# Less gravity vs. dark energy

• I
Gold Member

## Main Question or Discussion Point

One way to get the universe to expand is with dark energy that pulls at the matter of the galaxy separating it or equivalently for space-time to not be perfectly flat.

An alternative, in principle, would be for the gravitational pull between objects like galaxies and galactic clusters to be suppressed for some reason relative to the "GR without cosmological constant" prediction. Instead of being tugged apart from the outside, these objects would not be held together a strongly as expected. For example, perhaps gravity is a very long range, but not infinite range force, or perhaps dark matter absorbs gravitons thereby reducing the amount of gravitational pull that can escape the system with dark matter in it which could also address the "coincidence problem".

Lots of published academic work explores the dark energy possibility.

Is anyone aware of any peer review work/academic pre-prints that explore the suppressed gravitational field possibility as an explanation for the phenomena commonly attributed to dark energy?

This question is inspired by ideas sketched out on a back of napkin basis by A. Deur in published peer reviewed work, but the question is more general.

Related Beyond the Standard Model News on Phys.org
mfb
Mentor
Even without any gravitational attraction you wouldn't get an accelerated expansion as we observe it.

Gold Member
Even without any gravitational attraction you wouldn't get an accelerated expansion as we observe it.
Why not?

Isn't one gravitational force pulling two objects together combined with another pulling them apart equivalent to a single net force that is weaker than the gravitational force pulling the two objects together in the first scenario, and couldn't increasing distance over time cause this to accelerate if the fall off/suppression was related to distance? The scenario Deur explains doesn't do that, but one could imagine one where galaxies have KE from the big bang whose suppression by gravity falls off increasingly as they get further would would seem to look like acceleration (and might even change sign at some distance) (although even if it can't explain fully dark energy acceleration it could still quantitatively greatly reduce the amount of DE that was needed to get the same result).

A representative paper sketching out the idea without developing it very much is:

Implications of Graviton-Graviton Interaction to Dark Matter
A. Deur
(Submitted on 26 Jan 2009 (v1), last revised 6 May 2009 (this version, v2))
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.
Comments: Version published in Phys. Lett. B. Added material: 1) We explicited the steps leading from the Einstein-Hilbert Lagrangian to our simplified Lagrangian. 2) We showed how the Tully-Fisher relation emerges naturally from our framework. 3) We added a discussion on the approximations we used
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph)
Journal reference: Phys.Lett.B676:21-24,2009
DOI: 10.1016/j.physletb.2009.04.060
Cite as: arXiv:0901.4005 [astro-ph.CO]
(or arXiv:0901.4005v2 [astro-ph.CO] for this version)

The relevant portion of the linked article explains that:

Before concluding, we exploit further the QCD-gravity analogy, now on a qualitative level. The confinement of gluons inside a hadron not only changes the 1/r quark-quark potential into an r potential, but also causes two hadrons to not interact through the strong force [19] since there is no strong force carriers outside the hadrons. Similarly, the increased binding inside a galaxy would weaken its interaction with outside bodies. Such reduction of the strength of gravity is opposite to what we would conclude by explaining galaxy rotation curves with halos of exotic dark matter or with gravity modifications, and may be relevant to the fact that the universe expansion is accelerating rather than decelerating. This is currently explained by the repulsive action of a dark energy, see e.g. [2]. However, if gravity is weakened, the difference between the assumed Abelian force and the actual strength of the force would be seen as an additional repulsive effect. Such effect would not explain a net repulsion since it would at most suppress the force outside of the mass system (as for QCD). Thus, it would not be directly responsible for a net acceleration of the universe expansion. Nevertheless, it may reduce the need for dark energy. To sum up, the gravity/QCD parallel propounds that dark energy may be partly a consequence of energy conservation between the increased galaxy binding energy vs. the outside effective potential energy. This would implies a quantitative relation between dark energy and dark matter, which might explain naturally the cosmic coincidence problem [2].
Finally, just to be clear, I'm not interested so much in Deur's specific proposal here, so much as identifying if anyone else has explored conceptually similar concepts in academic articles. The hope is that someone in the crowd at Physics Forum BSM may have seen theories that I am unaware of explored somewhere credible.

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mfb
Mentor
Second Friedmann equation:$$\frac{\ddot{a}}{a} = -\frac{4 \pi G}{3}\left(\rho+\frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}$$

##\rho \geq 0## and ##p\geq 0##. With ##\Lambda=0## and ##G\geq 0##, we cannot get the positive ##\ddot a## we observe, even if we set the effective G to 0, which means absolutely no gravitational effect on the expansion of the universe.
Nevertheless, it may reduce the need for dark energy.
Sure, you need a smaller amount of dark energy if you reduce the effective G. So what. You still need it.

Gold Member
So what. You still need it.
Among other things, it dramatically reduces the total amount of stuff in the universe, because dark matter and dark energy cancel out, rather than adding to each other, and if there is a whole realm of possibilities that haven't even been probed theoretically at a superficial level, we could miss parts of parameter space that are interesting or important. For example, if you need much less dark energy than you would otherwise, you open up potential dark energy mechanism that otherwise wouldn't be worth considering because they are the wrong magnitude.

mfb
Mentor
Dark energy is 70% of the total energy density, and we know the cosmological constant with an accuracy of about 2%. Yes, if you remove all the impact of matter with a model*, its value would go down a bit, but you there is nothing magic about the current density. You would just get another value which we cannot calculate from deeper principles.

*that is problematic in the early universe which was matter-dominated

Among other things, it dramatically reduces the total amount of stuff in the universe, because dark matter and dark energy cancel out, rather than adding to each other,
Its important to stress that this depends on the measurement your making. They cancel out when measuring the expansion rate of the universe ( via supernovae 1a ) But when measuring the angular scale of the peak in the CMB anisotropy spectrum (which depends on the spatial geometry of the universe ) they actually add up.
See this combination plot to see how reducing both dark energy and dark matter won't fit the data:

http://supernova.lbl.gov/union/figures/Union2.1_Om-Ol_slide.pdf

mfb
Gold Member
See this combination plot to see how reducing both dark energy and dark matter won't fit the data:

http://supernova.lbl.gov/union/figures/Union2.1_Om-Ol_slide.pdf
This is right in the context of fitting lambda CDM parameters in a model dependent way. But, these conclusions are outside their domain of applicability in the context of a radically different model from the lambda CDM paradigm.

Gold Member
A new paper on point:

https://arxiv.org/pdf/1709.02481.pdf

A possible explanation for dark matter and dark energy consistent with the Standard Model of particle physics and General Relativity
A. Deur

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.
FWIW, I am not convinced that this approach to QG is truly consistent with GR, given canonical results from DSW but the difference between this and GR is quite subtle.

mfb
Mentor
I would like to see this going through peer-review...

Close to the Sun, ##\sqrt {GM/L}## is larger than in our galaxy. Nothing special has been observed there. Nothing special for binary pulsars either. Lunar laser ranging? The article doesn't even discuss all these things.

Gold Member
I would like to see this going through peer-review...

Close to the Sun, ##\sqrt {GM/L}## is larger than in our galaxy. Nothing special has been observed there. Nothing special for binary pulsars either. Lunar laser ranging? The article doesn't even discuss all these things.
Two previous articles in the same series (at least) have gone through peer review and been published. I suspect that this will follow. Honestly, I haven't had time to read the new article closely, but an important point in the prior articles had been a lack of spherical symmetry, which would explain, e.g., a lack of solar system observations and differences in apparent dark matter proportion based upon the extent to which elliptical galaxies are not spherical. https://arxiv.org/abs/1407.7496 Disk galaxies, obviously are not spherical except near their bulges, explaining the dark matter effect there, and clusters are modeled almost like dumb bells and are maximally asymmetric. But, I do not recall precisely how L is defined in this paper.

Basically, you need small galaxy scale masses and asymmetry before you get observable effects.

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