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Nature of Dark Matter

  1. Sep 13, 2012 #1
    If DM is immune to the electromagnetic force but not to gravity and the universe is homogenius, would we expect the find DM at the centre of normal matter bodies such as stars and planets trying to form black holes?
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  3. Sep 13, 2012 #2


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    Dark matter is not inside of stars or planets since we can measure their mass via their orbital characteristics. Dark resides in the space about these objects. We don't know what it is only that it shows up as it interacts with visible objects.

  4. Sep 13, 2012 #3


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    No. Unlike baryonic matter, DM is collisionless so it does not accumulate in the center of gravitating bodies. It just falls right through and passes out the other side.
  5. Sep 13, 2012 #4
    The only evidence we have for dark matter is gravitational? Is that all? Just the discrepancy between the mass of objects and their observed gravitational influence? Just seems kind of meager to me, not to mention taking into account the entire history of Astronomy and how we've been duped before in the past. Ptolemy and his epicycles first come to mind. The alternative theories make more sense to me. I hereby bet one dollar I'm right on this one too or is my support of these theories not consistent with the rules of this forum? I don't think so but maybe not.

    Oh yeah, ether wind too.

    I mean think about it: rather than slightly modify a simple, elementary concept created hundreds of years ago by frail H.sapiens, we'd rather just magically create an entire dark Universe and I personally do not agree with this approach.
    Last edited: Sep 13, 2012
  6. Sep 13, 2012 #5
    So DM contributes to the gravition force, but is not responsive to gravity? The Wiki suggests it clusters in a large ball around galaxies, what if not gravity causes it behave this way?

    Or is it that as it falls toward the centre of a body, the accelertion caused by gravity is enough to give it the escape velocity coming out the other side.
    Last edited: Sep 13, 2012
  7. Sep 13, 2012 #6


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    No one said that DM is "not responsive to gravity"!!!
    When things fall together they cannot stick together unless they have some way to shed the excess energy.

    Ordinary matter when it condenses gets hot and radiates away the excess energy of collision or condensation in the form of heat. DM cannot do this, so it is very interesting to think about how it can gather into clouds!

    Clouds of ordinary matter when they collide can get so hot that they even radiate X-rays! Temperatures can get as high as in the interior of stars. We see see some clouds of OM gas by their Xray emission. A great way for OM to dump excess energy, when it is gathering together by the self-gravitation.

    DM is certainly "responsive to gravity"! It just is rather limited in its ability to get rid of the excess "falling-together" gravitational energy and this handicap is why it tends to gather only in large clouds.

    Maps of DM concentration can be made by measuring optical effect on the backround that we see thru the clouds. Contour density maps. ("weak gravitational lensing") like looking thru ripply glass. Nice technique. Fun-house mirror-like distortion, but by transparent medium, not by reflection. Do you want a link to some of the maps?

    The point is that the physics of this type of matter leads to LARGE scale (not small scale) clumping.

    It is interesting how DM manages to collect or gather at all! How can it dump enough excess energy to even condense into LARGE clouds???

    If you pursue this line of thought you may be curious about this. What mechanism is available to "cool" self-gravitating clouds of DM? then keep on asking questions. Some intriguing stuff here :biggrin:
  8. Sep 13, 2012 #7


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    The modification would require more alterations of current theory than dark matter does.
  9. Sep 14, 2012 #8


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    The first part is correct. The second is as far from correct as you can possibly get.

    The reason why nearly all cosmologists are quite sure dark matter exists is because dark matter behaves quite differently from normal matter. It isn't just a matter of objects appearing heavier than the parts of them we can see. Dark matter doesn't form the same distribution as normal matter, because it is nearly collisionless and cannot lose energy due to friction.

    This simple fact causes dark matter to behave very differently from normal matter in a variety of situations, and those situations are all consistent with there being a significant component to the universe which acts very much like a weakly-interacting massive particle.

    None of the alternative descriptions of dark matter can match these observations.
  10. Sep 14, 2012 #9
    Also one of the major evidence for dark matter is that it changes the sound of the universe.

    Baryons conducts sound very well. So you take the "big boom" that happened with the big bang, and you end up with extremely strong sound waves that makes the universe very "clumpy". It turns out that the universe *isn't* that clumpy and the best explanation that anyone has come up with is that a lot of the universe is made up of stuff that doesn't conduct sound very well. I.e. dark matter.

    the magic google term for this is "baryon acoustic oscillations" and if you want to play with how the universe "sounds" different, there is this nice applet

  11. Sep 14, 2012 #10


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    Which ones? I think you'll find that the alternatives to particulate dark matter face significant shortcomings, and are arguably as, if not more, complex than CDM models.
  12. Sep 14, 2012 #11
    One thing that I have noticed is that a lot of the astrophysicists that are skeptical about dark matter do work in galactic clusters, and looking at their reasoning, they do have a point that there are things about LCDM that don't make sense at the level of galaxy clusters.


    1) when we talk about galaxy and cluster formation there are all sorts of complicated things that could happen so that a simple application of LCDM isn't expected to work.
    2) LCDM works really, really well for the early universe

    The reason for this is that when you talk about the early universe, you are talking about "linear" perturbations and things are pretty simple. As time passes the perturbations grow and by the time you get to now, it's pretty certain that there are very complicated things that cause the "simple linear" model not to work.

    So what it looks like to me is that there is some "missing piece" somewhere.
  13. Sep 14, 2012 #12
    Hardly. There is an entire cottage industry devoted to writing papers trying to find alternative to dark matter. If you want to discuss peer-reviewed papers that provide alternatives to dark matter, that's fine.

    If you go into "conspiracy theories" than that's not.

    The thing about ether is that we looked for ether and we didn't find it. As far as dark matter, we've seen it. People are putting together maps of dark matter based on gravitational lensing.

    The trouble is that the simple approaches just do not work. Also it's not a matter of agreement or disagreement. It's a matter of making things fit with the data.

    Things that we've seen for hundreds of years are *hard* to modify. A lot of cosmology in fact involves creating "tooth fairy" explanations for the data, but a lot of theory involves figuring out where you can hide a tooth fairy and where you can't. If your explanation is "bigfoot stole my lunch" and you are talking about the Pacific northwest, then maybe its plausible. If your explanation is "bigfoot stole my lunch" and you are talking about Times Square, then it won't work. One problem with tinkering with gravity is that we understand gravity very well, and there are some things that you just can't do with it. Whereas the useful thing about dark matter is that you can invoke it without running into major problems.

    Now all of this is very well covered by the literature, I think the one rule that works is "assume that astrophysicists aren't idiots" and if someone believes strongly that X is or is not the case, they have good reasons for it, and not go into conspiracy theories.
  14. Sep 15, 2012 #13
    The major alternative to dark matter are "alternative gravity" models. The one big difference between "alternative gravity" and "dark matter" is that we expect that different parts of the universe have different amounts of dark matter, whereas we expect that even modified gravity should behave the same in different parts of the universe.

    So if you see one galaxy behaving very differently than another, "dark matter" has an easy explanation. They have different amounts of dark matter. If you argue that gravity behaves differently in one galaxy than another, people will look at you funny. The big problem with modified gravity models with dark matter, is that no one has come up with a "simple rule" that unifies the behavior of different galaxies, and if you spend two decades looking for something and can't find it, you have to start considering the possibility that it isn't there.

    It's hard to tell when you can declare the game over, but I think with dark matter, it's pretty conclusive. Now as far as "dark energy", that's a totally different topic, and there modified gravity models are still in play.
  15. Sep 15, 2012 #14
    Ok twofish. I appreciate you taking the time to explain that to me so nicely. Seriously. But my understanding is that we do not have sufficient evidence to declare dark matter responsible for the observed discrepancies between mass and gravity:


    However I do not wish to derail this thread and perhaps I was wrong to argue aginst it here. Also, I know you guys aren't idiots. I respect and admire all of you. However, I can't ignore we've made many mistakes throughout the history of Astronomy and am skeptical that we just happen to finally have it right in our time.
  16. Sep 15, 2012 #15


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    That article is far, far too kind to the possibility that we don't have dark matter. The evidence for dark matter is really, really copious. Now, dark matter might not be as simple as a WIMP, though due to its simplicity that seems the most likely candidate. But the possibility of us merely misunderstanding normal matter, or misunderstanding gravity, has been ruled out pretty conclusively by observation.
  17. Sep 15, 2012 #16
    Ok, thanks for that. We just don't know what it is though from my understanding. That means we would have to discover some new type of matter. Just seems like a tough call to me. I remain skeptical but I'll accept it when we have some empirical evidence for this new substance.
  18. Sep 15, 2012 #17


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    Well, it was. But the science is pretty solid here. The best evidence, I think, is in the cosmic microwave background, which was emitted back before any structures in the universe formed, and as a result its behavior is independent of the messy physics that deals with the formations of galaxies and stars and the like. The physics back before the CMB was emitted were much simpler, and therefore the detection is much cleaner than worrying about the very complicated nearby universe.

    And what happened in the early universe was we had a situation where dark matter and normal matter behaved extremely differently, because normal matter experienced pressure, while dark matter did not (because dark matter doesn't interact much with anything, while normal matter at the time was a plasma, and plasmas interact very strongly with light). What we see when we look at the CMB is the imprint of pressure waves, whose effects are most clearly visualized by looking at the power spectrum of the CMB:

    Here you see the power spectrum as measured by WMAP. The first peak represents pressure waves that had just enough time in the early universe to collapse inward, but not enough time to bounce back.

    The second peak represents pressure waves that have a smaller wavelength, so that they could oscillate faster, and had just enough time to collapse inward then bounce back. Notice that it is vastly smaller than the first peak. Part of this is because there is an overall decreasing trend to the CMB power spectrum (because our image of it is a little bit blurry). But most of it is because of dark matter: dark matter experiences no pressure and doesn't bounce back.

    We know this because of the third peak, which is almost as big as the second. If we were just seeing the overall decreasing trend, then the third peak would be much smaller than the second. But instead it's nearly the same size, because it, like the first peak, also gets a boost from the quantity of dark matter: this peak represents pressure waves with even shorter wavelengths that had enough time to collapse, bounce back, then collapse again. The dark matter just collapsed once and sat there, so its contribution remains.

    The power spectrum goes on, to much shorter wavelengths, but WMAP doesn't image those shorter wavelengths very well, though other instruments have. And those other instruments show the same pattern: every even-numbered peak is suppressed compared to its odd-numbered neighbors.

    It's pretty much impossible to produce this kind of pattern without a weakly-interacting, low-temperature component of the universe that has substantial mass.
  19. Sep 15, 2012 #18
    Ok Chalnoth. I just don't understand the science and probably should have just stayed out of this. Unfortunately, I like Astronomy and it's difficult for me to not engage even at the risk of getting into trouble. But also I've become skeptical because the science is so deceptive as history has shown us.

    Would be nice to have a better handle on what precisely dark matter consists of.
  20. Sep 15, 2012 #19


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    What do you mean by "deceptive"? New discoveries happen all the time, but very rarely is something completely wrong or inaccurate to the point that we simply can't use it anymore.

    Absolutely. It's an exciting time in astronomy thanks to mysteries like this!
    On a side note, I just visited Kitt Peak National Observatory last night. They showed us the telescope where evidence for dark matter was first gathered by Vera Rubin.
  21. Sep 15, 2012 #20
    I got a big yard. When I stand in the middle, and look all about, it certainly looks flat to me.

    Also, they really do look like they're going backwards and forwards across the sky not to mention the big yellow one looks like it's moving across the entire sky each day from east to west. Andromeda looks like a difusse cloud until you have a nice telescope to resolve the detail and then realize the Milky Way is not the entire Universe. I seriously doubt we've reached the limits of that deception:)

    Nice! I would like to visit Palomar one day.
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