Deur's Work On Gravity

Background

MOND and other modified gravity theories that explain dark matter

A phenomenological toy-model theory developed in 1983 by M. Milgrom called MOND (for "Modified Newtonian Dynamics") has been used to successfully explain the weak field behavior of the gravitational force at all scales from Earth bound and solar system scales to the scale of galaxies and galaxy-satellite galaxy systems, although it tends to underestimate the non-Newtonian effects seen in weak fields in galaxy clusters.

Despite the fact that this theory is only a simple toy-model with a single universal acceleration parameter (a0 = 1.2 × 10−10 ms−2), it can reproduce the dark matter phenomena in all systems of galaxy scale or smaller, and can reproduce some, but not the full extent of, dark matter phenomena in galactic cluster scale systems.

This toy model version of the theory isn't relativistic, however, so its doesn't capture many of the distinctive features of General Relativity, although generalizations of its in a relativistic sense (e.g. TeVeS), and a variety alternative formulations that reproduce its core insight, have been attempted with mixed success.

Deur's Work On Gravity

One of the most promising is Alexandre Deur's efforts to identify quantum gravity effects in a graviton based theory of quantum gravity modeled for mathematical simplicity in the static scalar graviton case, exploiting analogies to quantum chromodynamics (i.e. the Standard Model's theory of the strong force that binds quarks and gluons based upon their color charge). This model is useful because, like the gluons of QCD (and unlike the photons of quantum electrodynamics, which is the Standard Model theory of electromagnetism), the gravitons which are the carrier bosons of a hypothetical quantum gravity theory would have gravitational interactions with each other. Graviton based quantum gravity, like QCD, is a non-Abelian force.

He concludes, by analogy to QCD, that graviton-graviton interactions lead to effects that mimic dark matter and dark energy in the weak fields of very massive gravitational sources to the extent that the systems are not spherically symmetric.

Despite the fact that he claims that his theory is consistent with General Relativity, however, this is not accurate, even though in the limit of spherical systems and strong gravitational fields, it closely approximates General Relativity's predictions.

Deur's "day job" is as a QCD physicist at Jefferson Labs, so he brings to the field a different set of mathematical tools and insights than the typical quantum gravity researcher, who typically starts with a study of classical General Relativity and then expands from there.

Why Is Deur's Approach Promising?

Deur's papers are not widely cited and, while they have been published in peer reviewed journals, have not received in depth investigation from other scientists in the astronomy and gravity field.

But, they remain very promising as a possible solution to the dark matter and dark energy problems.

Observational Evidence Supports Deur's Model

* There is strong evidence developed in the MOND context, that dark matter phenomena can be understood as modifications of conventional Newtonian approximations of gravity, as opposed to solutions with dark matter particles beyond the Standard Model. Because the formula that Deur develops is observationally almost indistinguishable from MOND in the circumstances where MOND works well, his theory benefits from this body of evidence. For example, new 21 cm 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 and 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 apparently 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.

* This quantum gravity theory overcomes problems arising from observational evidence that are associated with massive gravitons in other 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 mass fermion do not accurately reproduce the inferred dark matter halos that are observed.

Deur's Model Is Attractive Theoretically

* This explains dark matter and dark energy phenomena as a natural outgrowth of quantum gravity, with no "moving parts" that can be adjusted to make it fit the data in advance.

* Deur's theory provides a sound theoretical basis for the dark matter phenomena post-Newtonian gravity modifications that it utilizes, because they are derived from first principles, 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. It is not mere numerology or a purely phenomenological theory.

* While Deur's approach does not reproduce the conclusions of conventional classical General Relativity 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, except in ways generic to all quantum gravity theories (e.g. all quantum gravity theories with gravitons localize gravitational mass-energy, while classical General Relativity does not).

* 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 must be subtly flawed. This also explains why quantum gravity researchers trying to build a quantum gravity theory that exactly reproduces Einstein's equations have failed. They have tried to reproduce a slightly erroneous equation 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 mathematics are sound. Independent efforts corroborate the validity of the main simplification he makes relative to quantum gravity with a spin-2 massless graviton.

* 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. This coupling constant is basically Newton's constant G, although possibly in different units. Both the cosmological constant ad the universal acceleration constant of MOND can be derived, in principle, from 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, 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. Deur's formulation of the theory as a quantum field theory simplifies its integration with the Standard Model.

* Deur's theory explains the cosmic coincidence problem.

* 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.

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

A Quick Primer

One of the better and more intuitive introductions to his ideas is in this power point presentation. Some of the key points from that presentation are restated below:

Empirical parallels between Cosmology and Hadronic Physics

Cosmology (1)

Galaxies (or clusters of galaxies) have a larger mass than the sum of their known constituents.

Alternatively Gravity is stronger than we think for these systems.

Hadronic Physics (1)

Hadronic physics 2 quarks ~10 MeV, Pion mass 140 MeV 3 quarks ~15 MeV, Nucleon: 938 MeV

For non-relativistic quarks, this extra mass comes from large binding energy.

Cosmology (2)

Tully-Fisher relation: log(M)=γlog(v)+ε
(γ=3.9±0.2, ε ~1.5)
(M galaxy visible mass, v rotation speed)

Unexplained with dark matter. Assumed by MOND.

Hadronic Physics (2)

Regge trajectories: log(M)=c log(J)+b
(c=0.5)
(M, hadron mass, J angular momentum)

Cosmology (3)

Negative pressure pervades the universe and repels galaxies from each other. The attraction of galaxies is smaller than we think at very large distances.

Hadronic Physics (3)

Relatively weak effective force between hadrons (Yukawa potential) compared to QCD’s magnitude.

Deur's Quantum Gravity LaGrangian and QCD Compared

The gravitational Lagrangian that he develops is as follows:

ℒGR=[∂ψ∂ψ]+√G[ψ∂ψ∂ψ]+G[ψ2∂ψ∂ψ]+ΣGn/2[ψn∂ψ∂ψ]+√ψμνTμν

This is derived by expanding the ℒGR in term of tensor gravity field ψμν by developing gμν around the Minkowsky metric: gμν~ημν+G1/2ψμν+...

This compared to the QCD Lagrangian:

ℒQCD=[∂ψ∂ψ]+√4παs2∂ψ]+ 4παs4]

The first terms of each are Newtonian gravity and perturbative QCD respectively (in the static case). The next two terms of the respective Lagrangians are field self-interaction terms.

How strong are the gravitational self-interaction terms? 

This is a function, roughly speaking, of system mass and system size:

Near a proton GMp/rp=4×10-38 with Mp the proton mass and rp its radius. ==>Self-interaction effects are negligible:

ℒGR=[∂ψ∂ψ]+√G[ψ∂ψ∂ψ]+G[ψ2∂ψ∂ψ]+...  the stricken terms --> zero.

For a typical galaxy: Magnitude of the gravity field is proportionate to GM/sizesystem which is approximately equal to 10-3.

Basically, the most thinly spread the mass is in space, the stronger the self-interaction terms are relative to the Newtonian term of the Lagrangian. For gravitons, the profoundly weak strength of the Newton's constant means that self-interaction terms are only significant at immense distances where the mass is spread thinly.

In QCD, by comparison, the profoundly greater strength of the QCD coupling constant allows the self-interaction terms to be significant even at tiny distances on the order of 10-15 meters, despite the fact that the color charges are not spread thin.

Matter Distribution Shapes and Self-Interaction Strength

Essentially, self-interacting gravitons, rather than going off randomly in all directions, tend to veer towards direct gravitational fields between clumps of mass, making those fields stronger, while weakening the fields in the direction of empty space.

Isolated Point Masses

For two significant point masses with nothing else nearby, self-interactions cause the system to reduce from a three dimensional one to a flux tube causing the force between them to remain nearly constant without regard to distance.

Disk-Like Masses

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 FGis 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.

Spherically Symmetric Masses

If the mass is spherically symmetric, the self-interactions cancel out and the system remains three dimensional causing the force to have the 1/r2 form that we associate with Newtonian gravity.

Why do galactic clusters have so much more apparent dark matter than spiral galaxies? 

Because geometrically, they are closer to the two point particle scenario, in which galaxies within the cluster are the point particles that exert a distance independent force upon each other, rather than being spherically symmetric or disk-like.

Why does the Bullet Cluster behave as it does? 

Since gas dominates the visible mass of a cluster, the observation that most of the total (dark) mass did not stay with the gas appears to rules out modifications of gravity as an alternative to dark matter. But, actually, this isn't the case in a self-interacting graviton scenario.

Because it has a gaseous component that is more or less spherically symmetric, that component has little apparent dark matter, while the galaxy components, which come close to the two point mass flux tube paradigm which displays great inferred dark matter. So, the gaseous portion and the core galaxy components are offset from each other. The apparent dark matter tracks the galaxy cores and not the interstellar gas medium between them.

Annotated Bibliography

The first article in the series by Deur on gravity is:
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) (not published).

The first of his papers published in a peer reviewed journal is:
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)). 

Deur also makes a theoretical prediction which neither dark matter nor MOND suggest, which is born out by observation. This prediction is that non-spherical elliptical galaxies have greater deviations from general relativity without dark matter than more spherically symmetric elliptical galaxies do. This is found in a 2014 paper:
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.

Deur argues that most or all of observed dark energy phenomena results from gravitons being confined in galaxy and galactic cluster scale systems, which is what gives rise to the dark matter phenomena in this systems. The diversion of gravitons to more strongly bind matter in the galaxies leads to a small deficit of gravitons which escape the galaxy and cause galaxies and galactic clusters to bind to each other. It also neatly explains the "cosmic coincidence problem." He spells out this analysis in a 2018 pre-print (with an original pre-print date in 2017) which also examines cosmology implications of his approach more generally:
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” (August 14, 2018) (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 body text of this paper explains at greater length that:
The framework used in Refs. [3, 4] is analogous to the well-studied phenomenology of Quantum Chromodynamics (QCD) in its strong regime. Both GR and QCD Lagrangians comprise field self-interaction terms. In QCD, their effect is important because of the large value of QCD’s coupling, typically αs ' 0.1 at the transition between QCD’s weak and strong regimes [8]. In GR, self-interaction becomes important for p GM/L large enough (G is Newton’s constant, M the mass of the system and p L its characteristic scale), typically for GM/L & 10−3 [4]. In QCD, a crucial consequence of self-interaction associated with a large αs is an increased binding of quarks, which leads to their confinement. Refs. [3, 4] show that GR’s self-interaction terms lead to a similar phenomenon for p GM/L large enough, which can explain observations suggestive of dark matter. Beside confinement, the other principal feature of QCD is a dearth of strong interaction outside of hadrons, the bound states of QCD. This is due to the confinement of the color field in hadrons. While the confined field produces a constant force between quarks that is more intense than the 1/r2 force expected from a theory without self-interaction, this concentration of the field inside the hadron means a depletion outside. If such phenomenon occurs for gravity because of trapping of the gravitational field in massive structures such as galaxies or clusters of galaxies, the suppression of gravity at large scale can be mistaken for a repulsive pressure, i.e. dark energy. Specifically, the Friedman equation for the Universe expansion is (assuming a matter-dominated flat Universe) H2 = 8πGρ/3, with H the Hubble parameter and ρ the density. If gravity is effectively suppressed at large scale as massive structures coalesce, the Gρ factor, effectively decreasing with time, would imply a larger than expected value of H at early times, as seen by the observations suggesting the existence of dark energy. Incidentally, beside dark matter and dark energy, QCD phenomenology also suggests a solution to the problem of the extremely large value of Λ predicted by Quantum Field Theory [9].
Reference [9] in that paper is worth noting because is provides a simple and analogous solution to the gross disparity between the quantum mechanical expectation for the cosmological constant and its actual, tiny value. It is as follows:
Casher and Susskind [Casher A, Susskind L (1974) Phys Rev 9:436–460] have noted that in the light-front description, spontaneous chiral symmetry breaking is a property of hadronic wavefunctions and not of the vacuum. Here we show from several physical perspectives that, because of color confinement, quark and gluon condensates in quantum chromodynamics (QCD) are associated with the internal dynamics of hadrons. We discuss condensates using condensed matter analogues, the Anti de Sitter/conformal field theory correspondence, and the Bethe–Salpeter–Dyson–Schwinger approach for bound states. Our analysis is in agreement with the Casher and Susskind model and the explicit demonstration of “in-hadron” condensates by Roberts and coworkers [Maris P, Roberts CD, Tandy PC (1998) Phys Lett B 420:267–273], using the Bethe–Salpeter–Dyson–Schwinger formalism for QCD-bound states. These results imply that QCD condensates give zero contribution to the cosmological constant, because all of the gravitational effects of the in-hadron condensates are already included in the normal contribution from hadron masses.
Stanley J. Brodsky and Robert Shrock, "Condensates in quantum chromodynamics and the cosmological constant" 108(1) PNAS 45 (January 4, 2011).
The use of a scalar graviton approximation used by Deur is justified in a published 2017 paper:
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).

A paper in the same journal by independent authors confirms that scalar approximations can reproduce experimental tests:
We construct a general stratified scalar theory of gravitation from a field equation that accounts for the self-interaction of the field and a particle Lagrangian, and calculate its post-Newtonian parameters. Using this general framework, we analyze several specific scalar theories of gravitation and check their predictions for the solar system post-Newtonian effects.
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).

4 comments:

neo said...

would graviton graviton interactions analogy with QCD hold only when gravity is strong?

the weaker gravity is, the weaker graviton graviton interactions the less it is like QCD and more like QED

Deur's approach might make most sense in the strong QG regime, where gravitons behave like QCD such as a black hole, but i don't see how it explains the MOND regime

andrew said...

Graviton-graviton interactions are a second order effect (because gravitons don't have a lot of mass-energy relative to other things it could couple to). So, in a strong field, that second order effect (which is only present in non-spherical systems in any case) is overwhelmed by the first order effect of the gravitational pull from stuff other than gravitons.

The analogy with QCD holds because both carrier bosons are self-interacting.

QED has no self-interactions (it is abelian). Photons don't have electric charge and hence don't interact with other photons. And, weak force bosons (W and Z bosons) are so short lived (on average 10^-25 seconds) that they don't have much of an opportunity to interact with each other before they decay.

You do need a large source mass to get a discernible effect from graviton-graviton interactions, but you also need the strength of the gravitational field itself to be very weak so that the second order effects can become larger than the first order effects.

The easiest way to see this is to look at the terms of the relevant equations in the Power Point presentation.

neo said...

Lee Smolin's paper attempts to provide an outline why the Mond scale ao is close to the cc, Verlinde entropic gravity also takes note of this

MOND is a dark energy effect

how does Deur explain this apparent coincidence?

on Stacy's webpage a poster provided links to another theory that MOND is result of quantized inertia, rather than gravity.

andrew said...

"Lee Smolin's paper attempts to provide an outline why the Mond scale ao is close to the cc, Verlinde entropic gravity also takes note of this --- MOND is a dark energy effect -- how does Deur explain this apparent coincidence?"

In Deur's approach, the phenomena attributed to dark energy arise from the fact that gravitons that would head out into empty space to pull galaxies and galactic clusters together, instead stay inside those galaxies holding them together tighter that they would if they were not self-interacting. Thus, the "cosmological constant" is really due to a graviton deficit in empty space that is matched by a graviton surplus in systems that appear to have dark matter.

"on Stacy's webpage a poster provided links to another theory that MOND is result of quantized inertia, rather than gravity."

While this theory was proposed, it never really went anywhere and was hard to conceptualize. Our brains like thinking in terms of forces more than they do thinking in terms of inertia. Deur's approach, likewise, is conceptualized in terms of force fields.