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Can Relativity explain Dark Matter?

  1. Jul 16, 2009 #1
    I suppose the real title should be, "can relativity explain the NEED for Dark Matter"?

    Now I'm not a physicist, so if I get a detail wrong here or there please do not get stuck on that but rather try to answer the question...

    It is my understanding that dark matter is "needed" (in one case) to account for the stars on the edges of spiral galaxies not flying off into deep space. Without dark matter, the gravity of the galaxy could not hold the stars in place, as they are moving much too fast.

    Tests regarding Mercury's orbit proved that gravity slows the passing of time. Would that mean that, since the centers of galaxies are much more massive than the edges, the center of galaxies would appear to orbit slower when perceived from a distance and the outer stars moving relatively faster? If this is true, could this effect be strong enough to make it seem as if the stars on the edge are moving "too fast"?
     
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  3. Jul 17, 2009 #2
    Hi there,

    Like you said, without pointing out details of your question, I don't see any NEED for dark matter. Fact is that there a little need for anything, depending what you want to do with it. Enough philosophy.

    Coming to your question, dark matter is a theory that came from observing galaxies. With simple (ok not so simple) models using gravitationnal pull, observations showed that most galaxies are spinning faster than they should (keeping in mind our traditional gravity theory). Therefore,at the time, a question was brought to the attention of scientist: is our theory about gravity wrong, or is there something else that we don't see. The second was kept, since discovery of different very small particles were made. Therefore, we now believe that there must be some matter (alot for that fact) that is invisible to us (invisible matter would logically be dark matter). Therefore, the theroy behind dark matter emerge, from the dark. hahaha. little scientist humor.

    Cheers
     
  4. Jul 17, 2009 #3
    Thank you for your reply. I do understand all of the history of how the idea of dark matter came about. What I am asking though, is were relativistic effects taken into account during the measurements of these spinning galaxies? I won't go into what I half-*** know about sigma curves in spiral galaxies etc, but I do know that the problem arose from the speed of the stars on the edges of the galaxies. assuming that the mass of galaxies is concentrated in the core (if it is uniformly distributed throughout the galaxy then my question is moot), wouldn't relativistic effects show the stars in the center of the galaxy moving more slowly then measured. consequently, the stars on the outskirts would seemingly be moving faster. could that be the source of the problem. not that the stars ARE moving too fast, but that they seem to be because of relativistic effects.

    has that idea been considered when making these measurements? Or was our model of newtonian gravity only considered?
     
  5. Jul 17, 2009 #4

    Janus

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    Yes, of course any Relativistic effects were taken into account. Why would you even consider the idea that they wouldn't have? Do you honestly think that all the professionals in the field could have missed such an easy explanation?
     
  6. Jul 17, 2009 #5
    wow, no need to get excited. I'm not an astrophysicist and it's a simple question. a simple "yes, they were taken into account" would have sufficed. and I only was asking because i've only heard the phrase "simple gravitational theory" used when describing rotating galaxies. to me that sounded like only newtonian laws were being considered.

    separate question: do you know if the distribution of matter is even throughout galaxies or concentrated in the center? i ask because i have heard some people say it was evenly distributed while others say it is concentrated in the core.
     
  7. Jul 17, 2009 #6
    The first evidence for "missing matter" came from Fritz Zwicky's calculation of the mass of the Coma cluster of galaxies using the virial theorum in combination with the peculiar velocities of the cluster galaxies. This was in the 1930s well before Vera Rubin's measurements of flat rotation curves in spirals.

    srfriggin: The modelling of galaxy dynamics used to measure their mass uses Newtonian dynamics. There have been attempts to use general relativity to do this, in fact the claim was that the incorporation of GR did in fact fit the observed flat rotation curves without the need for dark matter (see http://arxiv.org/pdf/astro-ph/0507619v1"). The original argument was that the standard assumption of Newtonian dynamics being a good approximation of GR in the weak gravitational field regime (i.e. in galaxies, clusters of galaxies etc.) does not incorporate non-linear effects which may be important.

    These papers were discussed https://www.physicsforums.com/showthread.php?t=93486&highlight=Cooperstock"
     
    Last edited by a moderator: Apr 24, 2017
  8. Jul 18, 2009 #7
    Thank you Matt.o.

    Janus, see matt.o's response. seems like a question that isn't ridiculous to "consider".
     
  9. Jul 18, 2009 #8

    Chronos

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    Janus was merely pointing out the obvious. Astrophysicists tend not to miss the obvious explanations, but explain them more delicately. We really know nothing for certain, save we have historically been more wrong than right most of the time.
     
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