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inflector
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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
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.inflector said: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
inflector said: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
That's been my impression as well, though granted I'm not directly involved in that particular area.marcus said: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.
cristo said: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.
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.Constantin said: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.
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 said: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.
Perhaps, but only by adding a rather large number of extra parameters to the theory.Constantin said: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.
Exactly. Which becomes a problem at the time when it definitely should have been discovered, but was not there. Not earlier.Pretty much like the Higgs boson problem.
Haelfix said: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.
inflector said: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.
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.ohwilleke said: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.
rhody said: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...
andthere 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.
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."
MOND stands for Modified Newtonian Dynamics and is a theory that proposes a modification to Newton's laws of gravity to explain the observed discrepancies between the predicted and observed motions of galaxies.
The paper by Bekenstein presents a new formulation of MOND that incorporates the effects of dark matter and can better explain the observed galactic rotation curves.
The new formulation of MOND proposed by Bekenstein incorporates the effects of dark matter, which was not previously accounted for in MOND theories. This allows for a better fit to observed data and a more comprehensive understanding of galaxy dynamics.
There is a significant amount of evidence that supports the validity of MOND, including the observed rotation curves of galaxies, the bullet cluster collision, and the Tully-Fisher relation. However, further research and observations are still needed to fully confirm its validity.
MOND is a competing theory to the commonly accepted Lambda Cold Dark Matter (ΛCDM) model. While ΛCDM posits the existence of a non-interacting, non-luminous matter called dark matter to explain the observed discrepancies, MOND modifies the laws of gravity to explain these discrepancies without the need for dark matter. Both theories have their own strengths and weaknesses, and further research is needed to determine which one is more accurate.