Do gravitons interact with gravitons?

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KurtLudwig
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Summary
Do graviton-graviton interactions increase the gravitational binding of matter? Please read summary of "Implications of Graviton-Graviton Interaction to Dark Matter", arXiv:0901.4005v2 Astrophysics.
Specifically, how would graviton-graviton interactions increase the gravitational binding of matter?
I visualize that the beam of spacetime between two stars is very ordered, in that streams of gravitons from each star flow in very nearly anti-parallel paths. Will these gravitons interact?
Light beams do gravitationally interact, but very weakly: arXiv:gr-qc/9811052v1 by Faraoni and Dumse, "The gravitational interaction of light,..."
 
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Nonlinear effects of gravity (interaction between gravitons, in the particle picture) are completely negligible far away from compact objects. Otherwise they would be extremely prominent closer to these dense objects.
I visualize that the beam of spacetime between two stars is very ordered, in that streams of gravitons from each star
That doesn't make sense. Stars don't emit streams of gravitons.
 
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timmdeeg
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First reference in the OP:

https://www.sciencedirect.com/science/article/pii/S0370269309004870

To conclude, the graviton–graviton interaction suggests a mechanism to explain galaxy rotation curves and cluster dynamics.
It seems that the Graviton–Graviton Interaction yields the rotation curves of galaxies quite close to the observation, s. Fig. 3. How does this fit to
Nonlinear effects of gravity (interaction between gravitons, in the particle picture) are completely negligible far away from compact objects.
Is the Graviton-Graviton Interaction the authors are describing well accepted in scientific community?

To detect gravitons seems to be extremely challenging. But isn't it at least an indirect suggestion that - in the light of these calculations - they exist?
 
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I don't trust a model that only works in a single place. If it would work elsewhere, why wouldn't they discuss this?
 
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timmdeeg
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Indeed, it doesn't seem to be discussed with respect to the CMB, thanks.
 
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ohwilleke
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I don't trust a model that only works in a single place. If it would work elsewhere, why wouldn't they discuss this?

The paper cited is one of the earliest in a series of papers. Deur's approach has broad applicability (broader than MOND) to all types of galaxies (recognizing diversity in inferred dark matter amounts based upon the shape of ordinary matter distributions in a galaxy), to galaxy clusters including the Bullet Cluster, to the cosmic coincidence effect, to dark energy phenomena, and to the early formation of galaxies and certain other cosmology effects.

I think it has merit. Indeed, while my opinion as a lay person isn't really relevant, I think it is the most promising work on the questions of dark matter, dark energy and modified gravity published to date. If he's right, it is a Nobel Prize class insight, and the fact that several papers on this approach are published in peer reviewed journals (some with co-authors from the relevant field) and that it has made new observational predictions which have been confirmed means that it shouldn't be dismissed out of hand.

No publications or preprints challenge or dispute any of Deur's published work on gravity, and while simply being ignored isn't that praiseworthy, many other published papers purporting to explain dark matter phenomena with negligected non-Newtonian aspects of General Relativity (GR) not used by astronomers in estimating galaxy dynamics, using other aspects of GR, were the subject to quick published refutations. For example, it doesn't overlap at all with the gravitomagnetic effects claimed in G. O. Ludwig, "Galactic Rotation Curve and Dark Matter According to Gravitomagnetism" 81 European Physical Journal C 186 (2021) DOI 10.1140/epjc/s10052-021-08967-3 and by a few earlier authors, which have been criticized. Ludwig looks at the gravitational effects in GR arising from the motion of objects in a rotating galaxy that are neglected in the Newtonian approximation used by most astronomers in estimating galaxy dynamics, which critics argue are orders of magnitude too small to explain galaxy rotation curves. Deur is developing his work in a static approximation that ignores dynamic GR effects such as gravitomagnetism to make the calculations mathematically manageable.

This said, this approach, in part, because its author is primarily a QCD (quantum chromodynamics) physicist, rather than an astrophysicist or GR researcher, hasn't received much attention from other scientists in field: few other scientists have built on it, and no one has seriously examined and critiqued and reviewed it.

A 2014 power point presentation by the author provides a good introduction that is a bit less technical and hits the high points.

Graviton-graviton Interaction is universally accepted in scientific community as something that is present in any quantum gravity theory. See, e.g. Feynman, "Quantum Theory of Gravitation" (1963). See also here. There are gravitational field self-interactions in classical GR as well, although the structure of the typical form of the Einstein equations of GR obscure this effect by treating it differently than other sources of space-time curvature (because gravitational fields are not elements of the stress-energy tensor like other sources of space-time curvature). But the common assumption in the GR field (although not rigorously evaluated) is that this effect is small in galaxy scale systems. And, it is undisputed that it does cancel out in the case of spherically symmetric mass sources, which are the systems most carefully studied theoretically in GR.

Basically, Deur's argument is that this is a second order effect that is negligible near strong sources relative to primary graviton interaction which are much stronger at short distances relative to self-interaction effects, but that self-interaction effects are significant in extremely weak fields relative to primary graviton interactions, because the primary graviton interaction declines with distance proportionate to 1/r2 while the graviton self-interaction effect in a system with a disc-like geometry declines with distance proportionate to 1/r, so at a characteristic weakness of the field strength, the graviton-graviton interaction becomes stronger relative to the primary graviton interaction.

We know from observational tests of MOND (Modified Newtonian Dynamics developed by GR researcher Milgrom in 1983, creating a toy model formula to explain dark matter phenomena that is very well studied), that empirically, observed deviations from Newtonian gravitational approximations (which is what astronomers actually use to do models) which are attributed to dark matter, can be well described as occurring entirely via gravitational modifications in very weak gravitational fields. MOND has also made many ex ante predictions that have been proved correct, although it tends to underestimate dark matter phenomena at galaxy cluster scales and needs to be generalized to a relativistic version to accurately describe strong field gravitational effects, and is purely phenomenological rather than having a theoretical basis.

MOND investigators estimate the strength of the gravitational field below which non-Newtonian effects attributed to dark matter become relevant only in gravitational fields weaker than 1.2 x 10-10 ms-2. How weak is that? Assume that the only source of gravitational fields is the Sun. This is about 1/9th of the light year from the Sun, which is about 175 times the average distance of Pluto from the Sun. Pluto's average distance from the Sun is about 6 billion km). This is about 58 times more distant from the Sun that the heliosphere, which is a functional definition of where the solar system ends and deep interstellar space begins, that is about 18 billion km (120 astronomical units) from the Sun. As of February 2018, Voyager 1, the most distant man made object from Earth, was about 21 billion km from Earth, and Voyager 2, the second most distant man made object from Earth is about 17 billion km from Earth. Both were launched 43 years ago in 1977. These probes (which will run about of power around the year 2025), will reach this distance from the Sun about 2000 years from now in around 4000 CE.

The graviton self-interaction also cancels out if the geometry of the gravitational source is spherically symmetric (which the vast majority of theory papers in GR assume in their analyses at the outset). So, for example, it is almost non-existent in an interaction of a single star and a test mass, or in the vicinity of a nearly spherical elliptical galaxy.

If the mass is confined to a disk, the self-interactions cause the system to reduce from a three dimensional one to a two dimensional one, causing the force to have a 1/r form that we see in the MONDian regime of spiral galaxies.

In the geometries where Deur's approach approximate's MOND, the following formula approximate's the self-interaction term:

FG = GNM/r2 + c2(aπGNM)1/2/(2√2)r

where FG is the effective gravitational force, GN is Newton's constant, c is the speed of light, M is ordinary baryonic mass of the gravitational source, r is the distance between the source mass and the place that the gravitational force is measured, and a is a physical constant that is the counterpart of a0 in MOND (that should in principle be possible to derive from Newton's constant) which is equal to 4*10−44 m−3s2.

Thus, the self-interaction term that modifies is proportionate to (GNM)1/2/r. So, it is initially much smaller that the first order Newtonian gravity term, but it declines more slowly than the Newtonian term until it is predominant.

For two large point masses, you get a QCD flux tube-like solution which makes the self-interaction term stronger and explains why MOND underestimates dark matter phenomena in clusters where the geometry of the mass sources are best described as interacting large point masses.

This isn't the only work recognizing that dark matter phenomena behave differently based upon the shape of the underlying ordinary matter distributions involved. See also, e.g., Lorenzo Posti, S. Michael Fall "Dynamical evidence for a morphology-dependent relation between the stellar and halo masses of galaxies" Accepted for publication in A&A. arXiv:2102.11282 [astro-ph.GA] (February 22, 2021).

As noted below in the relevant papers section of this post, in an excerpt from one of Deur's papers, the CMB has not been worked out in this approach although general observations that suggest it may address the same issues that dark matter does are made. But, notably, a partially relativistic generalization of MOND (which Deur's approach tends to mimic in many systems) has reproduced the CMB of the LambdaCDM Standard Model of Cosmology with a MOND inspired theory.
We propose a relativistic gravitational theory leading to Modified Newtonian Dynamics, a paradigm that explains the observed universal acceleration and associated phenomenology in galaxies. We discuss phenomenological requirements leading to its construction and demonstrate its agreement with the observed Cosmic Microwave Background and matter power spectra on linear cosmological scales. We show that its action expanded to 2nd order is free of ghost instabilities and discuss its possible embedding in a more fundamental theory.
Constantinos Skordis, Tom Złosnik, "A new relativistic theory for Modified Newtonian Dynamics" arXiv (June 30, 2020).

So, it is very plausible the Deur's approach could also produce this result.

Deur's theory is attractive for a variety of reasons:

Observational Evidence Supports Deur's Model

* There is strong evidence, developed in the MOND context (and to a lesser extent in tests of other gravity modification approaches), that dark matter phenomena involved in galactic dynamics can be understood as a modification of conventional Newtonian approximations of gravity used in astronomy. This is in contrast to explanations of dark matter phenomena involving one or more dark matter particles beyond the Standard Model. Because the formula that Deur develops is observationally almost indistinguishable from MOND in spiral galaxies, in the circumstances where MOND works well, his theory benefits from this body of evidence.

* Deur's approach also makes predictions similar to MOND in other contexts. For example, new 21 cm background radiation observations, that are contrary to the predictions of the Lambda-CDM Model, also support Deur's theory.

* Deur's solution elegantly solves the galactic cluster problem of MOND by resorting to the differences in shape of clusters and their subparts, and the geometry between bodies attracted to each other in galactic clusters, and the arrangements of matter found in galaxies. Thus, it cures one of the main short fallings of MOND.

* Deur's solution predicts and explains a previously unnoticed relationship between the apparent amount of dark matter in an elliptical galaxy and the extent to which the galaxy is not spherical, which other modified gravity and dark matter particle theories do not.

* Deur's solution predicts and explains a previously unnoticed relationship between the thickness of a disk galaxy and the apparent amount of dark matter in a disk galaxy.

* This quantum gravity theory overcomes problems arising from observational evidence from correlated visible light and gravitational wave observations of black holes merging with neutron stars that shows that gravitational waves travel at a speed indistinguishable from the speed of light to high precision. This observation is inconsistent with massive gravitons in some modified gravity theories (e.g. many scalar-tensor or scalar-vector-tensor theories), because it utilizes only a single massless graviton.

* The Lambda-CDM Model does a great job of predicting the peaks in the cosmic background radiation of the universe, but does not do a good job of explaining dynamics of galaxies, or explaining why those dynamics are so tightly correlated with the distribution of baryonic matter in those systems. Simple cold dark matter models with a single "sterile" massive fermion do not accurately reproduce the inferred dark matter halos that are observed, nor do many more complicated dark matter particle theories. There are actually myriad discrepancies between observation and predicted behavior in the Lambda-CDM Model at the galaxy scale and some problems even at the galactic cluster scale. This is so even thought the Lambda-CDM Model is incomplete because it doesn't, by itself, explain how the cold dark matter in that matter came to have the very structured distribution that inferred dark matter distributions do observationally. I'll summarize the problems with Lambda-CDM at the bottom of this post.

Deur's Model Is Attractive Theoretically

* Deur explains dark matter and dark energy phenomena as a natural outgrowth of quantum gravity (although while a graviton concept transparently motivates the theory, it isn't inherently non-classical), with no "moving parts" that can be adjusted to make it fit the data in advance (although the relevant constants as applied haven't been worked out from first principles).

* Deur's theory provides a sound theoretical basis for an explanation of the dark matter phenomena with modifications of the Newtonian gravity approximation widely used in large scale astronomy contexts, that it utilizes, because it derives these modifications from first principles. It does so in a way that sidesteps the overwhelming calculation difficulties of doing the full fledged calculations of gravity with a spin-2 massless graviton that has been an insurmountable barrier to other quantum gravity theories, but without inducing significant systemic error in the systems to which the theory is applies (i.e. the differences between a spin-0 graviton theory and a spin-2 graviton theory in dark matter and dark energy contexts is slight except in gravitational systems that are far from equilibrium). It is not mere numerology or a purely phenomenological theory.

* While Deur's approach does not reproduce the conclusions of General Relativity as conventionally applied in the weak gravitational fields and spherically asymmetric systems where it dark matter and dark energy phenomena are observed, he does not make any assumptions about the properties of the graviton which are not utterly vanilla in the context of graviton based quantum gravity theories. None of the underlying assumptions from which this approach is derived contradict the underlying assumptions associated with General Relativity.

* Deur's approach builds on the common quantum gravity paradigm of gravity as QCD squared (strictly speaking Yang-Mills squared, but QCD is an SU(3) Yang–Mills theory).

* Basically, if Deur's approach ends up being correct, then the way that gravitational field self-interactions are incorporated into General Relativity in the Einstein's equations as conventionally applied are subtly flawed. This also explains why quantum gravity researchers trying to build a quantum gravity theory that exactly reproduces Einstein's equations as conventionally applied have failed. They have tried to reproduce a slightly erroneous operationalization of General Relativity, and the theoretical difficulties with doing this become more apparent in the quantum gravity context.

* Deur's background as a professional QCD scientist pretty much assures that his non-abelian gauge field mathematics are sound. Independent efforts corroborate the validity of the main simplification he makes relative to quantum gravity with a spin-2 massless graviton by using a static scalar graviton approximation (see, e.g. Diogo P. L. Bragança, José P. S. Lemos “Stratified scalar field theories of gravitation with self-energy term and effective particle Lagrangian” (June 29, 2018) (open access) (pre-print here)).

* Deur's solution is pretty much the simplest possible resolution of the problems of quantum gravity, dark matter and dark energy, because (1) it does so with no new particles (other than the graviton found in all quantum gravity theories), (2) no new forces, and (3) one fewer fundamental physical constants than the existing core theory of the Standard Model and General Relativity (without dark matter).

* The ΛCDM Model, also known as the Lambda-CDM Model, also known as the Standard Model of Cosmology, requires that 97.8% of the mass-energy of the universe be made up of never observed dark matter and dark energy, while Deur's theory relies entirely on Standard Model fundamental particles and massless gravitons.

* Many modified gravity theories assume new scalar and vector fields in addition to the tensor field of the graviton. Many dark matter particle theories require a new self-interaction force between dark matter particles or a new force governing interactions between dark matter and ordinary matter, or both. Deur's theory, in contrast, gives rise to no new forces or fields.

* This quantum gravity theory, in principle, replaces the three constants of general relativity plus MOND (Newton's constant G, the cosmological constant λ, and the MOND universal acceleration, a0) and replaces them with a single fundamental constant, the gravitational coupling constant (whose value has already been measured moderately precisely). This coupling constant is basically Newton's constant G, although possibly in different units. Both the cosmological constant and the universal acceleration constant of MOND can be derived, in principle, from G in this theory (although he has not done this derivation himself). In contrast, MOND adds one physical constant to the existing core theory, and dark matter adds at least one dark matter particle mass (and more masses in the dark matter sector such as a mass and coupling constant for a dark boson that carries a self-interaction or ordinary matter-dark matter interaction or both, are present in many versions of dark matter theories), one dark matter abundance constant, and other properties related to the dark matter particle. Modified gravity theories other than MOND (such as Moffat's MOG theory) often have even more new physical constants than MOND does.

* Deur's theory harmonizes gravity and the Standard Model with no particles beyond the Standard Model other than the massless graviton. The deep theoretical inconsistencies of the two models that make up core theory are eliminated (almost). Deur's formulation of the theory as a quantum field theory simplifies its integration as a quantum gravity theory with the Standard Model, which is also a quantum field theory.

* Deur's theory explains the cosmic coincidence problem in a very natural way.

* Deur's theory solves the conservation of mass-energy problem with general relativity's cosmological constant solution to "dark energy." Conventional general relativity theory, in contrast, accepts that gravitational energy is only conserved locally and not globally. In Deur's theory, dark energy arises from self-interacting gravitons staying within the galaxy at rates higher than they would in the absence of self-interactions which causes mass of the edge of a galaxy to be pulled more tightly towards the galaxy. Because these gravitons leave the galaxy at a rate lower than they would in the absence of self-interactions, the gravitational pull between galaxies is weaker than it would be in the absence of gravitational self-interaction. Thus, dark energy is due to a weaker pull between galaxies than in the Newtonian or cosmological constant free general relativity model, rather than due to having something pervasive in space pulling apart distant objects.

* Deur's theory is not plagued with tachyons, causation violations, ghosts, unitarity violations and similar defects that are common in efforts to modify gravity.

RELEVANT PAPERS

The related papers are:

The non-abelian symmetry of a lagrangian invalidates the principle of superposition for the field described by this lagrangian. A consequence in QCD is that non-linear effects occur, resulting in the quark-quark linear potential that explains the quark confinement, the quarkonia spectra or the Regge trajectories. Following a parallel between QCD and gravitation, we suggest that these non-linear effects should create an additional logarithmic potential in the classical newtonian description of gravity. The modified potential may account for the rotation curve of galaxies and other problems, without requiring dark matter.

A. Deur, “Non-Abelian Effects in Gravitation” (September 17, 2003) (pre-print not published).

Our present understanding of the universe requires the existence of dark matter and dark energy. We describe here a natural mechanism that could make exotic dark matter and possibly dark energy unnecessary. Graviton-graviton interactions increase the gravitational binding of matter. This increase, for large massive systems such as galaxies, may be large enough to make exotic dark matter superfluous. Within a weak field approximation we compute the effect on the rotation curves of galaxies and find the correct magnitude and distribution without need for arbitrary parameters or additional exotic particles. The Tully-Fisher relation also emerges naturally from this framework. The computations are further applied to galaxy clusters.
A. Deur, “Implications of Graviton-Graviton Interaction to Dark Matter” (May 6, 2009) (published at 676 Phys. Lett. B 21 (2009)).

We discuss the correlation between the dark matter content of elliptical galaxies and their ellipticities. We then explore a mechanism for which the correlation would emerge naturally. Such mechanism leads to identifying the dark matter particles to gravitons. A similar mechanism is known in Quantum Chromodynamics (QCD) and is essential to our understanding of the mass and structure of baryonic matter.
Alexandre Deur, “A correlation between the amount of dark matter in elliptical galaxies and their shape” MNRAS, 438, 1535 (July 28, 2014) .

We study two self-interacting scalar field theories in their high-temperature limit using path integrals on a lattice. We first discuss the formalism and recover known potentials to validate the method. We then discuss how these theories can model, in the high-temperature limit, the strong interaction and General Relativity. For the strong interaction, the model recovers the known phenomenology of the nearly static regime of heavy quarkonia. The model also exposes a possible origin for the emergence of the confinement scale from the approximately conformal Lagrangian. Aside from such possible insights, the main purpose of addressing the strong interaction here --given that more sophisticated approaches already exist-- is mostly to further verify the pertinence of the model in the more complex case of General Relativity for which non-perturbative methods are not as developed. The results have important implications on the nature of Dark Matter. In particular, non-perturbative effects naturally provide flat rotation curves for disk galaxies, without need for non-baryonic matter, and explain as well other observations involving Dark Matter such as cluster dynamics or the dark mass of elliptical galaxies.

A. Deur, “Self-interacting scalar fields at high temperature” (June 15, 2017) (published at Eur. Phys. J. C77 (2017) no.6, 412).


Numerical calculations have shown that the increase of binding energy in massive systems due to gravity's self-interaction can account for galaxy and cluster dynamics without dark matter. Such approach is consistent with General Relativity and the Standard Model of particle physics. The increased binding implies an effective weakening of gravity outside the bound system. In this article, this suppression is modeled in the Universe's evolution equations and its consequence for dark energy is explored. Observations are well reproduced without need for dark energy. The cosmic coincidence appears naturally and the problem of having a de Sitter Universe as the final state of the Universe is eliminated.
A. Deur, “A possible explanation for dark matter and dark energy consistent with the Standard Model of particle physics and General Relativity” (last revised October 22, 2019) (Proceeding for a presentation given at Duke University, Apr. 2014. Based on A. D. PLB B676, 21 (2009); A.D, MNRAS, 438, 1535 (2014). The published version is Eur. Phys. J. C 79, 883 (2019). https://doi.org/10.1140/epjc/s10052-019-7393-0 https://link.springer.com/article/10.1140/epjc/s10052-019-7393-0). The body text in this paper notes in one pertinent part:

CMB and BAO

Predicting the CMB and BAO is complex and model dependent, with many parameters. Like for large structure formation, a detailed investigation is beyond the scope of this article. However, one can remark that since the CMB main acoustic peak position depends on the Universe dynamical evolution, its calculation should involve Ω∗ M rather than ΩM: θ ' p Ω∗ M/zrec (with zrec ' 1100 the redshift at the recombination time). This yields θ ' 0.8 ◦ , in agreement with observations [16].

The end note citation is to G. Hinshaw et al., Astrophys. J. Suppl. 208, 19 (2013).

The discrepancy between the visible mass in galaxies or galaxy clusters, and that inferred from their dynamics is well known. The prevailing solution to this problem is dark matter. Here we show that a different approach, one that conforms to both the current Standard Model of Particle Physics and General Relativity, explains the recently observed tight correlation between the galactic baryonic mass and its observed acceleration. Using direct calculations based on General Relativity's Lagrangian, and parameter-free galactic models, we show that the nonlinear effects of General Relativity make baryonic matter alone sufficient to explain this observation.
A. Deur, Corey Sargent, Balša Terzić, "Significance of Gravitational Nonlinearities on the Dynamics of Disk Galaxies" (August 31, 2019, last revised May 18, 2020) (pre-print) Published ApJ 896 94 (June 17, 2020).

Observations indicate that the baryonic matter of galaxies is surrounded by vast dark matter halos, which nature remains unknown. This document details the analysis of the results published in MNRAS 438, 2, 1535 (2014) reporting an empirical correlation between the ellipticity of elliptical galaxies and their dark matter content. Large and homogeneous samples of elliptical galaxies for which their dark matter content is inferred were selected using different methods. Possible methodological biases in the dark mass extraction are alleviated by the multiple methods employed. Effects from galaxy peculiarities are minimized by a homogeneity requirement and further suppressed statistically. After forming homogeneous samples (rejection of galaxies with signs of interaction or dependence on their environment, of peculiar elliptical galaxies and of S0-type galaxies) a clear correlation emerges. Such a correlation is either spurious --in which case it signals an ubiquitous systematic bias in elliptical galaxy observations or their analysis-- or genuine --in which case it implies in particular that at equal luminosity, flattened medium-size elliptical galaxies are on average five times heavier than rounder ones, and that the non-baryonic matter content of medium-size round galaxies is small. It would also provides a new testing ground for models of dark matter and galaxy formation.
A. Deur, "A correlation between the dark content of elliptical galaxies and their ellipticity" (October 13, 2020) (pre-print).

We present a method to investigate the effect of relativistic corrections arising from large masses to the rotation curves of disk galaxies. The method employs a mean-field approximation and gravitational lensing. Applying it to a basic model of disk galaxy, we find that these corrections become important and magnified at large distances. The magnitude of the effect is sufficient to explain the galactic missing mass problem without requiring a significant amount of dark matter. A prediction of the model is that there should be a strong correlation between the inferred galactic dark mass and the galactic disk thickness. We use two independent sets of data to verify this.
Alexandre Deur, "Relativistic corrections to the rotation curves of disk galaxies" (April 10, 2020) (lated updated February 8, 2021 in version accepted for publication in Eur. Phys. Jour. C).

See also:
We investigate the possible existence of graviballs, a system of bound gravitons, and show that two gravitons can be bound together by their gravitational interaction. This idea connects to black hole formation by a high-energy 2→N scattering and to the gravitational geon studied by Brill and Hartle. Our calculations rely on the formalism and techniques of quantum field theory, specifically on low-energy quantum gravity. By solving numerically the relativistic equations of motion, we have access to the space-time dynamics of the (2-gravitons) graviball formation. We argue that the graviball is a viable dark matter candidate and we compute the associated gravitational lensing.
B. Guiot, A. Borqus, A. Deur, K. Werner, "Graviballs and Dark Matter" 11 JHEP 159 (June 3, 2020 revised September 3, 2020).
 
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ohwilleke
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Continuing the previous post, in evaluating the weight given to alternatives to the status quo one also needs to be weighed in light of Lambda-CDM, which has a multitude of problems, any one of which would be troubling, but which collectively points strongly to a major problem of crisis proportions with the paradigm of the so called "Standard Model of Cosmology."

* The gravitational lensing of subhalos in galactic clusters recently observed to be much more compact and less "puffy" than LambdaCDM would predict.
* A KIDS telescope observation of very large scale structure which shows it to be 8.3% smoother (i.e. less clumpy) than predicted by LambdaCDM.
* The Hubble tension that shows that Hubble's constant, which is a measure of the expansion rate of the universe, is about 10% smaller when measured via cosmic microwave background radiation (with a small margin of error) than when measured by a wide variety of measures at times much more removed from the Big Bang that the time at which the cosmic microwave background came into being.
* The DM halo shapes that LambdaCMD predicts are usually wrong (too cuspy and not in the NFW distribution predicted by the theory).
* The correspondence between the distribution of ordinary matter and inferred dark matter in galaxies is too tight; truly collisionless dark matter should have less of a tight fit in its distribution to ordinary matter distributions than is observed. This is also the case in galaxy clusters.
* It doesn't explain systemic variation in the amount of apparent dark matter in elliptical galaxies, or why spiral galaxies have smaller proportions of ordinary matter than elliptical galaxies in same sized inferred dark matter halos, or why thick spiral galaxies have more inferred dark matter than thin ones.
* It doesn't explain why satellite galaxies are consistently located in a two dimensional plane relative to the core galaxy.
* Not as many satellite galaxies are observed as predicted, or why the number of satellite galaxies is related to budge mass in spiral galaxies.
* The aggregate statistical distribution of galaxy types and shapes, called the "halo mass function" is wrong.
* Galaxies are observed sooner after the Big Bang than expected.
* The temperature of the universe measured by 21cm background radio signals is consistent with no dark matter and inconsistent with sufficient dark matter for LambdaCDM to work.
* Observations are inconsistent with the "Cosmological principle" that LambdaCDM predicts, which is "the notion that the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale.
* It doesn't do a good job of explaining the rare dwarf galaxies (that are usually dark matter dominated) that seem to have no dark matter.
* It doesn't explain deficits of X-ray emissions in low surface brightness galaxies.
* It predicts too few galaxy clusters.
* It gets globular cluster formation wrong.
* It doesn't explain evidence of stronger than expected gravitational effects in wide binary stars.
* There are too many galaxy clusters colliding at speeds that are too high relative to each other.
* It doesn't explain the "cosmic coincidence" problem (that the amount of ordinary matter, dark matter and dark energy are of the same order of magnitude at this moment in the history of the Universe since the Big Bang).
* There are potential unresolved systemic problems in current dark energy measurements.
* Every measure of detecting it directly has come up empty (including not just dedicated direct detection experiments but particle collider searches, searches for cosmic ray signals of dark matter annihilation, and indirect searches combined with direct searches). But it requires particles and forces of types not present in the Standard Model or general relativity to fit what is observed.
* It has made very few ex ante predictions and those it has made have often been wrong, while MOND has a much better track record despite being far simpler.
 
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ohwilleke
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Deur's approach to gravitational field self-interaction in lieu of dark matter once again has success, this time describing large scale structure formation.
We check whether General Relativity's field self-interaction alleviates the need for dark matter to explain the universe's large structure formation. We found that self-interaction accelerates sufficiently the growth of structures so that they can reach their presently observed density. No free parameters, dark components or modifications of the known laws of nature were required. This result adds to the other natural explanations provided by the same approach to the, inter alia, flat rotation curves of galaxies, supernovae observations suggestive of dark energy, and dynamics of galaxy clusters, thereby reinforcing its credibility as an alternative to the dark universe model.
Alexandre Deur, "Effect of gravitational field self-interaction on large structure formation" arXiv: 2018:04649 (July 9, 2021) (Accepted for publication in Phys. Lett. B) DOI: 10.1016/j.physletb.2021.136510

The of the paper conclusion explains that:
The consistency of the standard Λ-CDM model of the universe in explaining many observations that would be otherwise problematic is a compelling argument for the existence of dark matter and dark energy.
Yet, there are good reasons for studying alternatives to Λ-CDM, e.g. the lack of detection of dark particles, the dwindling support from theories beyond the standard model of particle physics, observations that challenge the dark matter model such as [19], lack of observations of Λ-CDM predictions such as the dwarf galaxy problem, or the Hubble tension [20]. The credibility of an alternative approach is enhanced if, like for Λ-CDM, it can consistently explain the otherwise puzzling cosmological observations.
One alternative approach proposes that these observations are explained by the self-interaction of gravitational fields in General Relativity. It naturally explains the galactic rotation curves [6]-[8], the supernovae observations suggestive of dark energy [10], the tight empirical relation between baryonic and observed accelerations [9, 19], the dynamics of galaxy clusters [6] and the Tully-Fisher relation [6, 10, 16].
The explanation is natural in the sense that a similar phenomenology is well-known in the context of QCD, a fundamental force whose Lagrangian has the same structure as that of General Relativity. Crucially, no free parameters are necessary, nor exotic matter or fields, nor modifications of the known laws of nature.
In this article, we checked whether the approach also explains the formation of large structures. We found that field self-interaction strengthens sufficiently the gravitational force so that the small CMB inhomogeneities can grow to the density presently observed.
Again, no free parameters were needed: the function that globally quantifies the effect of field self-interaction had been previously determined in Ref. [10] in the context of the a priori unrelated topic of dark energy.
 
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KurtLudwig
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After reading professor Deur's latest paper, arXiv:2108.04649v1, I was amazed by his insight into the mathematical similarities between gluon self-interaction in QCD and non-linear General Relativity by expanding the GR Lagraingian.
Prof. Deur's mathematical model fits the astronomical observations without dark matter and dark energy. Newton's equation accounts for just the first term of the expansion. The other terms account for the flat rotation curves, the CMB and the additional acceleration needed in the initial clumping of matter and other observed astronomical phenomena.
What are the opinions of physicists at Physics Forums?
 
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KurtLudwig
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Below is a quotation of the summary of the above paper:
IV. SUMMARY The consistency of the standard Λ-CDM model of the universe in explaining many observations that would be otherwise problematic is a compelling argument for the existence of dark matter and dark energy. Yet, there are good reasons for studying alternatives to Λ-CDM, e.g. the lack of detection of dark particles, the dwindling support from theories beyond the standard model of particle physics, observations that challenge the dark matter model such as [19], lack of observations of Λ-CDM predictions such as the dwarf galaxy problem, or the Hubble tension [20]. The credibility of an alternative approach is enhanced if, like for Λ-CDM, it can consistently explain the otherwise puzzling cosmological observations. One alternative approach proposes that these observations are explained by the self-interaction of gravitational fields in General Relativity. It naturally explains the galactic rotation curves [6]-[8], the supernovae observations suggestive of dark energy [10], the tight empirical relation between baryonic and observed accelerations [9, 19], the dynamics of galaxy clusters [6] and the Tully-Fisher relation [6, 10, 16]. The explanation is natural in the sense that a similar phenomenology is well-known in the context of QCD, a fundamental force whose Lagrangian has the same structure as that of General Relativity. Crucially, no free parameters are necessary, nor exotic matter or fields, nor modifications of the known laws of nature. In this article, we checked whether the approach also explains the formation of large structures. We found that field self-interaction strengthens sufficiently the gravitational force so that the small CMB inhomogeneities can grow to the density presently observed. Again, no free parameters were needed: the function that globally quantifies the effect of field self-interaction had been previously determined in Ref. [10] in the context of the a priori unrelated topic of dark energy.
 
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timmdeeg
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Deur's approach to gravitational field self-interaction in lieu of dark matter once again has success, this time describing large scale structure formation.

Alexandre Deur, "Effect of gravitational field self-interaction on large structure formation" arXiv: 2018:04649 (July 9, 2021) (Accepted for publication in Phys. Lett. B) DOI: 10.1016/j.physletb.2021.136510

The of the paper conclusion explains that:

One alternative approach proposes that these observations are explained by the self-interaction of gravitational fields in General Relativity. It naturally explains the galactic rotation curves [6]-[8], the supernovae observations suggestive of dark energy [10], the tight empirical relation between baryonic and observed accelerations [9, 19], the dynamics of galaxy clusters [6] and the Tully-Fisher relation [6, 10, 16].

If I understand it correctly the "self-interaction of gravitational fields" acts like a source gravity so that galactic rotational curves are flat. The sources of gravitational fields, energy density, shear stress and momentum flux are described by the stress-energy-tensor. So from this gravitational fields are not a source of gravity.

It seems you have been very busy with Deur's papers. I wonder if he discusses the self-interaction of gravitational fields as a source of gravity in the context of the stress-energy-tensor somewhere.
Any hint appreciated.
 
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ohwilleke
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If I understand it correctly the "self-interaction of gravitational fields" acts like a source gravity so that galactic rotational curves are flat. The sources of gravitational fields, energy density, shear stress and momentum flux are described by the stress-energy-tensor. So from this gravitational fields are not a source of gravity.

It seems you have been very busy with Deur's papers. I wonder if he discusses the self-interaction of gravitational fields as a source of gravity in the context of the stress-energy-tensor somewhere.
Any hint appreciated.
The self-interaction term effects in General Relativity arise on the left hand side of Einstein's field equations, not as an element of the stress-energy tensor like other sources of gravity. In a quantum gravity context, gravity is non-Abelian with gravitons interacting with each other. The details are explained by Peter Donis in a 2015 FAQ post which states:
The fundamental equation of GR is the Einstein Field Equation, which looks like this: Gab=8πTab Conceptually, what this equation says is ” “spacetime curvature = constant * stress-energy”. So the LHS of the equation is “gravity”, conceptually, and the RHS is the “source” that produces it. The key point here is that the RHS does not include any stress-energy due to gravity itself. That’s because there isn’t any; the “source” in the EFE does not include any “energy stored in the gravitational field”, because there is no coordinate-free way of defining any such energy, and the EFE is a coordinate-free, tensor equation.
At the quantum level, this means gravitons (the quantum particles associated with the massless, spin-two field) interact with other gravitons. At the classical level, it means that, since the EFE is nonlinear, the curvature can be present even when the “source” on the RHS of the EFE is zero, i.e., there can be vacuum solutions of the EFE that have curvature present. (Schwarzschild spacetime is an obvious example.) In other words, on this view, the answer to our question is “yes”: gravity *does* gravitate!
(1) In order to ensure the conservation of the source, the complete Einstein tensor, including *all* contributions from gravity, must appear on the LHS of the EFE; there is nothing left over to contribute to the “source” on the RHS of the EFE. So in this sense, gravity does *not* gravitate.

(2) Viewed as a quantum field, gravity is a massless, spin-two field, and the classical limit of the quantum theory of such a field is standard GR (based on the Einstein-Hilbert action for gravity). But this field interacts with itself; its field equation, at both the quantum and classical levels, is nonlinear. So in this sense, gravity *does* gravitate.
Deur's treatment of this is (so far as I can discern) not non-standard.

Newtonian gravity without general relativistic modification is used in most astronomy applications except around black holes, neutron stars and the Big Bang.

He examines a non-linear effect in General Relativity that is not present in Newtonian gravity. He argues that contrary to conventional wisdom, this particular non-linear effect is not negligible in cases where you have very weak gravitational fields derived from very large masses that are not spherically symmetrical.

See also paragraph 6 of post #6 above beginning "Graviton-Graviton Interaction . . . " and the links therein.
 
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timmdeeg
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The self-interaction term effects in General Relativity arise on the left hand side of Einstein's field equations, not as an element of the stress-energy tensor like other sources of gravity. In a quantum gravity context, gravity is non-Abelian with gravitons interacting with each other. The details are explained by Peter Donis in a 2015 FAQ post which states:
Thank you for your detailed response which clarifies my question.

You have listed lots of problems which seem to be unresolved by the Lambda-CDM model in #7. In my opinion the most prominent is the so called Hubble-tension. And I think it would be major step if graviton-graviton interaction could contribute in a quantitative way to explain the discrepancy between the values of the Hubble constant comparing CMB vs. today.
 
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ohwilleke
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Thank you for your detailed response which clarifies my question.

You have listed lots of problems which seem to be unresolved by the Lambda-CDM model in #7. In my opinion the most prominent is the so called Hubble-tension. And I think it would be major step if graviton-graviton interaction could contribute in a quantitative way to explain the discrepancy between the values of the Hubble constant comparing CMB vs. today.
Honestly, IMHO, the Hubble-tension is one of the least notable of the lot. There are lots of ways that discrepancies between two experimental measurements can be resolved (some of which are published, such as failure to properly account for dust in making red shift determinations, or in overlooked systemic errors), and previous Hubble constant measurement tensions of this magnitude have been resolved in the past.

My top three would be:

* The DM halo shapes that LambdaCMD predicts are usually wrong (too cuspy and not in the NFW distribution predicted by the theory).
* It doesn't explain why satellite galaxies are consistently located in a two dimensional plane relative to the core galaxy.
* The temperature of the universe measured by 21cm background radio signals is consistent with no dark matter and inconsistent with sufficient dark matter for LambdaCDM to work.
 
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timmdeeg
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Honestly, IMHO, the Hubble-tension is one of the least notable of the lot.
I got the impression that there isn't much hope to resolve it after having read Exploring the Hubble tension.

if we are going to throw it out, what do we substitute it with? The first thing is to take small steps away from the model, say by adding one parameter. For a while, you could say that maybe there is something like an effective neutrino species that might fix it, but a solution like this doesn’t quite fit the CMB data any more. I think the community may be split 50/50 between being almost ready to throw the model out and keeping working with it, because we have nothing better to use.
Systematics are always unknowns that may be there, but the level of sophistication of the analyses suggests that if there was something major then it would have come up. People do a lot of internal consistency checks; therefore, it is becoming increasingly unlikely that it is only due to dumb systematics.

My top three would be:

* The DM halo shapes that LambdaCMD predicts are usually wrong (too cuspy and not in the NFW distribution predicted by the theory).
* It doesn't explain why satellite galaxies are consistently located in a two dimensional plane relative to the core galaxy.
* The temperature of the universe measured by 21cm background radio signals is consistent with no dark matter and inconsistent with sufficient dark matter for LambdaCDM to work.
Here my top is satellite galaxies which they found also in a plane around Andromeda. This really amazing.
 
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  • #16
ohwilleke
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I got the impression that there isn't much hope to resolve it after having read Exploring the Hubble tension.
See e.g., Edvard Mortsell, Ariel Goobar, Joel Johansson, Suhail Dhawan, "The Hubble Tension Bites the Dust: Sensitivity of the Hubble Constant Determination to Cepheid Color Calibration" arXiv (May 24, 2021) which concludes in its abstract that: "systemic uncertainties related to the choice of Cepheid color-luminosity calibration method currently inhibits us from measuring H(0) to the precision required to claim a substantial tension with Planck data."
 
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  • #17
timmdeeg
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See e.g., Edvard Mortsell, Ariel Goobar, Joel Johansson, Suhail Dhawan, "The Hubble Tension Bites the Dust: Sensitivity of the Hubble Constant Determination to Cepheid Color Calibration" arXiv (May 24, 2021) which concludes in its abstract that: "systemic uncertainties related to the choice of Cepheid color-luminosity calibration method currently inhibits us from measuring H(0) to the precision required to claim a substantial tension with Planck data."
Thanks, very interesting. So lets wait for measurements of the Hubble constant from gravitational waves which are independent of supernovae measurements.
 
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timmdeeg
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http://dispatchesfromturtleisland.blogspot.com/p/deurs-work-on-gravity-and-related.html

Here is the penultimate comment:

the question Ethan was asked was,

Ask Ethan: Could Gravitons Solve The Mystery Of Dark Matter?

one point he makes relevant to Deur

"Given that strain amplitudes are typically things like ~10-20 or smaller, which itself requires a tremendous effort to detect, going 20+ orders of magnitude more sensitive is virtually unimaginable with the limitations of current technology. Whatever else might be true about gravitons, their self-interactions can be disregarded. "

So Deur's theory of graviton self-interaction is simply too weak an effect to explain galaxy rotation curves, which I suspect is the reason it's got zero citations. gravitational waves is a way to directly measure graviton self-interaction, and the effect is too weak as to be almost unmeasureable. graviton self-interaction can also be calculated, as gravity is an extremely weak force and gravitons are thought to be massless

I think the lack of citations is indeed weird. But the basic essence of this critics is so obvious that I can't imagine that Deur has overlooked this issue.
 
  • #19
ohwilleke
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http://dispatchesfromturtleisland.blogspot.com/p/deurs-work-on-gravity-and-related.html

Here is the penultimate comment:



I think the lack of citations is indeed weird. But the basic essence of this critics is so obvious that I can't imagine that Deur has overlooked this issue.
Gravitons still have mass-energy even though they lack rest mass. The locally insignificant force adds up because gravity is always attractive. So, in a large enough system a lot of little bits add up.

The lack of citations can be compared to obviously wrong approaches to get dark matter phenomena out of GR (like gravitomagnetism) which are swiftly responded to with critical papers. After about ten peer reviewed published papers (some with co-authors), if there were an easy rebuttal, you'd think someone would have done so by now. The lack of citation is frustrating and probably mostly sociology of science driven.
 
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timmdeeg
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The lack of citation is frustrating and probably mostly sociology of science driven.
May be, I can't judge that but quite certain a wrong calculation would have been criticized (as you say). And I am not sure if it is fair enough to say I don't deal with it because these papers haven't been cited.

Other people's opinion around here would be interesting.
 
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The locally insignificant force adds up because gravity is always attractive. So, in a large enough system a lot of little bits add up.
A qualitative statement doesn't become a quantitative statement through handwaving.
The lack of citations can be compared to obviously wrong approaches to get dark matter phenomena out of GR (like gravitomagnetism) which are swiftly responded to with critical papers. After about ten peer reviewed published papers (some with co-authors), if there were an easy rebuttal, you'd think someone would have done so by now.
Or maybe it's just so bad that no one can be bothered to write anything about it. "They ignore me, that means I must be right" is on the crackpot index for a reason.
 
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  • #22
ohwilleke
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A qualitative statement doesn't become a quantitative statement through handwaving.Or maybe it's just so bad that no one can be bothered to write anything about it. "They ignore me, that means I must be right" is on the crackpot index for a reason.
Crackpots are rarely professional physicists with ten papers published in peer reviewed journals in the field, some with co-authors who are likewise professional physicists. I'm not saying that it is bullet proof, and would far prefer that a specialist examine the claims critically (I've certainly dangled this body of work before people who are specialists in hopes of securing their attention).

But the analysis itself is not that wild and it is there in black and white to judge.
 
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KurtLudwig
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A qualitative statement doesn't become a quantitative statement through handwaving.Or maybe it's just so bad that no one can be bothered to write anything about it. "They ignore me, that means I must be right" is on the crackpot index for a reason.
Alexandre Deur is a physics professor at the University of Virginia and a researcher at Jefferson Lab doing nuclear research. I suggest to be more careful in using one's words.
 
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if there were an easy rebuttal, you'd think someone would have done so by now
There is more than one possible reason for the lack of rebuttal papers. One of them is that the paper (Deur's in this case) is actually right. Another is that its model is hard to check and nobody is willing to invest the time to do so because they judge that the probability of the model being right is too low to make it worth the effort.

I've certainly dangled this body of work before people who are specialists in hopes of securing their attention
And have you succeeded with any of them? If you haven't (which I suspect is the case or you would be giving us references to the papers the specialists published on this work after they decided it was worth their attention), then that is a data point in favor of the second reason I gave above.
 
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Alexandre Deur is a physics professor at the University of Virginia and a researcher at Jefferson Lab doing nuclear research. I suggest to be more careful in using one's words.
This is a straight argument from authority and carries no weight here. Credentialed scientists publish papers that are wrong all the time; that's a normal part of science.

Also, @mfb's comment was not directed at Deur, but at @ohwilleke, who was the one making the argument that @mfb paraphrased in quotes. @mfb was not claiming that Deur was making that argument, only that @ohwilleke was.
 
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  • #26
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quite certain a wrong calculation would have been criticized
Your Bayesian prior for the hypothesis that every paper gets read in detail and analyzed by somebody (other than its authors) is much too high.
 
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ohwilleke
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There is more than one possible reason for the lack of rebuttal papers. One of them is that the paper (Deur's in this case) is actually right. Another is that its model is hard to check and nobody is willing to invest the time to do so because they judge that the probability of the model being right is too low to make it worth the effort.


And have you succeeded with any of them? If you haven't (which I suspect is the case or you would be giving us references to the papers the specialists published on this work after they decided it was worth their attention), then that is a data point in favor of the second reason I gave above.
All true.

The end result, any time one primary lead researcher with respectable credentials reaches a result that is published in multiple peer reviewed journal articles, and others in the field, for whatever reason, neither investigate further or criticize a result, is that you are left in indeterminate state (although co-authorship of some recent articles helps move the threshold of credibility somewhat). His work also now has a few citations in other peer reviewed articles by independent authors, although predominantly in introductory surveys of various alternatives that have been attempted without a serious evaluation of those alternatives.

His strategy in several recent articles of (1) framing the results in terms of classical GR, rather than a scalar field approximation of a quantum gravity graviton self-interaction theory (in which the results follow more obviously), (2) shoring up observational confirmation of new predictions particular to the theory are also smart ones, and (3) securing co-authors, have been good ones.

There are certainly precedents for good results staying in that state for a fairly long time. But, it wouldn't take much to tip the balance one way or the other, e.g. a confirming article with a couple of co-authors from the GR/astronomy field in a more prominent journal that gets good press and a few favorable blog posts or tweets by big names, or a article with similar authorship pointing out a fatal flaw in the analysis, could do that. Likewise, even one publication in a top journal by Deur himself could really tip the tide, which one advocate on a editorial board of one of half a dozen journals or so could facilitate.

Almost all of the heavily researched alternatives have serious problems, with the head of the pack, LambdaCDM, being in particular trouble. The relentless march of astronomy observations and other evidence excluding more and more of the parameter space of the main alternatives week after week is also starting to generate a bigger pool of researchers coming up for air and looking for different paradigms.
 
  • #28
ohwilleke
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This is a straight argument from authority and carries no weight here. Credentialed scientists publish papers that are wrong all the time; that's a normal part of science.
The argument is not that credentialed scientists publishing in peer reviewed journals are never wrong. The argument is that when such scientists make these kind of publications that they are generally not advancing crackpot theories that are trivially wrong. Instead, these papers deserve to be taken seriously and evaluated on the merits rather than rejected out of hand in a way that one might for a paper advancing, e.g., "Vedic science".

In this case, the big threshold issue is whether Deur's estimation of the magnitude of the gravitational field self-interaction effect in weak gravitational field from very massive sources is order of magnitude correct or not, in the most basic case of a galactic rotation curve in a typical sized spiral galaxy.

The main gut reaction I've seen from skeptics of Deur's work, is that the magnitude of the non-linear effects by which GR differs from Newtonian gravity in weak gravitational fields should be negligible. But without much of an evaluation of that numerically.

If that threshold question is resolved favorably in a confirming analysis, then pretty much immediately, his approach becomes the most attractive game in town and should be making headlines. If not, the likelihood that it is a dead end is much greater. This is a pretty narrow question to evaluate.

I know the math and the physics well enough to know that if there is a problem that it is a fairly subtle one. None of the analysis is obviously wrong. But, I also know that I don't know that math and the physics well enough to rigorously and definitively evaluate its correctness, or to replicate the result from scratch with the hints provided from the way Deur tackles the problem. I'm an educated amateur, not a professional physicist, and definitely not a specialist in GR or astrophysics.

If he is right, this comes pretty close to a Holy Grail, Nobel-prize winning insight in astrophysics that turns the entire discipline upside down in a Copernican class revolution in how to think about cosmology, that solves almost all of the big outstanding problems in the field. Maybe this is too good to be true. But, a lot of far less promising BSM theories have gotten far more attention and have sustained it even after seriously observational problems with those theories have been identified.
 
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The argument is that when such scientists make these kind of publications that they are generally not advancing crackpot theories that are trivially wrong.
As I said, @mfb was not saying that Deur was making a crackpot claim. He was only saying that you were (the claim that, since everyone was ignoring Deur's theory, it must be right).

these papers deserve to be taken seriously
If that is true, then other papers citing these papers will appear, and gradually more physicists will recognize that the arguments have merit.

We have had a few discussions of the Deur papers in other threads, and my general takeaway is that his models are too complicated for a PF discussion to have much value. That's not to say the models are wrong, just that it doesn't seem to me like discussing them on PF is likely to go anywhere.
 
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the big threshold issue is whether Deur's estimation of the magnitude of the gravitational field self-interaction effect in weak gravitational field from very massive sources is order of magnitude correct or not, in the most basic case of a galactic rotation curve in a typical sized spiral galaxy
This is one threshold, but by no means the only one. A key weakness that I see in Deur's models as alternatives to dark matter is that they only address the specific case of galaxy rotation curves. But that is not the only case in which the dark matter hypothesis comes into play in the current Lambda CDM model of our universe. As far as I can see, Deur's models give no help at all in avoiding the dark matter hypothesis in, for example, the evolution of the early universe. His model depends on gravitational properties of isolated bound systems like galaxies and does not even apply to relatively uniform matter distributions like those in the early universe, before gravitational clumping became significant.
 
  • #31
ohwilleke
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A key weakness that I see in Deur's models as alternatives to dark matter is that they only address the specific case of galaxy rotation curves. But that is not the only case in which the dark matter hypothesis comes into play in the current Lambda CDM model of our universe. As far as I can see, Deur's models give no help at all in avoiding the dark matter hypothesis in, for example, the evolution of the early universe. His model depends on gravitational properties of isolated bound systems like galaxies and does not even apply to relatively uniform matter distributions like those in the early universe, before gravitational clumping became significant.
Actually, that isn't the case.

Early Universe Cosmology Evidence for Deur's Approach

The most recent paper that I cited above is precisely above the evolution of the early universe, a subject only addressed more tangentially in earlier papers:

Alexandre Deur, "Effect of gravitational field self-interaction on large structure formation" arXiv: 2018:04649 (July 9, 2021) (Accepted for publication in Phys. Lett. B) DOI: 10.1016/j.physletb.2021.136510

As the body text of that article explains:

1630639626129.png

Fig. 2. Time-evolution of a baryonic overdensity δ. The initial value of δ at the recombination time, t ≈ 3.7 Gyr, is 2 × 10−5. The band shows the evolution including field SI effects, with the central solid line corresponding to the nominal DM−1(t). The dashed line is the evolution without SI. In neither case, dark matter has been assumed.

The picture for structure growth that emerges from the result showed in Fig. 2 is that at early times, SI [Ed. i.e. gravitational field self interaction] did not influence the Jeans collapse mechanism since the initial overdensities were spherical. Mergings of overdensities would also not be significantly enhanced since density anisotropies would be too small to trigger the onset of SI. At later times, the overdensities lose their spherical symmetry due to mergers and radiative energy dissipation. The SI-enhanced internal binding then accelerates local collapses. As anisotropies become denser, merging rates increase, especially for overdensities that had so far retained their spherical shape. The quantitative analysis shows that the SI-enhanced gravitational interaction is sufficient to form structures reaching the present-day densities, without requiring dark matter. We have considered here the time-evolution of δ and not the related subject of matter's spatial distribution. As shown in Ref. [10], the same formalism also yields a position of the peak of the matter power spectrum of keq≃0.014 Mpc−1, in agreement with observations.

The cosmology conclusions in Reference 10 (a prior published paper by Deur, i.e.
A. Deur, Eur. Phys. J. C, 79 (2019), p. 883 arXiv:1709.02481) are as follows:

A. Supernova observations

Explaining supernova observations with GR’s self-interaction is the focus of this article. These observations are that the large-z (0.1 . z . 1.5) supernova apparent luminosities are dimmer, viz their apparent magnitudes are larger, than expected from a homogeneous and isotropic decelerating Universe. This is interpreted as evidence for an accelerating universe, i.e. for Λ > 0. However, we show in this section that lifting the approximations of homogeneity and isotropy can also explain the observations, while keeping Λ = 0. From the luminosity distance DL(z), Eq. (23), the apparent magnitude DL(z)H0 of events can be calculated. Assuming Λ = 0, taking H0 = 68 ± 1 km/s/Mpc, . . . we compute the band shown in Fig. 3. . . . Our calculation agrees well with the γ-ray bursts and supernovae data. There is no adjustment to these data, all the parameters in DM being constrained by observations of large structure evolution. Also shown in the figure are the calculations for the cases of a homogeneous and isotropic Universe with only matter (dotted line), that for an empty Universe (continuous line) and the ΛCDM (dark energy, cold dark matter) model (dashed line). The difference between the observations and the expectation from a homogeneous and isotropic Universe with Λ = 0 is clearer by forming a residual apparent magnitude:

r(z) = 5 log H0 DL(z) 1 + z − 5 log H0 z + z 2/2 1 + z , (27)

with the last term corresponding to the empty Universe case. Positive values of r(z) indicate fainter apparent luminosities than expected in the case of an empty Universe. They constitute the best evidence for Λ > 0. Our calculation of r(z) with Λ = 0 agrees well with the observations.

Screen Shot 2021-09-02 at 9.40.15 PM.png


FIG. 4: Residual between observed apparent magnitudes (γ-ray bursts: star symbol. Supernovae: other symbols) and their expectation from an empty universe. The continuous line is for the ΛCDM model. The band is the present work, without any free parameters adjusted to the γ-ray or supernova data.

We now outline how GR’s self-interaction may also explain the observations providing less direct evidence for Λ > 0.

B. Age of the Universe

Without Λ > 0, the calculated age of the Universe would be 11.7 ± 0.2 Gyr for the standard ΩM = 0.32 value and for H0 = 68 ± 1 km/s/Mpc. This conflicts with the measured age of the oldest stars, up to ∼ 13.5 Gyr. The ΛCDM model, with ΩΛ = 0.68 and the same H0 and ΩM values, yields 13.6 ± 0.2 Gyr. GR’s self-interaction also solves this problem, while keeping Λ = 0: Eq. (24) yields a compatible Universe age of 13.2 ± 1.7 Gyr.

C. Large structure formation

In a Universe without gravitational self-interaction or dark matter, large structures do not have time to coalesce. What happens in the self-interaction framework can be sketched as follow: As DM(z) departs from 1, viz as gravity weakens globally, energy conservation demands that the global weakening is balanced locally by an increase of gravity within the structures themselves, thus speeding up their formation compared to a Universe without self-interaction. Since DM(z) evolves following the formation of large structures, gravity strengthens locally with the same timeline. Because strengthening reproduces the dynamics of galaxies and clusters, the local effect of self-interaction is equivalent to the effect of dark matter. Furthermore, the position of the peak of the matter power spectrum is now given by keq = H0 p 2Ω∗ M(0)/aeq, with aeq the scale parameter at zeq. Assuming ΩBaryon = ΩM (no dark matter) and using Ω∗ M = ΩMDM yield Ω∗ M(0) ' 0.3, i.e. keq = 0.014, in agreement with observations. This suggests that the present approach is compatible with large structure formation.

D. CMB and BAO

The CMB main acoustic peak position depending on the Universe dynamical evolution, its calculation in the present framework involves Ω∗ M rather than ΩM. Thus we have now θ ' p Ω∗ M/zrec (with zrec ' 1100 at the recombination time), resulting in θ ' 0.8 ◦ , which agrees with observations. Predicting the smaller features of the CMB and the BAO is complex and, like for large structure formation, beyond the scope of this first article.

E. Other consequence

Field trapping naturally explains the cosmic coincidence, i.e. that in the ΛCDM model, dark energy’s repulsion currently nearly compensates matter’s attraction, while repulsion was negligible in the past and attraction is expected be negligible in the future. No natural explanation exists within ΛCDM for this apparently fortuitous coincidence. In the present approach, structure formation depletes attraction and thus, compensating it with a repulsion, viz dark energy, is unnecessary. Thus, there is no coincidence and hence no need for explanation.

It has long been known that modified gravity approaches, generically, tend to solve the "impossible early galaxies" problem of ΛCDM by speeding up structure formation. See, e.g., Llinares et al. 2008 ("we find that the large-scale structure evolution is faster in our revised MOND model leading to an even stronger clustering of galaxies, especially when compared to the standard LCDM paradigm."). See also Sanders 1998, McGaugh 1998, McGaugh 1999, McGaugh 2000, Sanders 2001, Nusser 2002, Stachniewicz & Kutschera 2002, McGaugh 2004, Skordis et al. 2005, Feix 2016, Khoury 2016.

Meanwhile, the ΛCDM model when compared with observational data at the larger cosmology scale also fails the test of the new EDGES 21cm wavelength radiation data that demonstrates the temperature of the universe at 180 million to 280 million years after the Big Bang. This is inconsistent with the ΛCDM model because the universe was much colder than predicted and is instead consistent, generically, with a no dark matter hypothesis.

Also, finally, last year, a relativistic modified gravity theory that reduces to MOND in the limit has reproduced the crown jewel of LambdaCDM, the CMB patterns that are observed. See Constantinos Skordis, Tom Złosnik, "A new relativistic theory for Modified Newtonian Dynamics" arXiv (June 30, 2020). And, given that Deur's model is very close to MOND at the galactic rotation curve level, the likelihood that Deur's approach can achieve the same result is much more plausible than it was two years ago. In another proof of the concept that modified gravity theories can get the CMB right, Moffat's MOG theory did so in 2014.

Other Non-Galaxy Rotation Curve Evidence

Furthermore, keep in mind that unlike many modified gravity theories, this isn't just galaxy rotation curves in addition to this recent paper on the early evolution of the universe discussed above.

It is also planar alignments of satellite galaxies. It is cluster dynamics and the Bullet Cluster. It is all dark energy phenomena (in a manner equivalent to dark energy that is not a pure cosmological constant as the data increasingly slightly favor).

It is providing a theoretical grounding from first principles for phenomena that other gravitationally based approaches to dark matter and dark energy phenomena have only described with phenomenological fits to observations.

Conclusion

One of the things that makes this approach so attractive is that it has a domain of applicability that is broader than almost any other gravity based approach out there right now, with the possible exception of Moffat's MOG theory (which manifestly deviated from GR and designed simply to fit observations without a clear theoretical foundation). MOND doesn't address dark energy or cluster scale phenomena correctly. Meanwhile, many other modified gravity theories proposed by GR researchers provide alternatives to the cosmological constant/dark energy, but don't solve dark matter phenomena.

Also, while there are areas of this approach that simply haven't been developed yet by the sole primary researcher working on this in addition to this day job in QCD physics (e.g. CMB, derivation of MOND constant from Newton's constant, wide binaries, external field effects), there are so far, no instances of clear contradictions with observation, or theoretical inconsistencies that have been identified.
 
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  • #32
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Getting back to the title of the thread. We really only have an effective field theory for gravitons. I think it is too early in our understanding of Quantum Gravity to make any definitive statement. It has been discussed before, discontinued in May, but for some reason revived on August 11.

The mentors will keep an eye on it to see if it adds value to the previous discussion.

Thanks
Bill
 
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  • #34
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Did you mean to link to this thread?

Sorry didn't notice it was the same thread. Changed the working to the mentors will keep an eye on if its revival is of any value.

Thanks
Bill
 
  • #35
timmdeeg
Gold Member
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After rereading Deur's recent article I came across this text passage where I seem to have a misunderstanding:

Likewise2 in gravitational systems, the increased binding due to GR's SI weakens gravity's action at large scale. If GR's SI is ignored, this weakening can then be misinterpreted as a large-scale repulsion, viz dark energy. The effect is time-dependent: as massive structures form, some gravitational fields become trapped in them, weakening their manifestation at larger scale. This implies a direct connection between dark energy and dark matter, particularly between dark energy and the onset of structure formation.
I "understand" that the " increased binding due to GR's SI" could account for dark matter. But how can the weakening of gravity's action (due to SI) account for the observed accelerated expansion of the universe?

If gravity is "just" weakened between the Galaxy clusters it's still attractive so how can this imitate (or replace?) the action of repelling gravity according to the Lambda-CDM model?

What am I missing?
 
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