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Dark Matter exists!

  1. Aug 27, 2008 #1
    I've recently come across some doubters as to the existence of dark matter. If you find yourself among that group, please read the following article on New Scientist and my comments. Then you can jump on the DM bandwagon with me!

    Dark matter and normal matter 'divorce' in cosmic clash:

    "Dark matter seems to have separated from normal matter in a mammoth collision between two galaxy clusters.

    The results bolster observations of a similar collision called the Bullet Cluster and put rough upper limits on how strongly dark matter interacts with other matter and itself. But so far they cannot rule out any of the leading dark matter candidates...

    ...Now, isolated clouds of dark matter have been observed in a collision between two massive clusters of galaxies lying 5.7 billion light years away."

    Check out the story here.

    There's a few important details the New Scientist article left out:

    First some background. This is the second time that we have seen two clusters collide, the first time was seen in the Bullet Cluster. In both cases, after the clusters collided the majority of normal matter is stripped from the clusters, and we see a separation between normal matter and dark matter.

    Now for the details. You probably saying to yourself, "stars collide and get swept up when clusters collide?, That's not right." And you'd be correct. When galaxies or clusters collide stars don't hit each other. The key to understanding the Bullet Cluster results is that most (~90%) of normal matter in a cluster resides in hot intergalactic gas. It's this gas that "hits" the gas in the other cluster. So the gas (i.e. 90% of normal matter) in the two clusters run smack into each other while the cluster's galaxies and stars and dark matter just sail on through leaving their hot gas far behind.

    Finally, using some techniques (weak lensing and maybe strong lensing too) based on the fact that light is bent by mass, we find the mass distribution (only the 2-D projection onto the sky) of this system. Then we see that sure enough, the dark matter just sailed on through with the galaxies and stars!

    These two results (this new cluster collision and the bullet cluster collision) are not compatible with MOND. MOND is an attempt to modify gravity so we don't need to postulate the existence of dark matter. However, MOND cannot explain these cluster collisions! In order to explain these cluster results, MOND (or some other gravitational theory) would need to say that the gravitational force vector does not point toward the mass creating that force! That is, in this new theory, I wouldn't be falling towards the center of the earth, but in some other direction! Highly unlikely.* Hooray for Dark Matter! If you're still not convinced or can't understand my explanation, then check out this excellent blog post by Sean Carroll at Caltech (it was written only about the Bullet Cluster).

    *Please ignore the pseudo-forces due to Earth not being an inertial frame of reference. Imagine a non-rotating earth that isn't orbiting anything.
  2. jcsd
  3. Aug 27, 2008 #2

    George Jones

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  4. Aug 28, 2008 #3
    Hi George,
    Last summer I glanced at the paper regarding the strange dark matter ring formed from a line of sight cluster collision. The impression that I got was that the separation between baryonic matter and dark matter isn't quite as established in the "ring" cluster (not really it's name) as it is in the bullet cluster.

    Regarding the "train-wreck" cluster, mmm I don't know. The system looks, in general, much more complicated than both the bullet cluster and this cluster I've mentioned. Perhaps there's much more to the story of this system than a simple collision, which is what the bullet cluster/this cluster seem be: a simple collision. This additional complexity makes it difficult to make any claims for or against collision-less dark matter.

    Still, these collisions are interesting stuff that MOND can't take into account. Nonetheless, the MONDian's claim that MOND can take the bullet cluster results into account in this paper. I have no idea how they can account for the gravitational field to point in any other direction than where the mass is that's causing the gravitational field, it's not very clear in the paper.

    It seems to me that these cluster collisions plus the wide array of observations--both rotation curves and cosmological observations (CMB power spectrum, structure formation, etc.)-- have put to rest question of MOND* vs dark matter. (that is, dark matter and general relativity win.)

    *in saying MOND, I'm not just referring to MOdified Newtonian Dynamics; I mean any gravitational theory other than General Relativity.
  5. Aug 28, 2008 #4
    Perhaps it might be helpful to consider that a modification of Newtonian dynamics (MOND changes the very foundation of acceleration, which is very strange) is not the same as a modification of the source of acceleration (MOG, CDM, which are still strange in a sense, but less "fudgy"). MOND in its original form does not naturally account for the lensing, the others do. The relativistic upgrade to MOND, TeVeS, supposedly accounts for lensing, but of course not everyone is convinced. Cooperstock and Tieu also have a model which attempts to model the contribution of non-linear gravitational interaction in the weak-field limit (where r >> R_S, as is in the galactic disc). Considering that non-linear gravitational interaction is the ROOT of general relativity, this also seems less "fudgy" than MOND. So really, not every non-DM model is a blatant attempt to destroy general relativity (ex: general relativity does not necessarily "win" or "lose", regardless of the validity of the supersymmetric standard model of particle physics). Recent work on the calculation of total mass-energy in the Cooperstock and Tieu model (by a third party) has conquered the (extremely naive) "refutation" regarding singular discs of negative energy.

    I'm sure someone with more experience would be able to explain it better.
    Last edited: Aug 28, 2008
  6. Aug 28, 2008 #5
    Aren't all these theories alternative theories? That is, don't they seek to replace GR with another theory? Thus aren't they (blatant) attempts to replace (or destroy) GR? Or are you saying that these alternative theories are similar to (or in the same spirit of) GR, thus they aren't really destroying it?

    I don't see how TeVes (which is what Moffat and Brownstein use to "explain" the bullet cluster results) or any other theory can account for a gravitational field that is pointing in a different direction than the mass that is sourcing it -- which is the requirement if you want to eliminate DM from the discussion.
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