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Dark Matter-What is it ?

  1. May 2, 2008 #1
    A high tech search is going on around the globe to spot what is known as dark matter about which the scientists admittedly are still in the dark.Certainly, whoever discovers the nature of the exotic dark matter would solve one of the greatest mysteries of modern science.Some say that they are tiny, exotic particles left over from the Big Bang some 13.7 billion years ago, but what actually is it???
     
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  3. May 2, 2008 #2

    cristo

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    Well, the short answer is that no-one really knows. John Baez's Physics FAQ might give you a little more info.
     
  4. May 4, 2008 #3
    I just came to know a bit of information about this. It seems that the prevailing theory about the dark matter is that it is made up of tiny exotic particles left over from the Big Bang some 13.7 billion years ago. I also read that the dark matter might have got its name because it does not give off either heat or light. Astronomers know it exists because of its gravitational tug-of-war with stars and galaxies.
     
  5. May 4, 2008 #4

    EL

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    That's a good summary!
     
  6. May 4, 2008 #5

    Chronos

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    I concur. Dark matter is because observational evidence insists it must exist. The evidence is overwhelming.
     
  7. May 7, 2008 #6
    The book "Dark Side of the Universe", subtitled "Dark Matter, Dark Energy, and the Fate of the Cosmos" is a super discussion of this topic.
     
  8. May 7, 2008 #7

    turbo

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    Dark matter probably does not exist. It is postulated to protect the model of gravitation put forth by Einstein in his theory of General Relativity. This model was an improvement on the Newtonian model of gravitation in some subtle ways, but Einstein was never happy with the implementation of GR and worked for a long time to improve it, without success. Does anybody here think that if Einstein had known about the existence of extra-galactic spiral galaxies, and their anomalous flat rotation curves that he would have invented DM to cover the failure of his model? I think not. I believe that he would have returned to the fundamentals of gravitation instead of inventing superfluous entities.
     
  9. May 7, 2008 #8
    You can't put words into Einstein's mouth and use it as an argument against mainstream astronomy, though can you? That doesn't really help.
     
  10. May 7, 2008 #9

    turbo

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    Mainstream astronomy is an entirely observational exercise because we cannot interact with the bodies being observed. I am not putting words into Einstein's mouth, since he wrote very extensively in his 1920 book on relativity, his 1920 Leiden essay on the the ether, and his 1924 essay "On the Ether" putting forth ideas that are not welcome today. If you are not familiar with these writings, I can help you out. I think that he was on the right path.
     
  11. May 7, 2008 #10
    I thought dark matter was obscured matter from our view.. take a nebula like the horsehead for example... all but the brightest stars behind it are obscured from our viewpoint.

    This is not dark matter. Dark matter seems to be the mass of invisible sub-atomic particles (virtual particles?) like neutrinos that we can't even detect yet. They are out there, and in number, and we can infer their existence from the rate of expansion of the universe. I think this played a hand in the cosmological constant?
     
    Last edited: May 7, 2008
  12. May 7, 2008 #11

    Wallace

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    We've moved beyond science being the process of extracting meaning out of the words of a single ancient figure (such as Aristotelean philosophy as it proceeded for over two millennium). Einstein was a genius who gave us amazing insight, however there are many many people, some of whom were contemporaries of Einstein and many of whom came after, that have shaped out view of relativity. We will do best to work with the observations and ideas we have now rather than trying to imaging 'what would Einstein think?'.
     
  13. May 7, 2008 #12

    turbo

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    Would you like to explain how you can infer the existence of Dark Matter from any of Einstein's work? Please remember that Einstein's work ~100 years ago did not envision any kind of universal expansion, so you'll have to explain that, too.
     
  14. May 7, 2008 #13

    Wallace

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    Isn't that my point? We can't infer dark matter, dark energy and plenty more 'from Einsteins work'. That's why we have to remember that hundreds of people have worked on relativity, not just Einstein. Fortunately using much of this work, we can say a good deal about those topics. I repeat what I said earlier, you seem to be focused on things only being true if they were said by Einstein and if he said something was false then it must be. This is not how science has worked since the 1600's.

    As for universal expansion, I can deal with that. It is a pop-sci mythconception, not science so don't get into knots about that one.
     
  15. May 8, 2008 #14

    turbo

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    Let me try to back up another step. When Einstein (and others) laid the groundwork for the relativistic model of gravitation, the nature of our visible universe was very poorly understood, and neighboring galaxies were still called "nebulae" and their natures were unknown. Newtonian gravitation explained the behavior of solar-system bodies pretty darned well, and the relativistic interpretation was marginally better.

    Here we are ~100 years on, and we have lots of wonderful observations. Vera Rubin discovered from spectroscopic studies that the rotation curves of spiral galaxies are much flatter than can be explained by GR's gravitational model. The MOND folks introduced modifications to the Newtonian gravitational model that not only fit the flat rotation curves very well, but also predicted the behavior of low-surface brightness galaxies over a decade before they were actually observed. This does not prove that MOND is correct, just that is predictive and therefor somewhat useful.

    On larger (cluster) scales we have observed that the gravitational binding forces are larger than can be expected from a GR model using visible matter. Zwickey knew this a long time back, so this is not a new problem. Lensing effects are also much stronger than we might expect by considering only the visible matter.

    We also have the Pioneer anomalies, showing unmodeled Sun-ward accelerations in spin stabilized spacecraft. Is it possible that neither the Newtonian nor relativistic models for gravitation are accurate on some scales? I think that we should be open to the possibility that these gravitational models might have a limited scale of applicability. Invoking the existence of exotic undetectable matter that only interacts gravitationally is a kludge. Even if it were a viable fix, it is advisable to look at what the MOND folks have done and ask what we could do to modify current gravitational theory to more closely accord with observations.

    I do not have a blind allegiance to Einstein or his work, despite what you might think. I am trying to point out that what he accomplished has to be seen in the context of scientific knowledge at that time, and inventing new entities (DM for one) to try to keep his relativistic model of gravitation predictive is short-sighted and likely counter-productive. Reading Einstein's tribute to Ernst Mach, I get the feeling that he would be very uncomfortable with this situation.

    "How does it happen that a properly endowed natural scientist comes to concern himself with epistemology? Is there no more valuable work in his specialty? I hear many of my colleagues saying, and I sense it from many more, that they feel this way. I cannot share this sentiment. ... Concepts that have proven useful in ordering things easily achieve such an authority over us that we forget their earthly origins and accept them as unalterable givens. Thus they come to be stamped as 'necessities of thought,' 'a priori givens,' etc. The path of scientific advance is often made impassable for a long time through such errors. For that reason, it is by no means an idle game if we become practiced in analyzing the long common place concepts and exhibiting those circumstances upon which their justification and usefulness depend, how they have grown up, individually, out of the givens of experience. By this means, their all-too-great authority will be broken."
     
  16. May 8, 2008 #15

    cristo

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  17. May 8, 2008 #16

    turbo

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  18. May 8, 2008 #17

    cristo

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    But that's an article from five years ago? The paper I linked to is from late last year, and was published in January this year. Here's a quote from the introduction:

    It doesn't make sense to counter a recent paper with an interview given five years ago!
     
  19. May 8, 2008 #18

    Kurdt

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    I don't think worrying about whether Einstein would have liked dark matter or not is answering the question in the thread which is (with slight modification), "What are the candidates for dark matter?".
     
  20. May 8, 2008 #19

    turbo

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    Newer does not equate to "more correct", especially in theoretical fields like cosmology. If you read through the Milgrom interview, you will see that theoreticians were trying to accommodate DM in MOND for years. The paper that you linked finds that the visible mass in their candidate galaxies is insufficient to explain the lensing observed. This would not be at all surprising if our gravitational theories need modification. The result goes hand-in hand with the stronger than expected cluster binding and stronger than expected cluster lensing.

    I do not believe that MOND is correct or universally applicable, though on galactic scales it has been remarkably predictive. Its real value may be in quantifying the failure of GR gravity on galactic scales.
     
  21. May 8, 2008 #20

    turbo

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    True, but Cristo answered that as well as can be expected in his reply. Nobody knows what particles could be undetectable and weakly interactive, except gravitationally. We can talk about the fanciful names that have been made up - like WIMP for weakly interacting massive particles - but putting a name to an undetectable entity is not the same as defining its nature.
     
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