Do e Observe Quantum Gravity Effects at Galactic Scales?

[marcus]

Do We Observe Quantum Gravity Effects at Galactic Scales?

http://www.arxiv.org/abs/astro-ph/0509163
Do we Observe Quantum Gravity Effects at Galactic Scales?
M. Reuter, H. Weyer
6 pages, 1 figure. Talk given by M.R. at the 21st IAP meeting "Mass Profiles and Shapes of Cosmological Structures", Paris, July 4-9, 2005; to appear in the proceedings

"The nonperturbative renormalization group flow of Quantum Einstein Gravity (QEG) is reviewed. It is argued that there could be strong renormalization effects at large distances, in particular a scale dependent Newton constant, which mimic the presence of dark matter at galactic and cosmological scales."

the version of QEG referred to in this paper is that published by Reuter and Lauscher in 2002
http://www.arxiv.org/abs/hep-th/0110021
Is Quantum Einstein Gravity Nonperturbatively Renormalizable?
O. Lauscher, M. Reuter
18 pages, 3 figures
Class.Quant.Grav. 19 (2002) 483-492

"We find considerable evidence supporting the conjecture that four-dimensional Quantum Einstein Gravity is 'asymptotically safe' in Weinberg's sense. This would mean that the theory is likely to be nonperturbatively renormalizable and thus could be considered a fundamental (rather than merely effective) theory which is mathematically consistent and predictive down to arbitrarily small length scales. For a truncated version of the exact flow equation of the effective average action we establish the existence of a non-Gaussian renormalization group fixed point which is suitable for the construction of a nonperturbative infinite cutoff-limit. The truncation ansatz includes the Einstein-Hilbert action and a higher derivative term."

here is an earlier paper about this, from October 2004. I started a PF thread about it when it came out, but that was a long time ago so I will add the abstract here

http://arxiv.org/abs/hep-th/0410117
Running Newton Constant, Improved Gravitational Actions, and Galaxy Rotation Curves
M. Reuter, H. Weyer
72 pages
PhysRevD.70.124028
"A renormalization group (RG) improvement of the Einstein-Hilbert action is performed which promotes Newton's constant and the cosmological constant to scalar functions on spacetime. They arise from solutions of an exact RG equation by means of a 'cutoff identification' which associates RG scales to the points of spacetime. The resulting modified Einstein equations for spherically symmetric, static spacetimes are derived and analyzed in detail. The modifications of the Newtonian limit due to the RG evolution are obtained for the general case. As an application, the viability of a scenario is investigated where strong quantum effects in the infrared cause Newton's constant to grow at large (astrophysical) distances. For two specific RG trajectories exact vacuum spacetimes modifying the Schwarzschild metric are obtained by means of a solution-generating Weyl transformation. Their possible relevance to the problem of the observed approximately flat galaxy rotation curves is discussed. It is found that a power law running of Newton's constant with a small exponent of the order 10^{-6} would account for their non-Keplerian behavior without having to postulate the presence of any dark matter in the galactic halo."

I see this one has 11 citations, several from other researchers who have taken up the idea

 

[wolram]

Please forgive the intrusion, this seems interesting to me, but the language
is to deep, i am sure this is important so will some one explain it in easy terms
please.

 

[marcus]

...this seems interesting to me, but the language
is too deep, i am sure this is important so will some one explain it in easy terms please.

you are not intruding, it's an appropriate question wolram. It seems interesting to me too. I cannot provide an adequate discussion, nor can I evaluate-----there may be something wrong with Reuter's approach, and even if there is, it is still attracting attention and being tried out by other researchers. Even if there are serious flaws or details that remain to work out it still appears SUGGESTIVE to a number of people.

Reuter has a scheme where the effective gravitational constant G is larger over very long distances

(and also smaller at microscopic distances than what we measure at ordinary lab scale-----the apparatus to determine G is difficult to miniaturize so it has not been measured at scales much less than 1 centimeter)

If G "runs", that is to say VARIES WITH DISTANCE, then maybe we don't need dark matter to explain galaxy rotation curves. Because if G is effectively larger at those large scales, maybe a smaller amount of matter (acting more strongly because of larger coupling constant G) is enough to explain the observed speeds and inferred accelerations.

This is likely to be a gross oversimplification. I haven't yet understood the real message. I suspect what he is talking about is not as simple as G varying with distance-----it could be something else (related to G) that is varying and it could be in response to something else (not distance). But at a superficial level that is roughly the idea.

 

[marcus]

another paper of theirs seems to be easier to read

http://arxiv.org/abs/hep-th/0410119
Quantum Gravity at Astrophysical Distances?
M. Reuter, H. Weyer
46 pages, 4 figures, to appear in JCAP
JCAP 0412 (2004) 001

"Assuming that Quantum Einstein Gravity (QEG) is the correct theory of gravity on all length scales we use analytical results from nonperturbative renormalization group (RG) equations as well as experimental input in order to characterize the special RG trajectory of QEG which is realized in Nature and to determine its parameters. On this trajectory, we identify a regime of scales where gravitational physics is well described by classical General Relativity. Strong renormalization effects occur at both larger and smaller momentum scales. The latter lead to a growth of Newton's constant at large distances. We argue that this effect becomes visible at the scale of galaxies and could provide a solution to the astrophysical missing mass problem which does not require any dark matter. We show that an extremely weak power law running of Newton's constant leads to flat galaxy rotation curves similar to those observed in Nature. Furthermore, a possible resolution of the cosmological constant problem is proposed by noting that all RG trajectories admitting a long classical regime automatically give rise to a small cosmological constant."

this paper is still not short but it is shorter than what they published in Physical Review series D. And it has fewer formulas and correspondingly more verbal explanation.
But don't get your hopes up, it is still not sure it is RIGHT and it is still somewhat vague and speculative, and it is still hard (for me at least) to understand.

the only thing objective I can point to is that other researchers are now jumping in on this----the citation numbers are rising on these Reuter papers, especially the ones that suggest some alternative to dark matter.

==========
this JCAP paper, from October 2004, has 16 cites many quite recent
the PhysRev paper, also from October 2004, has 11 cites, also several of them recent
the LATEST paper, from September 2005? too soon

 

[wolram]

Thank you MARCUS, it is all to coplex for me, i will wait for the tit bits, but
i am sure a modifed gravity theory will work one day.

 

[marcus]

Thank you MARCUS, it is all too complex for me, i will wait for the tit bits, but
i am sure a modifed gravity theory will work one day.

I sympathise Wolram---it is our common plight.

If you wouldn't mind telling me, it would save me doing a search: did you already start a thread in Astronomy about a paper by two Canadians I think they were, who claimed to get rid of the need for dark matter just using ordinary conventional vintage-1915 einstein gravity?

http://arxiv.org/abs/astro-ph/0507619

I hit this by accident and it is just the sort of thing you often come up with. they claim to have shown that dark matter isn't necessary just by ordinary general relativity. I find that quite puzzling.

 

[selfAdjoint]

The velocity profile matches are amazing. As I recall, when this was first posted on the Astro board, there were demurs of the nature of "dark matter does other things than determine the velocity profile" and "So-and-so has modified the effect of dark mattter on the profile". Obviously what astronomers should do now is subtract this effect from their observations and see if there is any role left for DM. I don't think they will be able to brush this work off.

 

[marcus]

The velocity profile matches are amazing. As I recall, when this was first posted on the Astro board, there were demurs of the nature of "dark matter does other things than determine the velocity profile" ...

I found the link to the thread you started about this:
https://www.physicsforums.com/showthread.php?p=692372#post692372

the Cooperstock and Tieu article

I'd really like to see some follow up on this!

 

[wolram]

http://www.fnal.gov/pub/presspass/press_releases/darkenergy_3-16-05.html

Sorry this is not a very good reference, i lost my files when my hard drive
died.

 

[marcus]

..., i lost my files when my hard drive
died.

sorry to hear that. I never lost a whole drive----several years email correspondence and things of that magnitude (it might feel a bit like electro-shock "therapy" would have, hopefully they don't do that any more) where a part of you is gone.

 

[ohwilleke]

Please forgive the intrusion, this seems interesting to me, but the language
is to deep, i am sure this is important so will some one explain it in easy terms
please.

There are two ways of explaining galaxy rotation curves. You can have dark matter or you can modify traditional rules of gravity. This paper uses a gravity modification approach. But, it does so using a quantum mechanical approach the general relativity, rather than the theories such as MOND which have been used in the past.

 

[wolram]

Come on guys, what is gravity should be so simple, i wish i never asked the question so long ago, give me a box of chocolates any day.

 

[selfAdjoint]

There are two ways of explaining galaxy rotation curves. You can have dark matter or you can modify traditional rules of gravity. This paper uses a gravity modification approach. But, it does so using a quantum mechanical approach the general relativity, rather than the theories such as MOND which have been used in the past.

What quantum mechanics? The paper has straight weak-field general relativity, and I sure didn't see anything else. Ohwilleke, on the other thread you had some technical objections to the developments in the paper; could you expand on that here?

As for the other objection, basically that observation has detected "dark matter" closer to the core than was thought, well doesn't that mean that the Newtonian gravity of this observed matter doesn't serve to account for the rotation profiles? It was originally put out in the haloes because that's where it served just that purpose. If we now have (1)observation of matter elsewhere, and (2) a mechanism that can account for the profiles without dark matter, then it would follow that the newly observed matter is not "dark matter" wouldn't it?

 

[Mike2]

What quantum mechanics? The paper has straight weak-field general relativity, and I sure didn't see anything else.

Just had a thought... If I remember correctly, some think that spacetime itself has entropy/information associated with it, the probability of the configuration compared to all the possible states that spacetime can have, especially if it is discrete... or something like that.

OK then, if there is entropy associated with spacetime, then isn't there also an energy associated with spacetime in general, maybe the cosmological constant or the Hawking radiation from the cosmological event horizon. If so, then could space itself be contributing to the gravitational field and give us more of an effect than we would otherwise think. And couldn't this explain all the effects of Dark Matter that we see at play on large scale objects? Thanks.

 

[Spin_Network]

Just had a thought... If I remember correctly, some think that spacetime itself has entropy/information associated with it, the probability of the configuration compared to all the possible states that spacetime can have, especially if it is discrete... or something like that.

OK then, if there is entropy associated with spacetime, then isn't there also an energy associated with spacetime in general, maybe the cosmological constant or the Hawking radiation from the cosmological event horizon. If so, then could space itself be contributing to the gravitational field and give us more of an effect than we would otherwise think. And couldn't this explain all the effects of Dark Matter that we see at play on large scale objects? Thanks.

And this is exactly what is reported in this new paper:http://arxiv.org/abs/gr-qc/0510015

Read it through, you may find what your looking for?

 

[Mike2]

And this is exactly what is reported in this new paper:http://arxiv.org/abs/gr-qc/0510015

Read it through, you may find what your looking for?

I searched the document and found no reference to dark matter.

If the cosmological constant or dark energy contributes to the Einstein field equations, then wouldn't there be more of a gravitational effect than otherwise? In such a case, isn't dark matter the same as dark energy?

I suppose that the wrapping due to the energy of the cosmological constant would pretty much cancel with no large gravitating bodies around. But in non-homogeneous regions, wouldn't the inhomogeneities become greater than otherwise?

 

[Kea]

another NoDarkMatter paper

A tensor-vector-scalar framework for modified dynamics and cosmic dark matter
R.H. Sanders
11 pages
http://arxiv.org/abs/astro-ph/0502222

Has PF mentioned this one? I just noticed it after a series of seminars here by Malcolm Longair, in particular discussing cosmological scale evidence for CDM.

 

[marcus]

A tensor-vector-scalar framework for modified dynamics and cosmic dark matter
R.H. Sanders
11 pages
http://arxiv.org/abs/astro-ph/0502222

Has PF mentioned this one? I just noticed it after a series of seminars here by Malcolm Longair, in particular discussing cosmological scale evidence for CDM.

PF has not, to my knowledge, mentioned this one.
Kea you could make a structured bibliography of all the Dark Matter-Huh? and Dark Energy-Hah! research papers. there is getting to be a bunch.

 

[selfAdjoint]

PF has not, to my knowledge, mentioned this one.
Kea you could make a structured bibliography of all the Dark Matter-Huh? and Dark Energy-Hah! research papers. there is getting to be a bunch.

Since this is a scalar-tensor theory, Garth may know something about it. Perhaps he will comment.

Second thought, this is maybe like the joke: "I visited the Great Wall of China last month". "Oh did you meet my friend? He lives in Shanghai."

 

[Garth]

Is Sander's A tensor-vector-scalar framework for modified dynamics and cosmic dark matter the same as Bekenstein's Relativistic gravitation theory for the MOND paradigm?
Bekenstein's TeVeS theory has been discussed on PF here.

It seems a rather complicated means to produce the MOND gravitational attraction. The question is: "How may it be tested?"

Garth

 

[Crazy Moron]

Quantum gravity says space is made of triangular structures. If the structures exist, what is the force holding the structures in place? Could that force be the greater force operating at short and long distances?

Space changes shape, and that causes gravity. Imagine a model where space is warped by the sun so that space is less dense close to the sun. Less dense space would mean the triangular structures are stretched larger. As the structures of space get farther away from the sun they are stretched less, gradually decreasing in size, making a more dense space. This model of space density would be like gravity.

What if space structure can change density? What would cause that structure? Another force causes the structure of space. Imagine a bubble surrounding the sun, which is a model of varying space density. It is an exact model, the same as gravity. The sun causes the density of space to change by acting on the force that structures space. But at a greater distance the sun’s gravity could be too weak to affect the structure of space. It loses its ability to change the shape of space.

Imagine the affect of the sun’s gravity suddenly running out, not being able to affect the structure of space. You enter space that is suddenly more dense, because the sun cannot affect it at all. Voyager would slow down in more dense space and galaxy arms would bend.

We are probably not looking at one changing force of gravity, but at two forces. One force holds space in its shape. The other force, gravity, warps space, but acts perfectly only over limited distances. And then, the other force, which causes the structure of space starts to have an effect.

 

[marcus]

Quantum gravity says space is made of triangular structures...

Dear Crazy, I am curious why you say that.

Do you have a journal article that says this? I mean some reliable (hopefully online) thing you can show us that says that in some specific approach to quantum gravity space is made of triangular structures. There are several approaches. What approach are you talking about?

 

[Kea]

Is Sander's A tensor-vector-scalar framework for modified dynamics and cosmic dark matter the same as Bekenstein's Relativistic gravitation theory for the MOND paradigm?

Hi Garth

No. Sanders says he has generalised an older idea of Bekenstein's (an article in Second Canadian Conference on General Relativity and Relativistic Astrophysics World Scientific) ie. phase coupling gravity. It has ideas in common with TeVeS. Sanders calls his model a 'preferred frame generalisation of phase coupling gravity'.

Reference: Bekenstein Phys. Lett. B202 (1988) 497

Kea

 

[Crazy Moron]

I read in an article about Loop Quantum Gravity that Lee Smolin and the people in Canada say they think space might be made of triangular structures. You could ask them. That's where I got it. But I believed it, and it stuck with me because I had come to the same conclusion: space must be made of triangular structures. They can only run in six directions, giving us the six missing dimensions of string theory.

But that is not as important, here, as the other thing I said. It was that space seems to have a fabric quality to it. Mass stretches the fabric to create gravity. The fact that the laws of gravity change unexpectedly might be due to the fabric quality of space (the quantum structure of space) starts to take over as the gravity of an object reaches a limit where it is no longer able to stretch or distort the fabric of space.

You can plug that idea into bending galaxy arms and it works. The arms are being dragged though a more dense space.

This idea could easily be called quantum gravity, in which another force underneath gravity, and beyond gravity is really what is holding the universe together.

The best proof that this quantum structure of space might be right is in the question of whether the laws of gravity are absolutely perfect when guiding spacecraft to places like Mars. If there is a small element of deviation in the laws of gravity in the solar system, that deviation can be generated out to galactic scales as a deviation in gravity. But if there is no deviation in the laws of gravity in a voyage to the moon or Mars, then maybe we are talking about two different forces. The underlying force, the literal structure of space starts to take over when gravity reaches a certain weakness.

 

[marcus]

I read in an article about Loop Quantum Gravity that Lee Smolin and the people in Canada say they think space might be made of triangular structures...

thanks for replying CM. I see better where you are coming from. As you indicate, Smolin is associated with LQG. But LQG does not say
that space might be made of triangular structures

one way people interpret LQG is to say that "spin networks" are the basic constituents of space-----but spin networks can contain 4-sided and 5-sided cycles---they don't have to be all triangles. Or 6-sided. A 2D spin network could, for instance, be all hexagons.

 

[marcus]

I found the link to the thread you started about this:
https://www.physicsforums.com/showthread.php?p=692372#post692372

the Cooperstock and Tieu article

I'd really like to see some follow up on this!

SelfAdjoint flagged this Cooperstock/Tieu article some weeks back, see the link. Then on 3 October, I spotted it and brought it up in this thread.
https://www.physicsforums.com/showpost.php?p=773139&postcount=6

Now it has gotten on slashdot.com and NEW.
A rebuttal has been posted on arxiv.
Here is the original cooperstock article:

http://www.arxiv.org/abs/astro-ph/0507619
General Relativity Resolves Galactic Rotation Without Exotic Dark Matter
F. I. Cooperstock, S. Tieu
Submitted to the Astrophysical Journal, 23 pages, 7 figures, 4 tables

"A galaxy is modeled as a stationary axially symmetric pressure-free fluid in general relativity. For the weak gravitational fields under consideration, the field equations and the equations of motion ultimately lead to one linear and one nonlinear equation relating the angular velocity to the fluid density. It is shown that the rotation curves for the Milky Way, NGC 3031, NGC 3198 and NGC 7331 are consistent with the mass density distributions of the visible matter concentrated in flattened disks. Thus the need for a massive halo of exotic dark matter is removed. For these galaxies we determine the mass density for the luminous threshold as 10^{-21.75} kg.m$^{-3}."

I will get the link to the rebuttal. It looks like a hot issue. If cooperstock is right it does away with one of the reasons for dark matter (without bringing anything else into the picture, like MOND)

 

[marcus]

Here is some discussion on slashdot.com, for what it's worth:
http://science.slashdot.org/article.pl?sid=05/10/10/1052224&tid=160

there is some discussion at Not Even Wrong
http://www.math.columbia.edu/~woit/wordpress/?p=276#comments
around comment #17 you see this:
---quote---
D R Lunsford Says:
October 10th, 2005 at 11:42 am
On the assorted link front, we see this:
http://arxiv.org/abs/astro-ph/0507619
GR explains what NG can’t. No dark matter!
-drl

ali Says:
October 10th, 2005 at 12:41 pm
DRL:
You might want to look at this response to the paper you mentioned. Apparently, their metric and assumptions aren’t self-consistent:
http://arxiv.org/abs/astro-ph/0508377

D R Lunsford Says:
October 10th, 2005 at 2:45 pm
...The rebuttal paper is by no means convincing. The posited matter distribution is not at all unreasonable, nor is it the only possibility. The remarkable thing is, someone actually bothered to solve the equations instead of invoking yet another hand-waving argument. I’m sure we’ll see a “re-rebuttal” soon!
-drl
----endquote---

the rebuttal paper:
http://arxiv.org/abs/astro-ph/0508377
Singular disk of matter in the Cooperstock and Tieu galaxy model
Mikolaj Korzynski
5 pages
"Recently a new model of galactic gravitational field, based on ordinary General Relativity, has been proposed by Cooperstock and Tieu in which no exotic dark matter is needed to fit the observed rotation curve to a reasonable ordinary matter distribution. We argue that in this model the gravitational field is generated not only by the galaxy matter, but by a thin, singular disk as well. The model should therefore be considered unphysical."

I have no sense of how this is apt to turn out. Just out of caution maybe it is a good idea to be skeptical of Cooperstock and Tieu. (but DRL tells us to expect a "rebuttal to the rebuttal"

 

[selfAdjoint]

have no sense of how this is apt to turn out. Just out of caution maybe it is a good idea to be skeptical of Cooperstock and Tieu. (but DRL tells us to expect a "rebuttal to the rebuttal"

DRL seems to be a fan. He is the one who brought it up on a comments thread that was on several other topics. I really like the Cooperstock and Tieu idea, but as far as I can see (not far!) the criticism is valid too. Ah well, wait and see again. This is the way of science.

 

[Garth]

But also see the thread in GA&C here.

Garth

 

[selfAdjoint]

But also see the thread in GA&C here.

Garth

Thanks for the link, Garth. Even though I was the one who started that thread, I hadn't been following it. A lot of very pertinent discussion! I like your "the galaxy has a disk" riposte. Maybe the rebuttal is just clashing idealizations, neither one completely correct?

What do you think of the $$\sqrt{G}$$ versus G issue the rebuttors bring up? I thought the square root was standard?

As for the time to condense the galaxies, my instinct would be not to push the dust cloud model too far in the early cosmology environment. It's too steady-state, wouldn't you say?