New Paper on MOND by Bekenstein

[inflector]

Has anyone read and digested "Alternatives to dark matter: Modified gravity as an alternative to dark matter" by Jacob D. Bekenstein?

http://arxiv.org/abs/1001.3876

 

[Chalnoth]

Has anyone read and digested "Alternatives to dark matter: Modified gravity as an alternative to dark matter" by Jacob D. Bekenstein?

http://arxiv.org/abs/1001.3876

As far as I'm concerned, the CMB and the Bullet Cluster have quite effectively killed MOND as a realistic alternative to dark matter.

 

[matt.o]

Plus the fact that you can't model any cluster's mass using MOND without the need for extra dark mass, generally postulated to exist in the form >2eV neutrinos.

 

[marcus]

Plus astronomers now map the clouds of dark matter using weak gravitational lensing. Some galaxies show more than average proportion of DM to ordinary. Dwarf galaxies tend to have a larger ratio, and the ordinary is distributed differently relative to the DM. And physical explanations have been worked out. The more DM observation, the more interesting variation shows up. It gets less and less reasonable to try to absorb all DM cases in some complicated MOND formula. My impression anyway---Matt O and Chalnoth feel free to correct.

Jacob Bekenstein is a brilliant theorist who made extremely important contributions (black hole thermo) in the 1970s, but he's an old guy now. As the father of the TeVeS improved version of MOND, he (along with Moti Milgrom) can be expected to be old-guy die-hards. It's only right that the father of a theory doesn't give up. But my impression is that pretty much everybody else has moved on by now.

There may in time be modifications of straight Einstein gravity. Sure. but not to explain away DM. Dark matter is here to stay.

 

[cristo]

Has anyone read and digested "Alternatives to dark matter: Modified gravity as an alternative to dark matter" by Jacob D. Bekenstein?

http://arxiv.org/abs/1001.3876

Regardless of whether MOND is judged to be "dead" or not, the paper you quote appears to be a chapter from a book, or maybe a short topical review article. Thus, I don't think there's anything "new" in the article.

 

[Chalnoth]

Plus astronomers now map the clouds of dark matter using weak gravitational lensing. Some galaxies show more than average proportion of DM to ordinary. Dwarf galaxies tend to have a larger ratio, and the ordinary is distributed differently relative to the DM. And physical explanations have been worked out. The more DM observation, the more interesting variation shows up. It gets less and less reasonable to try to absorb all DM cases in some complicated MOND formula. My impression anyway---Matt O and Chalnoth feel free to correct.

That's been my impression as well, though granted I'm not directly involved in that particular area.

 

[ohwilleke]

Regardless of whether MOND is judged to be "dead" or not, the paper you quote appears to be a chapter from a book, or maybe a short topical review article. Thus, I don't think there's anything "new" in the article.

The article doesn't contain "new" theories (TeVeS, the relativistic MOND theory is where it stops), but it is a newly written summation of the field referencing most of the important recent work done with the theory by others, apparently for a book. The new evidence and the new papers on the topic are considered at the end.

Some of the arguments that against Relativistic MOND from new evidence are refuted by Bekenstein; other problems (most notably the galactic cluster issue) are acknowledged. I'd read his take on the possibility of some sort of MOND theory having validity in this linked material as agnostic.

The article is worth reading because it is a fairly rare paper that takes the background out of the footnotes and really makes an assessment of a theory from the perspective of someone who has offered it up, rather than from a critic's perspective.

 

[Constantin]

I read some papers by John Moffat on modified gravity theory and they're quite interesting. I wouldn't consider this theory dead. Also dark matter is not really alive either as we're not any closer to finding it.

 

[Chalnoth]

I read some papers by John Moffat on modified gravity theory and they're quite interesting. I wouldn't consider this theory dead. Also dark matter is not really alive either as we're not any closer to finding it.

Most in the cosmology community consider the matter all but settled. There are still a few people investigating modified gravity for solving the dark matter problem, but they are a strong minority.

 

[Chronos]

As noted by Marcus, the bullet cluster put MOND in the ground. Plenty of other evidence [e.g., lensing] had already dug the grave, so it was a quick funeral. Objects in the universe do not 'know' how distant they are from other objects.

 

[Constantin]

The ones who side on dark matter find arguments against MOND and say it's wrong.
The ones who support MOND find arguments supporting it and explanations for lensing and all the other observed phenomena.
So there's a lot of biased thinking on both parts. Just the fact there are more people supporting dark matter doesn't prove anything, just the fact it's the "easier" explanation and was also the first one proposed.

The very big problem with dark matter is although there should be so much of it, we're not any closer to finding what it is. And the search has been going on for quite a while.
Pretty much like the Higgs boson problem.

 

[Chalnoth]

The very big problem with dark matter is although there should be so much of it, we're not any closer to finding what it is. And the search has been going on for quite a while.

If you look at realistic models of dark matter, there really isn't any expectation that we should have detected it by now. And the amount of it is also not in any way surprising.

 

[Constantin]

As I wrote earlier, I found some papers by John Moffat quite interesting.
He claims his Scalar–tensor–vector gravity (STVG) explains all the observed phenomena.

http://en.wikipedia.org/wiki/Scalar-tensor-vector_gravity
http://www.johnwmoffat.com/papers.html

 

[Chalnoth]

As I wrote earlier, I found some papers by John Moffat quite interesting.
He claims his Scalar–tensor–vector gravity (STVG) explains all the observed phenomena.

Perhaps, but only by adding a rather large number of extra parameters to the theory.

 

[Ich]

Pretty much like the Higgs boson problem.

Exactly. Which becomes a problem at the time when it definitely should have been discovered, but was not there. Not earlier.
As of now, there neither is a Higgs boson problem, nor a Dark Matter problem.

 

[MyOwnField]

There's a good discussion of some current MOND stuff and Bekenstein here: http://badphysics.wordpress.com/2010/03/07/mond/.

The whole thing actually sounds like it has some potential.

 

[Haelfix]

From the beginning, it was pretty clear that MOND and TEVES were theoretically unlikely and extreme longshots as replacements to Newtons Laws and GR. However they are absolutely fabulous in that they fit most rotation curves extremely well (in fact, its kind of unnerving just how well they really do).

People have called it Milgroms fabulous fitting formula. In short, people use it to get quick answers rather than going through a lengthy and complicated calculation, and for that they will continue to linger around.

 

[inflector]

However they are absolutely fabulous in that they fit most rotation curves extremely well (in fact, its kind of unnerving just how well they really do).

People have called it Milgroms fabulous fitting formula.

Seems to be empirical evidence that "something" is causing gravity to behave as if it had a 1/r drop off with distance for really low accelerations or perhaps long distances.

It seems to me that that is all MOND actually does, i.e. specify a 1/r drop in the strength of gravity which is why it then matches the rotation curves so well. Flat galaxy rotation curves is what you'd get with a 1/r dropoff in acceleration due to gravity.

 

[rhody]

After a brief explanation of my understanding on MOND, based on Lee Smolin's comments from:
"The Trouble with Physics", pages 206 - 214, for elliptical galaxies with stars in orbit, once acceleration exceeds 1.2*10^-8 cm/sec² Newton's law of gravitation breaks down, after this threshold the acceleration decreases by the distance versus the square of the distance.

This is the anomaly is I understand it.

On a smaller scale, in our planetary system, this law is again in question, this time by observations from both the Pioneer's 10 and 11 missions having been tracked (using Doppler shift techniques) for decades.
I did a search on this and could find no other sources for this on PF.
It turns out that these spacecraft do experience a deviation from projected trajectories by 8*10^-8 cm/sec², bigger than the galaxy anomaly by about a factor of 6.

The data has been independently scrutinized by scientists using the Aerospace Corporation's Compact High Accuracy satellite Motion Program, the result's agree with JPL's analysis.
There is speculation about temperature spacecraft anomalies, a small gas leak, and discussion about sending a probe into space specifically designed to remove any possible anomaly for comparison.

Summary:
spiral galactic scale: 1.2*10^-8 cm/sec² --> acceleration decreases by the distance versus the square of the distance
solar system scale: 8*10^-8 cm/sec² --> acceleration is observed as listed

In trying to fathom a force that may contribute to this, an attractive force, the only one that immediately pops into my head is the http://en.wikipedia.org/wiki/Casimir_effect" ., which has been demonstrated in laboratory conditions in (near vacuum conditions, with temperatures not matching those in space).

From experimental results I have read about they said the result of the experiment would be the equivalent of a single red blood cell of extra mass applying inward force to one of the plates. At first glance, hardly enough force to seem to have an effect. Having that framework as a reference, what effect would the Casimir Effect have on very large scales ? Are there any models I could be directed to to investigate ?

Thanks in advance...

Rhody...

 

[ohwilleke]

Seems to be empirical evidence that "something" is causing gravity to behave as if it had a 1/r drop off with distance for really low accelerations or perhaps long distances.

It seems to me that that is all MOND actually does, i.e. specify a 1/r drop in the strength of gravity which is why it then matches the rotation curves so well. Flat galaxy rotation curves is what you'd get with a 1/r dropoff in acceleration due to gravity.

The 1.2*10-8 cm/sec2 constant that describes when this effect empirically observed comes in is equally notable.

Basically, if you have dark matter, you have to explain why the 1/r drop and the constant factor dictate why it is arranged in a halo in the manner observed. Maybe this can be done. Maybe the constant is the key to discerning what WIMP characteristics we should be looking for. But, when you have this kind of phenomenological success and predictive ability, you need to theory to explain not just that there is dark matter out there, but that it is dark matter that acts in a very particular, highly constrained way in all but a very small subset of phenomena.

 

[ohwilleke]

There are lots of other ways to explain the Pioneer anomolies. For example, one of the most plausible possibilities is repeated physical collisions of satellites in motion with space dust too tiny to be observed in the solar system.

 

[Chalnoth]

But, when you have this kind of phenomenological success and predictive ability, you need to theory to explain not just that there is dark matter out there, but that it is dark matter that acts in a very particular, highly constrained way in all but a very small subset of phenomena.

First you have to show that it's actually peculiar. I'm pretty sure it just comes down to the fact that dark matter is largely dissipationless.

 

[rhody]

After a brief explanation of my understanding on MOND, based on Lee Smolin's comments from:
"The Trouble with Physics", pages 206 - 214, for elliptical galaxies with stars in orbit, once acceleration exceeds 1.2*10^-8 cm/sec² Newton's law of gravitation breaks down, after this threshold the acceleration decreases by the distance versus the square of the distance.

This is the anomaly is I understand it.

On a smaller scale, in our planetary system, this law is again in question, this time by observations from both the Pioneer's 10 and 11 missions having been tracked (using Doppler shift techniques) for decades.
I did a search on this and could find no other sources for this on PF.
It turns out that these spacecraft do experience a deviation from projected trajectories by 8*10^-8 cm/sec², bigger than the galaxy anomaly by about a factor of 6.

The data has been independently scrutinized by scientists using the Aerospace Corporation's Compact High Accuracy satellite Motion Program, the result's agree with JPL's analysis.
There is speculation about temperature spacecraft anomalies, a small gas leak, and discussion about sending a probe into space specifically designed to remove any possible anomaly for comparison.

Summary:
spiral galactic scale: 1.2*10^-8 cm/sec² --> acceleration decreases by the distance versus the square of the distance
solar system scale: 8*10^-8 cm/sec² --> acceleration is observed as listed

In trying to fathom a force that may contribute to this, an attractive force, the only one that immediately pops into my head is the http://en.wikipedia.org/wiki/Casimir_effect" ., which has been demonstrated in laboratory conditions in (near vacuum conditions, with temperatures not matching those in space).

From experimental results I have read about they said the result of the experiment would be the equivalent of a single red blood cell of extra mass applying inward force to one of the plates. At first glance, hardly enough force to seem to have an effect. Having that framework as a reference, what effect would the Casimir Effect have on very large scales ? Are there any models I could be directed to to investigate ?

Thanks in advance...

Rhody...

So much for the Casimir effect... rhody faceplants... again...

Not bumpiing an old thread, just reporting that the Pioneer orbit anomaly has a new explanation, backed by recent papers, see http://news.discovery.com/space/oh-pioneer-mysterious-anomaly-may-finally-be-solved-110414.html":

there was always the possibility that the culprit might just be heat. Specifically, heat from the plutonium inside the spacecraft s' generators, some of which got converted into electricity while the rest of it radiated into space. If it did so unevenly, radiating more heat in one direction than in another -- only a 5 percent difference is required -- that might be sufficient to give rise to the Pioneer anomaly.

and

When you do that, say Francisco et al., the anomaly magically disappears. His conclusion: "With the results presented here it becomes increasingly apparent that, unless new data arises, the puzzle of the anomalous acceleration of the Pioneer probes can finally be put to rest."

So never say never, but, it appears an uneven heat source has convinced at least a few scientists who are willing to publish to back their claims.

Rhody...