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Theories in Dark Matters:

  1. Oct 7, 2007 #1
    Hello everyone.

    I've been interested in Dark Matter ever since I read "His Dark Materials" trilogy which they are making a movie about... I was wondering, what are some other theories out there about what dark matter & dark energy is?
     
    Last edited by a moderator: Oct 8, 2007
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  3. Oct 8, 2007 #2

    cristo

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    It's a very good trilogy, but you should remember that they are fiction novels and, as such, include a lot of fantasy and religion.

    However, the search for the answers to the questions of "what are dark matter/energy" is indeed very interesting. For dark matter, you might be interested in reading this link in which the area is discussed. There are of course many other sites on the internet; some more trustable than others.

    We know far lessa about dark energy than we do dark matter. There are hundreds of theories out there, some more plausible than others, but we really don't know the answer yet. You may be interested in reading this page from the NASA website; but there are many more.
     
  4. Oct 8, 2007 #3

    EL

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    The currently most popular models for dark matter are different kind of WIMPs, since such particles seem to fit observations best. The main candidates arise from supersymmetric extensions of the standard model of particles, but plenty of other WIMP-candidates exist too.

    Regarding dark energy, the cosmological constant still fits the data well.
     
    Last edited: Oct 8, 2007
  5. Oct 9, 2007 #4

    Chronos

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    The dark energy model arose mainly out of the Perlmutter supernova study, where it was deduced expansion of the universe began accelerating at about half its present perceived age. See:

    http://www.lbl.gov/Science-Articles/Archive/dark-energy.html

    and the original paper:

    http/arXiv:astro-ph/9812133
    Measurements of Omega and Lambda from 42 High-Redshift Supernovae
     
  6. Oct 12, 2007 #5
    I read that trilogy, good books, not entirely sure why they would have convinced you to look into dark matter though. Personally i have reservations about how much dark matter/energy can tell us.

    the way i see dark matter and dark energy is as a conveniant scapegoat for modern astronomy. If theres something out there that cant be explained, i often feel astronomers tend to blambe it purely on dark matter or energy. I would like to point out that no-one has ever seen any dark matter or energy, it may not even exist. The current reason we think it is there could be due to errors in our original measurements. Maybe 70% of the universe is not made of dark energy, it just appears that way to us. Maybe gravity is not the predominant force in the universe and other magnetic/elecromagnetic forces can account for dark matter? To me the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity.

    The ugly fact is that no-one has ever detected or measured dark matter. While it is a very good method to try to answer some of the predicaments of modern cosmology it will do nothing to explain other mysteries in space such as the heating of the corona, accretion disc jets, Ultra-high-energy cosmic rays, Baryon asymmetry, the fact the solar wind has stopped completely for days, the pioneer anomaly, etc, all are legitimate questions that have no adequate answer. I very much doubt that dark mater can account for all of those observations.

    Lack of something cannot act as proof of its existance, that is a circular argument.
     
  7. Oct 12, 2007 #6

    SpaceTiger

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    This is incorrect. The effects attributed to dark matter are of very, very high statistical significance.


    Careful, you're delving into crank territory here. These possibilities have been ruled out for some time.


    It is still possible that a modification of gravity can explain the observations leading to the dark matter hypothesis, but things like the bullet cluster are making it increasingly difficult:

    http://arxiv.org/abs/astro-ph/0608407

    Furthermore, modified gravity theories have been having difficulty fitting the CMB.


    Where did you hear astronomers attributing accretion disk jets to dark matter? That's a pretty sizable collection of red herrings you've collected there.
     
  8. Oct 12, 2007 #7
    I'm aware they have nothing to do with dark matter, but thats the point i'm trying to make. A lot of people are under the impression that dark matter and dark energy are the only parts of the picture left to discover and only once we have learnt how they operate we will know fundamentally how the universe works. But there are many, many questions completely unrelated to dark matter.

    Dark matter is nothing to do with the problems i listed, thats why i dont think all the focus on dark matter is necessary.

    I am not arguing about the effects that we observe and have attributed to dark matter, just the assumption that we know it is any physical thing in the first place and not a unknown consequence of gravity at very large scales.

    What i'm trying to say (in a very roundabout way) is; If we can calculate the effects that this matter has to a very high statistical significance and are so sure its there, why do we have to assume these observed effects are due to completely new unknown particles, or a completely new type of energy, when it is probably more likely that our current model just needs adjusting.
     
    Last edited: Oct 12, 2007
  9. Oct 12, 2007 #8

    SpaceTiger

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    Dark matter is just one issue in one subfield of astronomy (that is, cosmology). A much bigger deal is made of it in the pop sci. literature than in the scientific community just because it sounds so exotic and mysterious. If you're suggesting that people outside of science should not spend so much time fretting over dark matter, I very much agree with you. If you're suggesting that astronomers spend too much time worrying about it, then I'd have to say that hasn't been my experience.


    Because it isn't more likely that it's due to a problem with our models of gravitation. The dark matter hypothesis is the leading one because it is the one most supported by the observational evidence. There are observable differences between a field of weakly interacting particles and a modification to the laws of gravity, as the paper I linked explains. If you don't understand the arguments being made there, please ask and someone here may take the time to help. That's what this forum is for.

    As for dark energy, I think many astronomers would readily admit that it's just a placeholder at the moment. We know that the universe is accelerating and that the acceleration is consistent with a cosmological constant, but there are still a lot of interesting possibilities (like modified gravity) that have yet to be ruled out.
     
  10. Oct 12, 2007 #9
    thanks, that clears it up, i'm glad to hear most astronomers dont spend too much time on a currently untestable thing. Thats the impression you get from most mainstream news sites. Hopefully some new method of detection comes up its just hard to think of what.


    There is one statement in that paper that i would dispute, nothing major, just some wording really.
    they say,
    "gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8-sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen."

    i would not have said "and thus proves that the majority of the matter in the system is unseen" rather "and implies that the majority of the matter in the system unseen ". Which i think is fair. They have not proved it really, there could be other explanations. You surely cant completely neglect the high forces that have been observed in plasmas via Z-pinch fusion in tests on earth?
     
  11. Oct 12, 2007 #10

    SpaceTiger

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    I would agree, actually, their language is a bit strong. In my experience, if given enough free parameters, one can always come up with an alternative explanation for something. As such, I imagine a sufficiently elaborate alternative gravity theory could explain the bullet cluster. At some point, however, you have to stop adding epicycles.
     
  12. Oct 13, 2007 #11

    Chronos

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    Cosmology is ruled by Occam's Razor - the simplest explanation is preferred. At present, dark matter comfortably leads all contending theories. Dark energy is still rather precariously perched on a small number of studies.
     
  13. Oct 16, 2007 #12
    I'm sorry, who's stories/trilogy?

    And a little uninvited commentary from a fiction buff. I love this "Dark matter/Dark Energy/Repulsive Matter" stuff, but lets not confuse Physics with Fiction. Larry Niven wanted to create the Ringworld, so he invented a fictional material called "Scrinth" to make it out of. Arthur Clark did the same with his Space Elevator, making it out of 100,000 mile long diamonds. These materials don't exist, and they probably never will.

    These new exotic forms of matter/energy are fun to play with, but they live solidly in the domain of fiction writers until the cosmologists prove them to exist. Now this does not mean that fictional thinkers and physicists can't inspire each other, but us fictionalists have to be careful not to get too attached to our wild ideas.
     
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