals.

 

[member 673059]

On the other hand, I think it should be possible to simply have a non-FLRW cosmological model with a depletion function without any reference to the underlying dynamics from which the depletion function is derived from.

 

[PeterDonis]

think it should be possible to simply have a non-FLRW cosmological model with a depletion function without any reference to the underlying dynamics from which the depletion function is derived from

The effects under discussion are in the dynamics of individual galaxies, which are not modeled using FRW spacetimes.

Also, while one can of course always insert a purely phenomenological function into one's model (this is what MOND does), without at least some kind of idea about underlying dynamics, one has no way of knowing whether such a function is actually physically reasonable.

 

[Adrian59]

Just to note that there appears to be two threads covering the same topic, this one and one started by kodama on Deur's gravitational thesis; but following on from post #43:

He was really arguing even in the quantum gravity papers that it was the self-interaction of the field that produces the effect.

He comes at it by analogy to QCD which is, of course, formulated as a quantum theory. And, the logic of why it should have that effect is a lot more obvious when formulated in quantum form and in a way that can exploit known analogies in QCD.

I have read the reference posted by timmdeeg, below:

In this paper

On the Invention of Dark Matter and Dark Energy Craig Mackay, Institute of Astronomy, Cambridge, UK.*

the author argues with the Einstein-Hilbert Lagrangian (page 6), whereby the nonlinearities take the self-interaction into account.

I have to say that this is the paper I would have liked to have written! Craig Mackay is an established academic from a world renowned institution. This is not a contribution that should be ignored in this debate.

 

[Adrian59]

I know I am responding to some older comments here, but I am trying to understand the terminology. It appears that at times some comments, and even some of the references are not clear. My understanding is the original use of gravito-magnetism (or GEM) were theories that did contain a definite electromagnetic component to explain galaxy dynamics, and that strictly speaking this does not apply to consideration of non-linear gravitation, as below:

I suspect Deur wasn't cited since this paper is based on Ludwig GEM proposal rather than GR self-interaction. I asked Stacy McGaugh about Ludwig proposal and he regarded it as rubbish, while GEM is real and experimentally verified by Gravity probe B, it's far too weak to explain dark matter phenomena on his blog by orders of magnitude.

Or put another way:

In General Relativity, the gravitational field a.k.a. curvature of space-time arising from gravity, doesn't necessarily arise from the stationary rest mass of nearby matter. It can arise from anything that goes into the stress-energy tensor, and the relationship between the source and the field strength (curvature magnitude) can have a non-linear relationship to the size of the mass-energy that is the source of the field (curvature).

I suppose one could be purest and say that electromagnetism has an associated energy, so must contribute to the energy-momentum tensor, but at these scales I would think like Stacy McGaugh that this was negligible. So I would consider the term gravito-electro-magnetism etc reserved for theories that make the claim of a direct electro-magnetic effect.

 

[PAllen]

I know I am responding to some older comments here, but I am trying to understand the terminology. It appears that at times some comments, and even some of the references are not clear. My understanding is the original use of gravito-magnetism (or GEM) were theories that did contain a definite electromagnetic component to explain galaxy dynamics, and that strictly speaking this does not apply to consideration of non-linear gravitation, as below:

Wrong. GEM is simply a way to write the Einstein Field Equations of GR in a way that shows analogies to Maxwell's equations. It is neither a different theory, nor does it have any relation to EM fields. It is proven to be equivalent to first order Post Newtonian approximation of GR, which is all that is needed to be used for the most precise solar system computations. In your prior post, you mention a paper quoting a Post Newtonian correction term. This term is implicitly contained in the GEM formalism.

Or put another way:
I suppose one could be purest and say that electromagnetism has an associated energy, so must contribute to the energy-momentum tensor, but at these scales I would think like Stacy McGaugh that this was negligible. So I would consider the term gravito-electro-magnetism etc reserved for theories that make the claim of a direct electro-magnetic effect.

Again, this is all a misunderstanding of what GEM is.

 

[PAllen]

Re the Ciotti paper: Can general relativity play a role in galactic dynamics?
Provides a pretty thorough counter-argument to it IMO.

I'm surprised this wasn't worked out before. Linearized gravity is well known. You can't on one hand tell me that gravity propagates at a finite speed and on the other tell me it's irrelevant at cosmological distances. Trivially, there's frame dragging inside a spherical shell of mass in GR that has absolutely no connection to anything Newtonian. The cavalier approach to turning a weakly hyperbolic set of equations into an elliptic set has always to struck me as odd. Cooperstock has an example using the van Stockum cylinder of dust: https://doi.org/10.1142/S021827181644017X

It doesn't have to explain every use of dark matter to be valid. It should be a signal to take approximations to GR with far deeper care. Numerical relativity is sorely needed.

This paper is claimed to be thoroughly refuted by https://arxiv.org/abs/2303.06115, so the debate goes on. It establishes that the homogeneous solutions are irrelevant and endorses the Ciotti paper's conclusions.

 

[kodama]

. It is proven to be equivalent to first order Post Newtonian approximation of GR, which is all that is needed to be used for the most precise solar system computations. In your prior post, you mention a paper quoting a Post Newtonian correction term. This term is implicitly contained in the GEM formalism.

does

It is proven to be equivalent to first order Post Newtonian approximation of GR, which is all that is needed to be used for the most precise solar system computations. In your prior post, you mention a paper quoting a Post Newtonian correction term. This term is implicitly contained in the GEM formalism.

does Post Newtonian correction term also implicitly contained self interaction term

 

[PAllen]

does

does Post Newtonian correction term also implicitly contained self interaction term

"self interaction term" has many meanings to many people. I am discussing something much more well defined. In the 4th paragraph on p. 5 of https://www.ast.cam.ac.uk/sites/default/files/Invention_DM&DE_Rev1201_011221.pdf, a term in the Einstein-Infeld-Hoffman equations (that implement first order Post Newtonian approximation) is mentioned. This term is implicitly included in the GEM formalism. For any other question, you will need to be much more specific. IMO, GEM is sufficient to model any conceivably observable differences galaxy dynamics between Newtonian and non-Newtonian (assuming GR and nothing else as the gravity theory) and it suggests there are none of any significance.

 

[kodama]

"self interaction term" has many meanings to many people. I am discussing something much more well defined. In the 4th paragraph on p. 5 of https://www.ast.cam.ac.uk/sites/default/files/Invention_DM&DE_Rev1201_011221.pdf, a term in the Einstein-Infeld-Hoffman equations (that implement first order Post Newtonian approximation) is mentioned. This term is implicitly included in the GEM formalism. For any other question, you will need to be much more specific. IMO, GEM is sufficient to model any conceivably observable differences galaxy dynamics between Newtonian and non-Newtonian (assuming GR and nothing else as the gravity theory) and it suggests there are none of any significance.

Deur theory that proposed self interaction of GR is strong enough to explain MOND over Post Newtonian correction term

 

[PAllen]

Deur theory that proposed self interaction of GR is strong enough to explain MOND over Post Newtonian correction term

In my opinion, Deur theory is an alternate gravity theory suggested by analogies between the GR Lagrangian and QCD. This is fine, except he claims that certain results are standard GR with no convincing evidence - against much evidence to the contrary ( no method starting from GR equations reproduces his results). I just wish he would advocate for his results as a form of MOND.

 

[kodama]

In my opinion, Deur theory is an alternate gravity theory suggested by analogies between the GR Lagrangian and QCD. This is fine, except he claims that certain results are standard GR with no convincing evidence - against much evidence to the contrary ( no method starting from GR equations reproduces his results). I just wish he would advocate for his results as a form of MOND.

how could Deur theory be modified suggested by analogies between the GR Lagrangian and QCD. to get it to work as a form of MOND.?

 

[Adrian59]

Wrong.

I was asking for clarification (questions are never wrong only the answers), by suggesting alternatives, so which alternative is wrong? The trite response could be both, but I think that I have exposed the problem that the term is being used in possibly three different contexts: 1) Einstein weak field approximation; 2) a more specific combination of gravity and Maxwellian electromagnetism as in Ludwig's paper; and 3) to, erroneously, describe Deur's approach, see the Barker et al paper, referenced in this thread.

You appear to be opting for 1), but in the references, authors are using the term differently: hence my post!

 

[Adrian59]

This is fine, except he claims that certain results are standard GR with no convincing evidence - against much evidence to the contrary (no method starting from GR equations reproduces his results).

Have you seen any papers by Cooperstock. He uses GR to describe the rotation curves of galaxies without any modification, see refs below:

1) Cooperstock, F. and Tieu, S. (2005). ‘General Relativity Resolves Galactic Rotation without Exotic Dark Matter’. arXiv:astro-ph/0507619.

2) Carrick J. and Cooperstock, F. (2012). ‘General Relativistic Dynamics Applied to the Rotation Curves of Galaxies’. Astrophysics and Space Science; 337, Iss. 1: pp 321–329.

3) Cooperstock, F. and Magaheis, N. (2015). ‘Galactic mapping with general relativity and the observed rotation curves’. arXiv:1508.07491v1.

 

[PAllen]

how could Deur theory be modified suggested by analogies between the GR Lagrangian and QCD. to get it to work as a form of MOND.?

Maybe MOND is not the right word. I mean the broader class of modified gravity theories. In this case, a new theory suggested by GR and QCD analogies.

 

[PAllen]

Have you seen any papers by Cooperstock. He uses GR to describe the rotation curves of galaxies without any modification, see refs below:

1) Cooperstock, F. and Tieu, S. (2005). ‘General Relativity Resolves Galactic Rotation without Exotic Dark Matter’. arXiv:astro-ph/0507619.

2) Carrick J. and Cooperstock, F. (2012). ‘General Relativistic Dynamics Applied to the Rotation Curves of Galaxies’. Astrophysics and Space Science; 337, Iss. 1: pp 321–329.

3) Cooperstock, F. and Magaheis, N. (2015). ‘Galactic mapping with general relativity and the observed rotation curves’. arXiv:1508.07491v1.

Except that most GR experts disagree with Cooperstock, and recent papers quoted in this thread cite Cooperstock and claim to rule out his claims.

FYI: I've been aware of Cooperstock's work for many years, and was initially hopeful it would work out.

 

[PAllen]

I was asking for clarification (questions are never wrong only the answers), by suggesting alternatives, so which alternative is wrong? The trite response could be both, but I think that I have exposed the problem that the term is being used in possibly three different contexts: 1) Einstein weak field approximation; 2) a more specific combination of gravity and Maxwellian electromagnetism as in Ludwig's paper; and 3) to, erroneously, describe Deur's approach, see the Barker et al paper, referenced in this thread.

You appear to be opting for 1), but in the references, authors are using the term differently: hence my post!

Right after the word "wrong" was a paragraph of explanation. Instead of responding to this word, perhaps respond to what you don't understand in the following explanation.