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No dark matter after all?

  1. Nov 16, 2011 #1
    I read an article explaining how all the stars in the milky way revolve around the galactic center at the same velocity, which is obviously different than the way the planets in this solar system revolve around the sun. They say this is because the C.O.M. isnt located in the galactic center, but spread throughout the entire milky way due to dark matter.

    They went on to talk about how Einstein disproved Newton when having to deal with quantum level objects and continued on to say that perhaps newtonian physics isnt entirely correct galactic sized systems. ie...it is only really valid "medium" sized stuff(apples, planets). Perhaps a variation of Newton is required for really big stuff? Ill be the first to admit this author isnt the most reliable, I just thought it was an interesting take on DM.

    So if someone could come on here and laugh at me and tell me why Im wrong that would be great. Astrophysics isnt my specialty so I dont really know better
     
  2. jcsd
  3. Nov 16, 2011 #2
    You are correct in assuming the Newton's laws aren't always correct in astrophysics. For example, in the presence of very strong gravitational fields (such as that from a galaxy, or galactic center) Newton's laws of gravity are a poor approximation. Einstein demonstrated this as well, when he published his theory on General Relativity.

    This is why scientists use Einstein's equations for relativity, not Newtonian gravity, to do the sort of calculations you are talking about. Even with these corrections, however, it still appears that most of the mass in galaxies is matter which we cannot see.

    This matter has actually been indirectly viewed; I myself have seen an image taken of a galactic merger, where gravitational lensing (the distortion of light due to ulta-strong gravitational fields--in case you did not know the term) was observed in areas outside that of the visible merger. Most astronomers have concluded that this was evidence of the dark matter from the two galaxies merging in the area surrounding the visible merger.
     
  4. Nov 16, 2011 #3

    phinds

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    How can a center of mass be "spread throughout ... "??? That just doesn't make any sense ... it is a self-contradictory statement. The MASS is spread throughout, the CENTER of mass is ... wait for it ... in the CENTER.
     
  5. Nov 16, 2011 #4

    DaveC426913

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    Inaccurate, but the sentiment is correct.

    The orbital velocity drops off with the distance from the centre (as with planets) - it's just that with galaxies, it doesn't drop off as fast as it should.

    There are proposals for a form of modified gravity, yes. But they have problems of their own, and can't compete with the mounting case for Dark Matter.

    There are multiple lines of evidence that point to Dark Matter beyond just galactic orbital velocity. Look up Bullet Cluster.
     
  6. Nov 16, 2011 #5

    phinds

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    Dave, I just can't understand why people on this forum like you more than me.

    Here I give a nice crisp sarcastic answer, and you just ramble on and actually EXPLAIN things.

    '
    '
    '

    Oh ... wait ... I think maybe I'm beginning to see my problem :blushing:
     
  7. Nov 16, 2011 #6

    DaveC426913

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    Yah, I caught that too. Didn't see how correcting it would get him closer to answer. He was right enough. :wink:
     
  8. Nov 18, 2011 #7
    When they calculate the mass of the galaxies, are they taking into account all the objects that are no longer visible? Black holes, red and white dwarfs, and other objects and material.
     
  9. Nov 18, 2011 #8

    phinds

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    Probably depends on who "they" are, but yeah I think the intent is always to include everything. This is how dark matter was discovered ... there just isn't enough OTHER stuff (by a LONG shot) to account for how the galaxy rotates, so if "they" thought they had left out black holes and other things, it would have been a different story.
     
  10. Nov 18, 2011 #9

    DaveC426913

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    Yes. Actually BHs and dwarfs are some of the more visible artifacts. Less visible artifacts are cold gas and dust, of which there is quite a lot. But it too is taken into account.
     
  11. Nov 21, 2011 #10
    If they did account for "everything", then it would be a guess. There could be many black holes wandering through the galaxy with nothing around them. The same could be true for old neutron stars, and burned out stars.

    I'm not saying they are wrong, just playing the devil's advocate about mass they may have overlooked.

    This raises other questions beyond the original topic, but if and when we do explore space, there will be a need to detect objects of mass that do not emit light far in front of the path of the spaceship.
     
  12. Nov 21, 2011 #11

    DaveC426913

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    1] Our models do not predict that interstellar space is littered shoulder-to-shoulder with black holes and neutron stars. To be enough to account for the missing mater, we'd have to be wading through a veritable fog of objects. And not one of them is anywhere near the Sol system. So now we must have a galaxy full to the brim with invisible objects yet we live in a special bubble that is empty.
    2] They would shine with infalling matter as they pass through gas and dust. Nothing happens in a vacuum. Even in a vacuum. :biggrin:
    3] And even if they were miraculously invisible, we would most definitely see them via gravitational close encounters.
     
  13. Nov 21, 2011 #12
    If there is no activity around a black hole, they are undetectable unless there is some new form of detection I am not aware of.
     
  14. Nov 21, 2011 #13

    DaveC426913

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    1] No space in a galaxy is free from gas and dust. No black hole is going to be free from an accretion disk.
    2] And yes, there is a form of detection that is quite detectible: gravity. We would see their dances with nearby objects.
     
  15. Nov 21, 2011 #14
    Then why is there no accretion disk around the supermassive black hole at the center of our galaxy?
     
  16. Nov 21, 2011 #15

    DaveC426913

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    What makes you think there isn't?

    We don't have the resolving power to see it directly. We are able to deduce the size of the object at the centre of our galaxy by observing the objects orbiting it and knowing their perihelion. This sets a maximum possible size on the object, which is not larger than Saturn's orbit. Since we also know its mass, we can conclude that there are no other known objects that are as dense as that except black holes.
     
  17. Nov 21, 2011 #16
    There is no images or evidence of an accretion disk that I am aware of. The only visible evidence is the 15 year infrared light study of the motion of the stars around the black hole.

    If there is any images of an accretion disk, I would like to see them! :)
     
  18. Nov 21, 2011 #17

    DaveC426913

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    Regardless, invisible black holes are not a viable candidate for the missing mass in galaxies.
     
  19. Nov 21, 2011 #18
    Microlensing searches for such objects (otherwise known as MACHOs - Massive Compact Halo Objects) come up well short of the amount of matter needed to explain the missing mass.

    There is more info on the wiki page:

    http://en.wikipedia.org/wiki/Massive_compact_halo_object" [Broken]
     
    Last edited by a moderator: May 5, 2017
  20. Dec 8, 2011 #19
    There is an area of research on "modified gravity" to explain the galaxy rotation problem. Some theories are better fleshed out and stand up to criticism more than others. Any some interpretations of these ideas even INCLUDE dark matter, so it doesn't necessarily rule it out completely (there are several phenomena attributed to DM). But some contend that with modifications to gravity, no dark matter is needed.

    Probably the oldest and most published one is Modified Newtonian Dynamics, or MOND, proposed by Mordehai Milgrom. It has been expanded to tensor-vector-scalar gravity, or TeVeS, to incorporate relativistic effects. More recently, John Moffat proposed his modified gravity (MOG) with scalar-tensor-vector gravity (STVG) (...seriously, could they not have come up with a less similar name?!?). Both of these theories have successes and failures at predicting various things that DM has been used to explain.
     
  21. Dec 8, 2011 #20

    phinds

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    You are jumping to an unfounded conclusion. Absence of evidence is not evidence of absence.
     
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