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 Post subject: The end of Mond?
PostPosted: Wed May 16, 2007 9:23 am 
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Does the apparent discovery of clustering dark matter mark the death knell for all the various Mond theories?

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PostPosted: Wed May 16, 2007 11:52 pm 
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Dear PaulS
Are you in any way referring to something in the June "Astronomy Now"?
I must admit to being quite a fan of Mond if only because it seems relatively simple. Having read several articles including the new one "The Great Cosmology Debate" in Astronomy Now a simple discussion between four cosmologists, it seems to me that currently cosmology is in a bit of a mess. One chap seems to be suggesting that dark matter might exist in a variety of forms, doing different things. Personally as far as I am concerned it is possibly just as plausible that Gravity varies throughout the universe. The gravitywe know might just be special to our particular part of the universe. Or it could be that our gravity particularly applies to our neighbourhood but the same gravity might be found in one or other far parts of the universe. I am not totally convinced myself that existing computer models of galaxies are correct. I am not even totally convinced that cosmologists understand\know the rotaion characteristics of galaxies or the real mass of galaxies with respect to the the numbers of stars and dust and gas in galaxies. Indeed the amount of dark matter seems to almost make the amount of ordinary matter hardly worth studying insomuch as it might have an insignificant effect on the way galaxies work. Certainly until recently there have been a variety of models related to galaxies and their collisisions which made no considerations of dark matter. Including dark matter in the equations might (?) make a nonsense of everything cosmologists know (or thought they knew about colliding galaxies).
Mond may well prove to be wrong (?).
But for me the alternatives currently just seem to be a bit of a mess.
But then I am just a simple guy.
Best of luck from Cliff


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PostPosted: Thu May 17, 2007 9:55 am 
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CLiff, I've not revcieved the AN June issue yet, my comments relate to the New Scientist article I linked to. But I look forwards to reading it.

As far as I understand it, the simplicity of Mond has fallen apart because it has been successively bastardised to deal with empirical observations over the last 20 years or so.

Firstly to take account of effects like gravitational lensing it has an explanation of gravity that's definitely more complicated than Einstein's - the Mond sub-theory to do with tensors and the like. I forget what the sub-theory is called - TeScV or something similar.

More recently, afaik effects like the article describes can't be accounted for in a Mond with only baryonic matter. So the theory has had to be married up with the supposition of a particular form of exotic particle.

And if as forecast there's gravitational lensing evident where there are no galaxy clusters then Mond will have to accept that there's a bit of dark matter hanging about.

Hardly simple any more, I think.

I know what you mean when you say ordinary matter hardly seems worth studying any more :) But I for one am convinced that there's more than the eye (or Hubble Scope or Chandra etc) can see!

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PostPosted: Thu May 17, 2007 12:16 pm 
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Paul S wrote:
I forget what the sub-theory is called - TeScV or something similar.

TeVeS.

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PostPosted: Thu May 17, 2007 12:52 pm 
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davep wrote:


Thanks Dave, that's the one. I think tensors is where my ability to comprehend maths / geometry in astrophysics comes to a dead halt :oops:

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PostPosted: Thu May 17, 2007 11:54 pm 
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Dear PaulS
I could start by saying that I do not like the suggestion that Mond has been bastardised. But I will not, because I think it better to be blunt and to the point rather than be long winded (as I usually am!).
Personally, I am not really worried about such bastardisation (assuming that is what it is or might be). Anyway if an out and out string (or one of the string derivatives) theorist suggested the Mond bastardisation I would think they must be joking (in view of the number of variants related to strings). Of ourse each variant has its own particular supporters.
I think it occurred to me long ago (but possibly wishful thinking on my part) that gravitation might not be constant throughout the universe.
I certainly thought that might be the case with respect to the speed of light ie it might not be constant across he whole universe. (could make an interesting discussion(?)).
The possibility that the original Mond has needed modifying does not worry me one jot. I personally find it far more palatable than the need for the greater part of the universe to be something that is not even really known to exist. Dark Matter and Dark Energy are far weirder to me than the possibilty that somewhere at some unknown distance from Earth gravity is different from what we know it. And indeed at some other distant location may be different again.
I am very happy to accept current cosmologists ideas that suggest that an object I might have imaged is X billion light years from Earth. But I take that distance with a pinch of salt - and I do not think in terms of the 3 percent error that astronomers have apparently now achieved in with respect to measuring the distance of nearby stars (within 3000 light years), I mean the possibility of big time errors with regards the more distant astronomical objects. Although to be fair I presume serious cosmologists still talk about Z values or whatever and using light years for things extreme in our universe is reall just for the benefit of us lot (ordinary amateurs).
Best of luck from Cliff
I think TWO previous posts I sent tonight have gone the way of some other unpopular hypothesis\theories - disappeared into oblivion.


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 Post subject: Re: The end of Mond?
PostPosted: Thu Feb 24, 2011 2:05 am 
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Paul S wrote:
Does the apparent discovery of clustering dark matter mark the death knell for all the various Mond theories?


There's a new article on the Nature web site, which suggests MOND supporters may be fighting back. See link for details:

http://blogs.nature.com/news/thegreatbeyond/2011/02/post_73.html

Quote:
The predictions of a theory proposed as an alternative to dark matter have been verified in a new class of objects, according to a study currently in press. The results seem unlikely to convince astrophysicists to abandon dark matter as one of the cornerstones of the standard model of cosmology; but suggest a direction in which new work is urgently needed.

The alternative theory is Modified Newtonian Dynamics, or MOND, proposed in 1983 by astrophysicist Moti Milgrom of the Weizmann Institute of Science in Israel. Like dark matter, MOND was hypothesized to explain why galaxies remain in one piece when observations of the rotation speeds and estimates of the mass of their luminous contents suggest they should fly apart. Rather than assuming the universe contains 5/6 invisible dark matter, Milgrom proposed a modification to Newton’s laws that kicked in at the low acceleration scales of stars in galaxies, and that strengthened the attractive force of gravity, having the effect of dark matter without imagining a mysterious extra substance. MOND successfully predicted the observed rotation speeds of galaxies and gained supporters but its adherents dropped off dramatically after observations of the cosmic microwave background (CMB) – the so-called “echo of the Big Bang” -- first by the balloon experiment BOOMERanG and then by NASA’s Wilkinson Anisotropy Probe (WMAP), provided stunning confirmation of predictions from the dark matter theory.


The paper referred to in the article: A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies, by Stacy McGaugh, is available here.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/ ... 3913v1.pdf

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Last edited by david entwistle on Fri Feb 25, 2011 2:27 am, edited 1 time in total.

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PostPosted: Thu Feb 24, 2011 11:03 pm 
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Dear David
Interesting !
I'm trying to keep an open mind whilst bearing in mind I am a very amateur fan of "Mond".
However, I read in a recent New Scientist (29th Jan I think? ), that I gather that supposedly in hopes of getting the general public on-side and avoiding the controversies that demoting Pluto from its old "planetary" caused, a new internet site as been set up whereby anyone can make suggestions as to how "a galaxy" should be defined. Apparently there is currently no official definition of what actually constitutes a galaxy.
However, the NS article also seems to say that some professional astronomers\cosmologists have made suggestions which involve Dark Matter being included in their galaxy definitions.
Now I personally have no intentions of making any specific galaxy definition myself for two reasons :-
1) I'm not sufficiently knowledgable myself to make a valid suggestion.
2) I have a bee in my personal bonnet that humans in general (scientists included) seem to need to give very specific definitions to everything.
That said with regards to the above mention that some astronomers\cosmologists suggest that a galaxy defintion should include mention of Dark Matter I'd like to make two points :-
1) their inclusion of Dark Matter suggests that the existence of Dark Matter is now conclusively proved. (I wonder ?)
2) that all galaxies have a "portion of" Dark Matter. (Now I, possibly mistakenly, have thought that many galaxies are supposed to have Dark Matter within them, but some Galaxies may not have any Dark Matter.
Now that may simply result in some objects currently called galaxies losing that status. On the other hand if Dark Matter does prove to be a red herring ??????????
Best wishes from Cliff


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PostPosted: Fri Feb 25, 2011 2:26 am 
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Cliff wrote:
However, I read in a recent New Scientist (29th Jan I think? ), that I gather that supposedly in hopes of getting the general public on-side and avoiding the controversies that demoting Pluto from its old "planetary" caused, a new internet site as been set up whereby anyone can make suggestions as to how "a galaxy" should be defined. Apparently there is currently no official definition of what actually constitutes a galaxy.

However, the NS article also seems to say that some professional astronomers\cosmologists have made suggestions which involve Dark Matter being included in their galaxy definitions.


Hi Cliff,

Yes, I saw that New Scientist article too - as I mentioned, largely down to your occasional references to NS, I've started getting it again. The New Scientist article, When is a group of stars not a galaxy? is available on line here.

Quote:
Are there impostors lurking among the many millions of galaxies identified so far? No one can give a clear answer because there is as yet no formal definition of what a galaxy is. But a pair of astronomers are now putting the question of what defines a galaxy to a public vote, in the hope of reaching a consensus and avoiding the sort of controversy that surrounded Pluto being stripped of its status as a planet.


Although I appreciate you may not be taking part, the survey, for anyone interested in taking a look, is here.

I also see that there are a number of recent reference to an article in Nature detailing measurements taken by the Herschel Space Telescope which claims to put tighter limits on the actual amount of dark matter required, in the galactic halo, to trigger galaxy formation. Here are a sample:


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PostPosted: Fri Feb 25, 2011 2:03 pm 
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Dear David
Thanks for providing t the further references.
I do like New Scientist magazine but I have to admit I don't believe everything it says. If I tried to go into more detail with regards NS gen it would stop me doing various things that give me most fun.
You are quite right in thinking I won't get involved in the forthcoming "galaxy fracas" myself.
Pluto is still Pluto as far as I'm concerned. The designation of solar system objects has got far too complicated for me now. If I took these matters seriously I think I'd drop doing astronomy all together.
Best wishes from Cliff
PS I think the astronomical content of NS has been poor in recent weeks.


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PostPosted: Sat Feb 26, 2011 8:26 am 
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The American Association for the Advancement of Science (AAAS) web site have a nice summary of the recent Stacy McGaugh paper here.

Quote:
Thousands of physicists, astrophysicists, and astronomers are searching for dark matter, mysterious stuff whose gravity seems to hold the galaxies together. However, an old and highly controversial theory that simply changes the law of gravity can explain a key property of galaxies better than the standard dark-matter theory, one astronomer reports. That claim isn't likely to win over many skeptics, but even some theorists who favor the standard theory say the analysis hands them a homework problem they should solve.

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PostPosted: Sun Feb 27, 2011 1:00 pm 
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Dear David
It's all a bit\lot beyond me really, but it does look rather like the mainstream believers in Dark Matter have as many problems with their own ideas (which they seem inclined to ignore) as the Mond lot.
Personally I have no problem in thinking it possible that things like gravity may vary throughout "my" infinite universe.
I know time varies; in general it flies faster as you get older.
Best wishes from Cliff


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PostPosted: Sun Feb 27, 2011 1:05 pm 
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Time flies very much faster as you get older, at least it seems so too me.

It's all relative you know.

Regards, David.


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PostPosted: Sun Mar 06, 2011 12:05 pm 
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Dear al(L)
Back to what I think is something directly related to the original tpoic.
I am no expert on these cosmological issues but I am based by my simplistic mind.
If I recall correctly (I am not sure about this so please comment if anyone knows differently or disagrees with what I say) some years back when computer simulations came into being, cosmologists began producing simulations of merging galaxies showing how different galaxies merge. If I recall correctly those early simulations were supposed to be quite realistic but as far as I know didn't take "the dreaded Dark Matter" into account.
So if supposed Dark Matter is exists in the profuse amounts it supposedly does, how come cosmologists mamaged to come up with apparently reasonable explanations of how the Universe behaved without so little knowledge of this mysterious substance.
Dark Energy is another weird thing that adds to the puzzles.
I can understand in my own simplistic way that when galaxies collide very few stars collide and tend to pass but the gases in the two or possibly more galaxies colliding, bump into each other and tend to merge. Then eventually the two seperate oppositely moving groups of stars may be dragged back towards each other (possibly back to the "combined" gas cloud.
However, what do the two lots of Dark Matter do when there two galaxies collide ?
Do the clouds pass through each other like the stars or do the clouds crunch like the gases of the two galaxies apparently do ?
I cann't say I have researched these matters but to date I have never seen any mention of what happens or might happen to the dark matter when galaxies colloide, in the astronomy magazines I read.
Best wishes from Cliff


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PostPosted: Sun Mar 06, 2011 6:32 pm 
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Cliff wrote:
Dear al(L)
However, what do the two lots of Dark Matter do when there two galaxies collide ?
Do the clouds pass through each other like the stars or do the clouds crunch like the gases of the two galaxies apparently do ?
I can't say I have researched these matters but to date I have never seen any mention of what happens or might happen to the dark matter when galaxies collide, in the astronomy magazines I read.
Best wishes from Cliff


What happens to the dark matter (i.e. do dark matter particles interact with other dark matter particles?) will depend on the type of particles actually comprise dark matter - something which is currently unclear.

By observing the results of galaxy collisions and of galaxy cluster collisions and seeing what has happened to the dark matter (which they locate via its gravitational lensing effect), astronomers can detect whether the dark matter has become separated from the interacting normal matter or has stayed with it - this sets limits on the types of particle that can make up dark matter.

Here are links to two reports from recent years :

http://www.msnbc.msn.com/id/14453775/ns ... nce-space/

http://www.newscientist.com/article/dn1 ... ision.html


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