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A question on Dark matter

  1. May 29, 2010 #1
    I know that Newtonian mechanics predicts dark matter due the the fact that the sun (and other stars) is orbiting around the centre of the galaxy much faster than expected

    But i was just wondering if Einstiens general theory of relativity predicted this aswell?
    Does it "say" there is as much dark matter as Newtons theory?
    or are they about the same?

    Just wondering
    Thanks : )
  2. jcsd
  3. May 29, 2010 #2
    They don't "predict" dark matter... they predict a certain result, which does not match what people observe. So people have come up with the idea that there must be hidden matter so make our observations fit the equations. The other possibility is that there is no dark matter and the equations are not perfect yet.

    However there is some other evidence for DM besides just the velocity of stars on the edges of galaxies.
  4. May 29, 2010 #3
    Do the 2 theories calculate the same orbiting velocties?
    i.e. as a result of having the same orbital velocities conclude that there is the same amount of DM in the universe (The Galaxy)?
  5. May 29, 2010 #4
    You probably want to look at http://www2.phys.canterbury.ac.nz/~dlw24/" [Broken]'s work:
    Last edited by a moderator: May 4, 2017
  6. May 29, 2010 #5
    Oh this is a prof from my uni .. Geuss i should go talk to him :O
    Thank you guys
  7. May 31, 2010 #6


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    Yes. Wiltshire's work has nothing to do with that.
  8. May 31, 2010 #7
    Thanks Ich.. i was looking for ages through his papers and journals at uni and couldn't find anything

  9. May 31, 2010 #8


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    Wow - quick reply. I had to google for cesiumfrog's http://www2.phys.canterbury.ac.nz/~dlw24/universe/summary.html" [Broken], and it's about something completely different, and very speculative, too.
    Gravity in Galaxies is relatively weak, so it doesn't matter for all practical purposes whether you use GR or the Newtonian approximation. Except that the latter is much easier to apply.
    Last edited by a moderator: May 4, 2017
  10. Jun 1, 2010 #9
    That's a great idea! Please do and tell us what you learn.

    The OP's question was to what extent does the inferred dark matter ratio depend on whether the observational data (galactic luminosity and redshift curves) are interpreted under the framework of Newtonian physics or GR. Ich, can you site other sources that have addressed this question? The fact is that there has been a difficulty in identifying the theoretically correct averaging procedure to describe grainy matter distributions in GR properly.

    Do you have any evidence to support your claim that the Newtonian approximation does not make any relevant practical difference from GR here?
  11. Jun 1, 2010 #10


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    There has been a paper a few years ago which claimed a difference between GR and Newtonian analysis. It has been shown to be errorneous. I forgot both the paper and the replies, maybe someone else remembers it?
    Sure. The total mass of a galaxy is ~ 5*10^11 Msun, with a Schwarzschild radius of ~0.1 Ly. So we're talking about a potential of order ~10-6 to 10^-5. OTOH, we have velocity measurements with an accuracy of order ~0.1. No way for GR to produce significant corrections.
  12. Jun 1, 2010 #11
    I couldn't get hold of him, he's vbusy (which makes sense :P )
    But, i read his journals and papers in the library and couldn't find anything refering to this? Not to say he didn't as all his journals werent there.. I'm not sure.. Hopefully he will be my PhD supervisor, when i answer this question tehehe

    So I will let you know, when i solve it xD
  13. Jun 1, 2010 #12

    D H

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    To add to what Ich already gave:

    The orbital velocities of stars, clusters, and satellite galaxies about a central galaxy should be tiny. Instead, they are just small (compared to c). For example, NGC3198 has orbital velocities that are about 150 km/s from 4 to 30 kpc from the central core. 150 km/s is 0.0005 c, which is too small for any significant relativistic effects to be involved.
  14. Jun 1, 2010 #13
    When you put it that way, I understand
    Thank you :)
  15. Jun 1, 2010 #14


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    Compelling evidence for dark matter is provided by the bullet cluster papers. That is about as good as observational evidence gets. Zwicky noticed the missing mass problem almost a century ago. Virial theory confirmed his suspicions. MOND is merely a drive by shooting victim. Dark matter is not going away any time soon.
  16. Jun 1, 2010 #15
    Can any of you experts tell me the correct way to use GR to model a galaxy? Obviously one does not use a Schwazschild metric except in a trivial worst approximation, since that would treat the bulk of the galaxy as a vacuum. Since GR is nonlinear, we know that the field of two stars is not quite simply the sum of the individual fields of either star in isolation. Do any of you know how much difference this makes? Have any of you seen the calculation attempted?

    Isn't it obvious to every thinking person that one will arrive at different numbers for the quantity of dark matter depending on whether one interprets that data using one theoretical framework or another subtly nonequivalent framework? So how is it invalid (nay, threatening) to investigate how much those numbers differ? You know, actually bothering to check quantitatively how good the approximation is? I don't think performing such exercises warrants the label "speculative", if anything then "speculative" would be asserting a particular outcome of such exercises without bothering to have anyone perform them carefully. I'm certainly not claiming that Newtonian physics is a terrible approximation: the papers I've seen only reported about a factor of 2 discrepancy, which should be perceived as no threat to the dark matter dogmatists. I'm only advocating placing more weight on nuanced application of scientific method (in this case, reading of published calculations that exist on the topic) than on simple hunch.

  17. Jun 1, 2010 #16
    What you do is to expand things out into a power series

    GR = Newtonian + (something) x + (something) x^2 + (something) x^3 + .....

    Then you look at what x is and how big it is, and x is v/c and for the purposes of galaxy rotation it's not big enough to make any difference. The good thing about this sort of argument is that could apply even if GR is wrong.

    Also if you show that x is big enough to be important, but x^2 isn't, then you end up with what is called the post-Newtonian formalism and that happens to be linear.

    Except that you won't. As long as your theory of gravity approximates Newtonian gravity then it's not going to matter. Now you could be in a situation where your theory of gravity *doesn't* approximate Newtonian gravity in which case you get into the world of MOND models.

    What you can show is that as long as things are "subtly" different, things aren't going to matter. Things have to be very different for things to matter.

    People do that. The trouble with that is that it can be done in two paragraphs and it's so easy to do that it's not worth writing a paper about it. Now what *would* be worth a paper is if you could stare at the basic argument and show that it's flawed.

    Be my guest :-) :-) :-)

    That's interesting since there is a whole series of papers that argues that Newtonian physics *is* a terrible approximation to galaxy rotations. If you want galaxy rotations to be a gravity effect, you have to argue that Newtonian gravity is wrong at galactic scales. This isn't a crazy thing to do. I mean, we know that Newtonian gravity is very wrong at cosmological scales, and we know it's pretty good at solar system scales.

    You don't want nuance. This is something that you want to be blunt about. You don't want complex math, you want a simple straightforward argument, and that turns out to be that v/c << 1.

    Also the calculation to check if GR matters or not is a trivially simple one, and it's so simple that no one is going to publish a paper about it. Now if you can think of a reason why the basic argument is wrong, then *that* would be worth a paper.
  18. Jun 1, 2010 #17
    Also astrophysicists like these sorts of simply arguments because they are quick. If it turns out that GR makes a difference then you are going to be spending the next months/years/decades figuring out the details.

    So a five minute argument that says "it's not going to matter" is something that is very useful. It also tells you that there are three possibilities 1) dark matter 2) some theory of gravity that is totally unlike newtonian gravity or 3) some fundamental problem with the quick argument.

    The problem with these arguments is that they never quite get published because it's too easy and not worth publishing.
  19. Jun 1, 2010 #18


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    It strikes me as odd that you rather call us dogmatists than consider the possibility that the problem really is easy enough to be settled with a few numbers. There are weak fields (~10^-6), slow velocities (~10^-3), so perturbation theory will work. That means that the respective corrections are themselves of order 10^-6, 10^-3.
    That's not magic, it's science. And if you don't like the result, because there's a dogma or two in your thinking that DM must be non-existent, well, then come up with a better argument. This one won't work.
  20. Jun 1, 2010 #19
    That was good for a chuckle, thanks. This came immediately to mind:
    [PLAIN]http://www.acsu.buffalo.edu/~dpadgett/ackbar.jpg [Broken]

    I find the dark matter result disturbing, but as Ich has said it is because of previous dogmatic thinking on my part. The more I learn about it (often in such forums as this) the more I understand how limited the options are. I would put some money on WIMPs, but in general, if DM didn't exist it would be almost inconceivable at this point. GR is just too good, and observations all seem to agree on the basic issues.
    Last edited by a moderator: May 4, 2017
  21. Jun 5, 2010 #20
    I apparently can't repeat this enough: I do not think such a thing. I don't understand how you have misread so extremely.
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