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Validity of Randall–Sundrum model

  1. Mar 24, 2010 #1

    This board got yet another new non-physicist, me! Besides from being largely a layman when the formulas get too complicated, I've been very interested in physics pretty much for all my life. I just never taken it to a (semi-) professional level, and became an IT guy instead. So, you're not dealing with some esoteric nutjob here that just came to know what physics is, but most of my insight came from popular science sources, such as various popsci-books, wikipedia articles, TV documentaries, lecture videos on youtube/fora.tv and such. Oh, and please forgive me my occasional slip ups, I am german, and might spell a few things wrong, or produce grammatically interesting challenges :)

    My actual question. I've always found string theories rather appealing, and like to spend some philosophical thoughts on it (this is one case, where the hard math is well beyond me, so I unfortunately am limited to philosophical thinking here), and came up with some things that, as I found out recently, are pretty well described by the http://en.wikipedia.org/wiki/Randall–Sundrum_model" [Broken] (more precisely RS2). Now, before I spend more potentially stupid thought on this, I'd like to have a hint at to how much this theory is accepted as viable or possible among the ST guys, or in general even. Is there anything that makes this theory cumbersome, and puts it onto the brink of disregard?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 24, 2010 #2


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    Welcome Medium9. One thing that I can think of right off the bat is that it's a 5D theory. I'm guessing it's possible to embed RS2 within M-theory (a full 11D framework), but I'm not an expert. I'll think of more later...
  4. Mar 24, 2010 #3
    The reason I ask for is, that I try thinking of shapes our universe might have (locally), that makes gravitational influences called dark matter, possible to originate from other universes (branes) in a way we could deduct where this influence originates from.
    Currently, this would be my proposal for DM in the halo of galaxies, that causes outer regions to rotate unexpectedly:
    http://img641.imageshack.us/img641/7335/branes.png [Broken]
    The black line would be a 2D representation of our universe, seen from the side, and it's curvature caused by the mass of some galaxy. The blue line would then be another close-by universe with another clump of matter (just to avoid the term "galaxy" here), that is a bit larger than the galaxy in our universe.
    It in this case would not come as a surprise, that large quantities of matter would gather at the same spot, but couldn't collide because the brane keeps them separate. The larger potential in the center of the foreign universe "clump" would case the mass to be too far away to have an observable influence on ours, but the halo of that clump might thus be closer to our universe, enough to have an impact.

    That ultimately would aim at the question: Do all galaxies show this rotation anomaly, or is it just some? And: Are there also galaxies that show a reverse effect, which could indicate that these may serve as large counterparts to a smaller clump in another universe, effectively creating dark matter there?
    There also then arises the questions of how many other branes there might be. If it is only one, and there are no fast-halo galaxies in our brane, this could also indicate that mass may bend space only in specific directions, which would introduce a term of orientation to branes.

    I also thought about possible shapes if there are no other branes, but where ours could be of any form in a higher dimensional super-space, where the matter of our own universe could serve as dark matter. But that escapes my brain capacity, since this wouldn't be possible to do in a +1 dimensional super-space, and thus no longer imaginable in 3D with planes as substitutes for branes.

    I also want to stress here, that I am not a professional physicist, not even studying it - but it's a long term field of interest of mine. Thus I cannot in any way express this mathematically. These are merely philosophically induced thoughts, and I am looking for a confirmation (or the opposite) of the possibility of such things in a ST framework for quite some time now.
    I only would very much like to know if I completely stepped out of physics into the realm of wishful thinking with this, or if it is worth spending more thoughts and time on this.
    (Also, I am a non-native English speaker, and thus might lack some words to make a spot on and short description/question. If anything is unclear, please ask!)
    Last edited by a moderator: May 4, 2017
  5. Mar 25, 2010 #4


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    I don't know off hand if your idea will work, however, I greatly encourage this kind of activity. Just be careful to take actual observational evidence into account when crafting your model.

    A couple points. One of the popular approaches to understanding the galaxy rotation curves is to do away with dark matter and instead play around with the gravitational theory itself (google MOND). However, one thing that these theories miss, is that dark matter does more for us than just give wacky galaxy rotation curves. For example. dark matter appears to have been needed to aid in the formation of structure in the early universe. Any theory of alternative gravity that attempts to do without dark matter must not only explain the rotation curves, but also account for all the other things dark matter does for us.

    Secondly, with respect to your suggestion, I would make one comment regarding its naturalness. Just to be sure I understand, you're saying the following: You have two parallel branes such that the matter distribution on a hidden brane affects the behavior of the matter distribution on our brane in a way that we can't account for by only taking the mass on our brane into account. It seems to me that for this to work, one would at the very least need to have very similar (if not identical) matter distributions on each brane. While this is in principle possible, it's highly suspicious and would need to be understood.

    Lastly, it doesn't look like your idea relies on RS, but is rather based on a fairly generic brane world.
  6. Mar 25, 2010 #5
    Thanks for the reply!

    You understood my... well, it's probably not even sane to call it a theory, but for the lack of a better word, theory right. I would think of constant (and ongoing) gravitational mutual influence between branes, making the distribution of matter in each dependent on the others. Thus, large matter pools should, if looked at from the "outside" be close by. Because matters of different branes couldn't clash, it would still be possible to have "free chunks", and regions that oscillate/orbit each other. Also, in my mind branes won't necessarily be flat with bulges caused by mass. They might as well be crumpled in any thinkable way, and mass contributing too.
    As far as my knowledge goes, this wouldn't be broken by any observed effect which is thought to be caused by DM, the rotation curves were merely a rather simple example.

    What I was hoping a little for was, that if the math put any constraint to these thoughts, it might be possible to derive some loose predictions, which might be falsifiable through observation in space.

    What lead me to refer the RS was, that this was so far the only theory I found, where multiple branes and closed-string gravitons roaming between these are explicitly named. I agree that my thoughts alone may be thinkable in any generic braneworld, but I simply was so happy to find actual scientists formulating what I thought up without that much background ;).
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