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Galaxy motions -> hidden superstructure (DM!)

  1. Sep 11, 2004 #1

    Nereid

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    Motions in nearby galaxy cluster reveal hidden superstructure, a Chadra PR, goes on to say: "A nearby galaxy cluster is facing an intergalactic headwind as it is pulled by an underlying superstructure of dark matter, according to new evidence from NASA's Chandra X-ray Observatory. Astronomers think most of the matter in the universe is concentrated in long large filaments of dark matter and that galaxy clusters are formed where these filaments intersect." The cluster is Fornax.
     
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  3. Sep 11, 2004 #2
    Perhaps more evidence to revise previous theories? :surprised
     
  4. Sep 12, 2004 #3
    If this were correct, it would seem to shed some light on what dark matter really is. Could it simply be that dark matter is ordinary matter, but is of low density so that no stars can form? Thus only where intersections or higher concentrated regions of dark matter come together star production and galaxies eventually form?

    Theories of dark matter being dark objects like comets, stars that aren’t luminous, rocks, or black holes, may not actually be what dark matter is…
     
  5. Sep 12, 2004 #4

    Chronos

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    It is pretty simple to me. Dark matter is the most logical solution to observational evidence.
     
  6. Sep 12, 2004 #5

    Nereid

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    This is quite close to the generally accepted models Vast! However, the difference is that DM is certainly not ordinary (baryonic) matter - galaxies etc formed at the intersections of the filaments because the overall mass density was high enough that interactions among the baryonic matter quickly resulted in that matter 'cooling' (you can think of it as losing gravitational energy due to emission of photons). Baryonic mass also formed galaxies etc along the filaments, but not at the same rate.
    In the most popular cosmological models, DM can't be comets, rocks, pebbles, gas, or even BHs ... a) because there's no observational evidence for several of these hypothesised forms (gas, dust, stars), and b) because a model with mass in the form of only baryonic matter doesn't resemble the present day universe at all (there are some alternative models - e.g. Garth's SCC - where this inconsistency might disappear).
     
  7. Sep 12, 2004 #6
    Imagine Neried. A overall geometry that details evrything that we can see with predictability? Okay. So I am dreamer :cry:

    The isometric relations that one might find in the cosmos, in relation to the orbitals always serves as a nice catelogue reference, when we look at these events. But like the dreamer one has to imagine that there is a geometry that underlines all the basis of exploration that is curently taken from the cosmological pallete to the one we so have a hard time understanding below.

    So how shall the quantum geometry of the universe, take us to the one below? Is this possible in your doggone eyes :smile:
     
  8. Sep 13, 2004 #7

    Chronos

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    Dark matter? Come up with a better explanation and collect your Nobel. Nereid correctly points out there is no better explanation. Is anyone happy with dark matter? No. The facts remain. There is no better explanation.
     
  9. Sep 13, 2004 #8

    Nereid

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    Free Falling is the title of HEASARC's piccie of the week (yes, the same Chandra image of the Fornax cluster).

    There are some good sites on the links on this page, good for a fun few hours.

    HEASARC: High Energy Astrophysics Science Archive Research Center.
     
  10. Sep 13, 2004 #9

    Garth

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    The spectra of distant quasars reveal a Lyman Alpha forest - the fingerprint of intergalactic hydrogen in which is also the spectral signature of a relatively high degree of "metallicity" of which only 20% can be explained as galactic outflow. Is not this then the observational evidence for the Dark Matter? In the form of cold gas?

    The fact it is normally attributed to something exotic is because the standard BB model produces only a maximum of 4% of critical density in baryonic form, and the density required by cluster dynamics and gravitational lensing is about 20% - 30% critical density. However this higher value is in the range produced by the 'freely coasting ' model.
    - Garth
     
  11. Sep 13, 2004 #10

    turbo

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    Well, you know how I feel about this one. Dark matter is the most convenient way to reconcile GR with observations that cannot be explained by GR. It is hardly the most logical.

    There is no plausible explanation of its make-up. There is no logical way to explain how how dark matter can distribute itself so obediently in every circumstance where GR needs it, so GR can stay predictive with observations in galactic rotation rates, cluster structure, etc. Please remember that this powerful stuff is entirely invisible, despite decades of searching by very inventive, talented people.

    When a phenomenon is entirely unconstrained by causality, and is entirely undetectable, yet performs in intelligent ways to save GR, we should not celebrate our ignorance. We should ask ourselves "Where is GR broken?" We should ask "What does GR NOT model correctly, that can explain the convoluted behavior of the dark matter needed to fix GR?" I believe that the solution may be found in the places that GR cannot go - QED.

    This is not to say that GR may not be found to be predictive and accurate after its deficiencies are explained and corrected. After all, Newtonian dynamics are predictive and accurate for a LOT of real-world stuff. If we look at the progression of theories explaining planetary movements up though Newton, and Einstein, and then declare that GR is the end-all, we are both being presumtuous and inapprorpriately smug. Science is defined by the processes though which theories are expressed, modeled, tested, and refined.

    Einstein's theories have already been refined and redefined in many ways. It is interesting to see how many GR folks are willing to accept these modifications, and yet will believe six impossible things before breakfast (regarding dark matter) to keep GR alive. When folks outside the GR field try to model the problems, the GR folks decry their attempts as "fringe" or "unconventional", while clinging to these outrageous "leap of faith" concepts necessary to keep it alive. It's pretty sad.

    The conventional GR view is "GR is true, so Dark Matter must exist because if GR is true, we need it to explain the observations." That kind of thinking is wrong in so may ways! Epicycles (like dark matter) are an obvious measure of just how the standard model is broken, and should lead us to challenge the fundamentals of the theory and find a better solution. We can either strive to learn, or we can stagnate. Choose.

    The glove is down! :mad:

    OK, Chronos, it's not like I could be mad at you, but you may have to supply some math.! :smile:
     
    Last edited: Sep 13, 2004
  12. Sep 16, 2004 #11

    Chronos

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    I am reverse engineering this question.

    Describe models that are more logical and why they are more logical.
    OK, I will try, how about dark matter is matter that does not emit detectable EM?
    Agreed, dark matter always seems to be exactly where it needs to be according to GR.
    Agreed. That is maddening.
    Good question. Why does dark matter just happen to be right where GR predicts it should be?
    The evidence GR is not broken appears to be far more convincing than the evidence that it is broken.
    The need for such a solution has not yet been established, so far as I can see.
    What deficiencies?
    Agreed.
    Please clarify. What modifications have GR folks accepted and impossibilities believed before breakfast are you refering to?
    What 'leaps of faith'?
    Dark matter, or something like it, is observational fact. Did the standard model forbid dark matter before it was observed? No. Does dark matter forbid the standard model? No.
    Assumes facts not in evidence.
     
  13. Sep 16, 2004 #12

    Garth

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    The freely coasting model does not need DM or Inflation.

    Your suggestion that Dark Matter is just that, dark ordinary matter does not fit as the BBN allows only a maximum of 4% baryonic matter when you need over 20%. It must therefore be, according to GR, not only Dark but unknown, some exotic non-baryonic species.


    1. Inflation - no Higgs boson
    2. Dark Matter - see above
    3. Dark Energy - what the heck is this supposed to be? Anybody's guess is as good as anybody else’s; there is certainly no shortage of suggestions, all of which of course are completely unsubstantiated.
    4. No large angle fluctuations in the CMB WMAP data, is the universe flat and infinite or not? Perhaps we all do live on a football (soccer-ball) then!
    5. False vacuum fine-tuned to one part in 10^(102). (Anthropic argument again perhaps?)
    6. Densities of Dark matter, energy and baryons all approximately equal (to within an order of magnitude) but why? Oh! I forgot; Anthropic explanation called for!
    7. The small value of the false vacuum energy is unstable to quantum corrections - if interpreted as a small positive cosmological constant then it is incompatible with String theory.
    8. Galaxy mass profiles predicted by the standard theory have too pronounced a cusp at small angles and a too steep galaxy luminosity function.

    Perhaps the claim that we are in the epoch of the 'end of cosmology' is a little premature - just as it was at the turn of the 19th/20th centuries.

    - Garth
     
    Last edited: Sep 16, 2004
  14. Sep 16, 2004 #13

    turbo

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    Gotta go to work, but let me give you my take on this point quickly. Dark matter does NOT happen to be right where GR predicted it. GR did NOT predict dark matter - neither its existence, its function, not its distribution. None of this is in GR. It all has to be put in by hand to fix deficiences in GR that cause observations to disagree with its predictions.

    "Dark matter" was dreamed up to explain deficiencies in GR. The deficiencies were observed and measured, and then conventional physicists said "what could cause this?" They agreed that extremely large fields of matter might cause the effects, if they were distributed "just so". The problem is that each instance in which "dark matter" is invoked to fix GR has a special "just so" distribution that has to be put into GR by hand to save it. This is not science. It is religion - blind belief.

    Dark matter's properties and distributions were tuned to make GR's "predictions" more accurate. Instead of probing these problems, some GR adherents are claiming that this artificially restored predictive ability is proof that dark matter exists! If there is a more glaring example of circular reasoning in "science", I'd love to see it. :rofl:
     
  15. Sep 16, 2004 #14

    selfAdjoint

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    AFAIK, the problems that dark matter was introduced to solve had nothing to do with GR in particular. The motions of the galaxies were computed from Newtonian approximations, which was fine because they are so slow and spread out. So the animus you apparently direct at GR should be aimed at Newton instead.
     
    Last edited: Sep 16, 2004
  16. Sep 16, 2004 #15

    Garth

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    turbo-1 although we tend to agree on this question I have one niggle..
    Not all religion is blind belief.
    - Garth
     
  17. Sep 16, 2004 #16

    Garth

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    I agree - the fact that it is believed that DM exists is down to Newton and it could be cold gas/mini primordial black holes/bricks or whatever. It hasn't been identified and the limits on each form are getting smaller so it is a problem.

    The fact that it is believed to be some exotic non-baryonic substance WIMPs etc. is down to GR because, as I said above, BBN requires the baryonic density to be no more than 4% (even that is pushing it) whereas there is a requirement for >20% of DM.

    So for GR BBN to be consistent with galactic/cluster rotation rates and gravitational lensing it has to invoke a totally unknown species of matter, a form of mass that has evaded discovery after more than two decades of intense research.

    It is in this sense that GR is deficient.

    - Garth
     
  18. Sep 16, 2004 #17

    turbo

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    You are absolutely right. I should have said the "Standard Model" and not GR, which is only a part of the SM (if an important one). I tend to lump them together mentally because the inapplicability of GR to QED is a blind spot that might hide the mechanism for the "misbehavior" of galaxies and galaxy clusters that necessitated the invention of dark matter. Thank you for the clarification.
     
  19. Sep 16, 2004 #18

    turbo

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    I intended no offense to those of faith. My point was that there is an air of 'sanctity' about the Standard Model that allows otherwise logical people to abandon the scientific method in this instance, and believe in dark matter.
     
  20. Sep 16, 2004 #19

    Garth

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    No offence taken!

    The air of 'sanctity' you mention is worrying, it manifests itself by a certainty that the evidence will not bear. "Precision cosmology", "the concordance model", “the end of cosmology”, even "the end of physics" and "knowing the mind of God", have all been banded about.

    Of course the WMAP data set is very detailed and rich in information, the question is how should that information be interpreted, as that interpretation is theory dependent?

    Garth
     
    Last edited: Sep 16, 2004
  21. Sep 16, 2004 #20

    Nereid

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    Perhaps we should try harder to get to the bottom of the apparent differences of opinion here ...

    Perhaps we start with the assertion above that folk like Tegmark have 'abandoned the scientific method' - in what way do you feel they have done so?

    Then we could consider 'believe in dark matter' - if we are following the scientific method, then our beliefs are irrelevant (fairies, dark matter, the existence of turbo-1); what counts is whether we can construct hypotheses and theories which are a) internally consistent, b) consistent with QM and GR within the appropriate domains of applicability, and c) {most important} consistent with good observational and experimental results. In these respects, I submit that appropriate hypotheses incorporating cold, non-baryonic dark matter are good hypotheses. Further, AFAIK, no other hypotheses satisfy these three conditions as well; for example, MOND is explicitly inconsistent with GR, and Garth's SCC (or the more general 'freely coasting' cosmologies) has quite some work still to do address the full range of observations and experiments (e.g. large scale structure, the SZE (I'm not 100% sure of this one), and primordial abundance of nuclides (the 'Indian team's paper is, IMHO, far from satisfactory in this regard)).

    If you consider that DM hypotheses do not satisfy one or more of these criteria, please say so (and give details); if you are aware of other hypotheses which don't include DM but do satisfy all these criteria as well as the concordance models, please provide details.

    Note to Mentors: could you give us an indication please as to whether disagreement with the current state of cosmology - based on personal distaste for certain concepts - is appropriate here? I feel it's similar to some of the posts - since moved to TD - expressing dislike for SR or GR.
     
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