dark matter and concentric circles

[CPL.Luke]

is it just me or does dark matter (and dark energy) seem like a way of fudging a theory in order to make it work?

 

personally the idea of some mystical dark matter keeping our galaxy together is reminiscent of the concentric circles model of the solar system, add to that the fact that the dark matter models keep on becoming more and more complex as new observations remove possible sources of dark matter.

 

I've been starting to think that the entire problem rests on our own incorrect view of gravity, and that some other theory like modified newtonian dynamics is where the real solution lies. This also got me thinking that black holes may also be a relic of an incorrect theory of gravity. Which would explain why alot of the mathmatics surrounding them gets funky (we still can't explain why there's a giant beam of particles streaming out perpendicular to the accretion disc near the speed of light).

 

 

at the very least a new theory of gravity that describes some of these phenomenon would make for a far more elegant view of the universe and as history has shown the more elegant and simplistic views in science are most often correct

 

sory for the bad grammar and writing, but its late and I've had to much coffee

[timo]

Yes, a new, working theory of gravity that does not need dark matter sounds nice (I´ll stick do dark matter, I know almost nothing about the attempts to explain dark energy). But there´s one thing that you should keep in mind:

 

-> Dark matter is nothing but matter which we can only observe due to its gravitational interaction. What good reason do we have to suppose that there is no matter like that?

 

There´s a lot of possible candiates for dark matter and the reasons why some are more or less ruled out are probably manifold. Some might be disfavoured by current cosmological models. Cosmological models depend on our current understanding of gravity and for the case of explaining dark matter also on our current understanding of particle physics or even guesses of what a possible extension of our current model of particle physics might be. The issue with our current understanding of particle physics is that most people actually expect it to fail for some extreme conditions - conditions that appear at some time in the history of the universe. I don´t know how prone cosmological constraints on dark matter are to that failure of particle physics.

 

In fact, some candidates for dark matter directly come from extensions of our current model of particle physics. You might put "altering our current model of gravity" on the same level as "altering our current model of particle physics", but there is two reasons that make altering the latter seem favourable to me:

1) As I already stated, we actually expect our current model to fail at some point. In other words, we suppose that it´s just an approximation of what a model of particle physics should look like. But there is no real reason to assume a failure of our model for gravity.

2) Dark matter is often supposed to be some new type of yet-unknown matter. In the last ~30 years we have discovered/proven a lot of new particles that were not known before and don´t play any role in everyday-phyics at all because they only appear in very extreme situations and/or have very little interaction with other matter. It does not seem too unlikely that there´s the "dark matter" particle and we just don´t know it, yet.

 

As a last point I would like to note that to my knowledge, there simply is no promising model to replace General Relativity. I don´t know much about MOND, but let alone from the name I would assume that it is not compatible to Special Relativity. The incompatibility of Newtonian Gravity with SR was the reason to come up with GR, after all.

 

Summary: Of course a perfectly working model of gravity which does not need dark matter to explain our observations would be great. But besides that our current model of gravity does seem to work great, I don´t see any promising candidates for an alternative. No one knows what the future will bring, but at the current state I tend to favour dark matter approaches over modified gravitational models.

 

 

EDIT: While I tried to be objective in above, I should be as fair as to mention that I am heavily biased on that topic for multiple reasons. You should probably keep that in mind when considering what I said.

[CPL.Luke]

well there is a new version of mond that is supposedly compatible with GR, and predicts gravitational lensing. MY reason for posting this is that I just read about the theory in discover magazine (it was on the front cover) and apparently the theory breaks down like this

 

 

F=Ma is an approximation and the real version is

F=Ma^2/a_0

 

where a_0 is a constant. This original idea was created adhoc in the 80's and used to some success in getting the proper rotation curves for galaxies, however it was rejected by the main stream for its adhoc nature and that it wasn't relativistic.

 

however recently a version of it came out called tensor vector scalar gravity or TeVeS which is relativistic and has a lower end limit of the above formula and turns into general relativity at the upper end.

 

note: I don't have the mathmatical background to say that it looks promising and I also think that the F=ma^2/a_0 is to adhoc to be really considered without some mechanism for a_0 being what it is.

 

 

and on the particle end, I could buy it if the dark matter was strewn throughout the galaxy and just didn't have an electrical charge and thusly we wouldn't see it glow in the electromagnetic range. However now that its been revealed that the dark matter would have to form halo's outside of the galaxy I can't buy into it. I simply can't buy that that much matter is out there without having attracted enough charged particles to it for us to see the halo glow.

 

PS atheist why are you biased on this issue?

[Martin]

...As a last point I would like to note that to my knowledge' date=' there simply is no promising model to replace General Relativity. I don´t know much about MOND, but let alone from the name I would assume that it is not compatible to Special Relativity. The incompatibility of Newtonian Gravity with SR was the reason to come up with GR, after all.

 

EDIT: While I tried to be objective in above, I should be as fair as to mention that I am heavily biased on that topic for multiple reasons. You should probably keep that in mind when considering what I said...[/quote']

 

PS atheist why are you biased on this issue?

 

everybody gets to be biased, and if they know it, so much the better:-)

Maybe people SHOULD have biases.

Maybe it is not a fault. you train scientists to have hunches as well as knowledge, and you pay them to have educated intuitions

and then you balance out what different ones say!

 

just for balance, if you want, I will give you some dope on Bekenstein's TeVeS mond. I don't have a horse in this race---happy either way.

TeVeS IS compatible with Special Rel,

and it IS diffeomorphism invariant----i.e. general covariant----just like Gen Rel.

 

what other people sometimes talk about is the ORIGINAL 1980s MOND invented by Moti Milgrom. It had some ad hoc kludge character and some flaws which Jacob Bekenstein fixed.

 

Somebody named Moffatt also has a version of MOND with some virtues similar to Bekenstein TeVeS. (I sort of lump Moffat's together with Bekenstein's, both are up-to-date circa 2005 relativistic monds)

 

Bekenstein TeVeS is testable within the solar system (recent Bekenstein Magueijo paper about this and online video of Perimeter Institute seminar talk)

 

it does the GALAXY ROTATION CURVES just like the crude original 1980s MOND

 

it also does GRAVITATIONAL LENSING so as to come out looking about as good on that as dark matter

 

it may not look as good as dark matter in explaining clusters of galaxies---not sure about this.

 

the main thing is it has to be TESTED against standard Gen Rel to see which gives the best predictions for inside solar system stuff (outer planet probes etc)

 

there is really no use taking sides. it is just two scientific theories that make predictions that one can compare with observations---see which is wrongest or which is rightest.

 

I will get some links about the prospects for testing

http://arxiv.org/abs/astro-ph/0509519

A Primer to Relativistic MOND Theory

http://arxiv.org/abs/astro-ph/0602266

MOND habitats within the solar system

http://arxiv.org/abs/gr-qc/0605141

Time Delay Predictions in a Modified Gravity Theory

[CPL.Luke]

martin it seems you misunderstood my tone, I was asking why he was biased out of general curiosity, not to antagonize.

[swansont]

is it just me or does dark matter (and dark energy) seem like a way of fudging a theory in order to make it work?

 

 

Perhaps. But remember that "fudging" also led to the discovery of the neutrino.

[timo]

and on the particle end, I could buy it if the dark matter was strewn throughout the galaxy and just didn't have an electrical charge and thusly we wouldn't see it glow in the electromagnetic range. However now that its been revealed that the dark matter would have to form halo's outside of the galaxy I can't buy into it.

Given that the visible matter evidently does form structures on different scales (solar systems, galxies, clusters, superclusters), I would acutally EXPECT dark matter to also form any kind of structures. Afaik, computer simulations for the distribution of dark matter have been done and do show structure formation. Dunno which model assumptions went into the models, though.

 

Wim de Boer gave some possible explanation for the distribution of dark matter (which is basically a computer simulation of a cloud of gas gravitationally attracted by a galaxy, as far as I can see that). Maybe his talks (which all seem to be the same one augmented to the time he had) might be interesting for you: http://www-ekp.physik.uni-karlsruhe.de/~deboer/

Lemme add the personal comment that I´ve heard the talk twice. First time I was absolutely impressed that someone has actually "seen" dark matter. Second time I was a bit more sceptical for some details which hopefully are minor. One would be the question why dark matter distribution is the cause of some dark matter being attracted by an already-existing galaxy - why doesn´t the structure formation of dm and ordinary matter happen at the same time?.

 

I simply can't buy that that much matter is out there without having attracted enough charged particles to it for us to see the halo glow.

Perhaps the other charged matter attracted it more? I cannot answer the question but structure formation is a topic where I would trust simulations much more than intuition.