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Bill Thomson, Harold Jeffreys and Dark Stuff

  1. Apr 21, 2007 #1
    Is it by now conclusively established that the universe is made mostly of exotic dark matter and dark energy? I need reassuring that there is no escape from this still startling conclusion.

    The following cautionary tale will explain why I have the temerity to ask such a question:

    In the late 19th century geologists had become convinced (correctly as it turned out) that the Earth is a great deal older (several Gyr) than the 6000 years that the Biblical calculator Bishop Ussher had estimated. Geologists had a great deal of evidence to support their conclusion.

    Then along came Bill Thomson (a prestigous physicist, aka Lord Kelvin) who proved, using impressive mathematics not understood by most geologists, that the Sun couldn't possibly be older than about 0.1 Gyr, and also that the Earth was not much older than that!

    Of course the discovery of radioactivity and fusion has long since resolved this conflict in favour of geology.

    Then in the early 20th century the prestigous mathematician/physicist Harold Jeffreys proved, again with impressive reasoning not familiar to the geological community, that the continental drift ideas of the geologists Wegener and Du Toit were nonsense.

    Of course the validation of plate tectonics by J.T. Wilson has since reversed this verdict in favour of the early proponents of continental drift.

    These examples show that even the most prestigous scientific folk are sometimes deceived by sly nature.

    The most prestigous scientific folk since WW II have been nuclear physicists, for very good reasons. And it is they who have explained to cosmologists, a different breed of scientists, how the observed elemental abundances of the lighter elements, placed in the context of their own area of expertise, nuclear physics, put an upper bound on the amount of baryonic matter in the universe. The fact that this bound is far below the critical density of stuff required to explain our flat universe has since made cosmologists jump through many hoops about exotic unobserved forms of dark matter/energy.

    Is it not possible that somehere in the setting of this upper bound sly nature has deceived those prestigous nuclear physicists? This is a cynical possibility suggested by an ignorant someone who is neither a mathematician, nuclear physicist, cosmologist or geologist.

    I don't suppose that nature would sink so low as to deceive folk clever enough to invent the bomb ... but I'd be grateful if contributers to this forum could reassure me that there is in fact plenty of independent evidence of nature's good character.
     
    Last edited: Apr 21, 2007
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  3. Apr 21, 2007 #2

    marcus

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    No.

    It has not been conclusively established.
     
  4. Apr 21, 2007 #3

    Garth

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    Only when the DM particle(s), DE itself and the Higgs Boson/Inflaton are discovered in the laboratory (LHC?), their properties measured and found to be concordant with cosmological constraints will we know what we are talking about, and then we will be able to say the existence of these entities has been "conclusively established".

    Until then cross checking the observations in as many ways as possible is the only means of verifying their existence. However these checks are always 'theory dependent' and may lead to degeneracies in the interpretation of the data.

    One possible way of escaping from the '4%' baryonic matter restriction is the strictly linearly expanding universe, but you have to explain how to deliver such an expansion see A case for nucleosynthesis in slowly evolving models .

    One theory that might have delivered this expansion has just died a death with the GP-B geodetic result!

    Garth
     
    Last edited: Apr 21, 2007
  5. Apr 21, 2007 #4

    marcus

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    If you would put the question in a more easy-going fashion, like Is there a current consensus that... then you could get a more reassuring answer.

    Is there a current consensus among professional cosmologists that, if one goes according to classical 1915 General Relativity, most of the world is dark stuff?
    Yes but.

    the history of science is full of cases where current consensus changes. this current consensus could merely be an artifact of the coincidence that nobody has yet thought of a satisfactory modification of classical Relativity.

    To get rid of the dark stuff, one would need to do find the right way to modify GR.
    Indeed there are scores of people working on doing just that! Do you want a list of URL? Perhaps you already know the writings of some of them*.
    These people are just as qualified and reputable as the majority. The only difference is they are in the minority, so they are not part of the consensus.


    *people trying out ways to obviate the need for DE or DM or both:
    Bekenstein, Moffat, Wiltshire, many others I would have to do a search to get names
     
  6. Apr 21, 2007 #5

    Garth

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    You could start with Alternative theories being tested by Gravity Probe B. :smile:

    Garth
     
  7. Apr 21, 2007 #6

    marcus

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    Personally I would not want to get into that can of worms. What it looks like to me is that these days one can not SWING A CAT without hitting one of the people in this minority who are trying modifications of gravity to dispense with dark energy

    I just went to arxiv and yelled modified gravity and got, e.g., this

    http://arxiv.org/abs/0704.2406
    Dynamics of the universe in the modified unimodular theory of gravity
    Authors: Robert D. Bock
    (Submitted on 18 Apr 2007)

    Abstract: The equations that govern the dynamics of the universe in the modified unimodular theory of gravity are derived. We find a mechanism for inflation in the early universe without postulating a false vacuum state during the first $10^{-35}$ seconds after the Big Bang. In addition, we find a natural explanation for the acceleration of the universe without resorting to dark energy.
    ==================

    http://arxiv.org/abs/0704.2520
    Dark energy from modified F(R)-scalar-Gauss-Bonnet gravity
    Authors: Shin'ichi Nojiri, Sergei D. Odintsov, Petr V. Tretyakov
    (Submitted on 19 Apr 2007 (v1), last revised 20 Apr 2007 (this version, v2))

    Abstract: The modified F(R)-scalar-Gauss-Bonnet gravity is proposed as dark energy model. The reconstruction program for such theory is developed. It is explicitly demonstrated that the known classical universe expansion history (deceleration epoch, transition to acceleration and effective quintessence, phantom or cosmological constant era) may naturally occur in such unified theory for some (reconstructed) classes of scalar potentials. Gauss-Bonnet assisted dark energy is also proposed. The possibility of cosmic acceleration is studied there.
    =========

    the search produced 595 papers, of which 12 were quite recent---posted April 2007.

    Of those 12 that appeared this month, I chose two and pasted abstracts here as sample.

    It is clear that there is a RUSH going on to try to modify the law of gravity (classic GR) so that one will not need Dark stuff. Many people are rushing in to try their hand at this.

    Obviously these people (although a minority) do not think that DE has been "conclusively" confirmed.

    But it would be a waste of time and eyesight to try to keep abreast of this research IMHO
     
  8. Apr 21, 2007 #7

    Garth

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    There is of course MOND as a way of also dispensing with exotic DM, although I think the case for DM is far more established.

    If it does in fact exist the question then is; "What is it?"

    Garth
     
  9. Apr 21, 2007 #8

    marcus

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    "far more" established, unsure what it means, far more than minus infinity is still minus infinity
    because don't think DE established at all (just a working consensus)
    I withhold judgment---remain skeptical of Darks

    Last year the famous NASA press conference where a guy I actually know :bugeye: Maxim was one of the presenters, advertised as the final blow to MOND---those bullet galaxy collision weak lensing findings.

    but after that the MOND people found reasons to say those observations were compatible with MOND----there was that paper with the catchy title "Can MOND take a bullet?" (and survive, that is.)

    Situation too complicated. Too much work to assemble all relevant sources.
    MOND people are NOT looking only at DM, they also look at DE, some indication that a mond-like modification might eliminate need for both kinds of fudge.

    I just happened to have these two links (don't presume to say they are especially relevant but I'll put'm down anyway)
    http://arxiv.org/abs/astro-ph/0601581
    http://arxiv.org/abs/gr-qc/0512109
    another recent sample. I give the abstract for this one just to illustrate:
    http://arxiv.org/abs/astro-ph/0605322
    Dark energy, MOND and sub-millimeter tests of gravity
    I. Navarro, K. Van Acoleyen
    (Submitted on 12 May 2006)

    "We consider modifications of General Relativity obtained by adding the logarithm of some curvature invariants to the Einstein-Hilbert action. These non-linear actions can explain the late-time acceleration of the universe giving an expansion history that differs from that of a pure cosmological constant. We show that they also modify the Newtonian potential below a fixed acceleration scale given by the late-time Hubble constant times the speed of light. This is exactly what is required in MOND, a phenomenological modification of the Newtonian potential that is capable of explaining galactic rotation curves without the need to introduce dark matter. We show that this kind of modification also predicts short distance deviations of Newton's law at the sub-mm scale and an anomalous shift in the precession of the Moon's orbit around the Earth, both effects of a size that is less than an order of magnitude below current bounds."
     
    Last edited: Apr 21, 2007
  10. Apr 21, 2007 #9
    For completeness, it's probably worth noting that some people have suggested that it might be possible to get accelerated expansion (or, at least, the appearance of acceleration) straight out of GR with no dark energy. The idea is that variations in the density of matter in the universe can actually cause significant effects in terms of the expansion, due to the non-linearity of the GR field equations.

    See, for example, http://arxiv.org/abs/astro-ph/0506534

    Also, it's worth noting that, as easy as it seems to be to find modifications of gravity that might act like dark stuff, it tends to be even easier to find modifications of the standard model of particle physics that include massive particles that don't interact through the strong, weak, or electromagnetic forces - i.e. candidates for dark matter.
     
  11. Apr 21, 2007 #10

    Garth

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    The case for Dark Matter is established on the nearly flat rotation curves of spiral galaxies, the virial mass of galactic clusters, the gravitational lensing of distant quasars by galactic clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter is vital in large structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background.

    These different types of evidence suggest that there is far more matter in galaxies, galactic clusters and the universe as a whole than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component".

    The question is: "What is it?"

    The matter component that does interact with electromagnetic radiation, stars, gas and dust, comprises only 0.003 of the critical density. The matter component comprises 0.27 of the critical density, and the maximum baryon density that standard BBN can produce is 0.04 of the critical density.

    Hence 0.037 of the critical density is dark baryonic matter and, according to standard model BBN, 0.23 of the critical density is exotic non-baryonic dark matter. It is this that needs to be identified in the laboratory, alongside the Higgs Boson/Inflaton and DE to confirm that model.

    We may not know what the 0.23 dark matter is, but there are multiple reasons to believe that it is there in some form or other.

    Garth
     
    Last edited: Apr 21, 2007
  12. Apr 21, 2007 #11

    Wallace

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    I agree that DM and DE are not conclusively proven, though I would point out the same could be argued about electrons (or protons or many other things) but that comes done to semantic objections I have to the use of the word 'proof' in empirical science.

    DM and DE I would think are the most likely answers to the questions they were postulated to solve that we currently have, but not as likely to be the correct answer as electrons etc are to be for the reasons they were postulated.

    Pedantry aside, have a read of A Direct empirical proof for the existence of dark matter. If you want a more general level summary an excellent explanation by Sean Carroll does a far better job than I could.

    This result is very compelling!
     
  13. Apr 21, 2007 #12

    marcus

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    That is the paper by my dive-buddy Maxim Markevitch!
    I mentioned that earlier. "Bullet cluster galaxy merger" weak lensing observations. Big NASA ballyhoo!

    It was the most convincing demonstration to date that you could not explain effects just as well or better with MOND.

    Yes, and I remember reading Sean Carroll's popularization on his blog, when it came out.

    However Wallace I've seen several papers since then suggesting that competent MOND people have answers, have shown MOND more compatible with those observations than people indicated in the initial reaction.

    So I think it is wiser to reserve judgment---"the wheel's still in spin."
     
  14. Apr 21, 2007 #13

    Wallace

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    Ahh yes, sorry I missed your earlier reference to those papers.

    I have my suspicions about the MONDish theory and the bullet cluster papers though. They are hard to read and it is not clear qualitatively how you get the effects seen in the bullet cluster with MOND. I'm always suspicious of an argument that can't be summarized clearly for even a technical audience.
     
  15. Apr 21, 2007 #14

    marcus

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    I think it is highly respectable for someone to now consider the case settled and believe that MOND is dead as a substitute for dark matter. (as long as they respect my disbelief)
    What we have here is perhaps a difference in temperament leading to an acceptable range of attitudes.

    My attitude is to be skeptical both of DM and DE and also (paradoxically enough) of MOND and other modified gravity.

    I am refusing to consider anything as convincingly established for the time being.

    I would be delighted (as would you I assume) if a "dark matter particle" were found or some extension of the standard model provided a clear and probable favorite candidate. But I would also be delighted if some (simple elegant) technical improvement of the law of gravity provided explanations of DE and DM effects.

    I am happy to see research proceeding in both directions and I don't want to favor one or the other.

    What I appreciate in you and some others is an undogmatic quality. I am interested in what you Wallace think especially, in fact, because you seem to leave me room have a different attitude.

    (and oh yeah, most MOND strikes me as ugly, overcomplicated, and hard to followthru and be sure about---long way to go to make it elegant---but Im glad they are trying)
     
    Last edited: Apr 21, 2007
  16. Apr 22, 2007 #15

    Wallace

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    The thing about MOND is that it initially made sense and seemed symmetrical to DM to explain galaxy rotation and cluster dispersion velocities. So we found that things were moving too fast for the amount of stuff we could see to hold it is using standard gravity. The two reasonable answers were then that either gravity works differently to the way we think (MOND) or there is more stuff there than we can see (DM). DM for various reasons seemed more likely but either possibility is most certainly possible and the answer to the problem given by each theory was logical and understandable.

    However the bullet cluster seems so much harder for MOND to explain. A valid theory of gravity (to me) must obey one simple condition, that of the isotropy of space. To be clear, in this context what I mean by this is that a law of gravity should tell you what acceleration you experience due to the presence of some mass given how far that mass is from you only, not the direction to that mass. I simply cannot see how any theory of gravity that satisfies this condition can be compatible with the bullet cluster result, even one made up entirely to explain the bullet cluster results without caring about the rest of the Universe.

    I simply cannot see how a law of gravity that obeys the isotropy of space* could possibly make the gravitational potential of a body not lie where the body actually is!

    I don't think the bullet cluster will be the last word on DM, for starters another system showing the same kind of result that was studied by a different research group, and preferably with a different telescope (the second is unlikely in the near future!) would be ideal to confirm the results, however if you take the observational result as unproblematic (i.e. they didn't stuff up the data analysis) then I really don't see how anything other than a DM type hypothesis can explain the results.

    I would stress though that when I say I can't see how it could be done I don't mean that it can't be done at all! If anyone can suggest how I would love to hear it since I can't see a clear explanation in any bullet-cluster and MOND paper I've read. If you have to BTWBS and can't simply summarise the principle I will, as stated above, be very suspicious of the result!

    * Note that what I mean by isotropy of space is the basic symmetry principle, not anything to do with the Cosmological principle of the isotropy of the the Universe
     
    Last edited: Apr 22, 2007
  17. Apr 22, 2007 #16
    A succint and decisive reply. No pussyfooting ... I like that .. thanks.

    Thanks for the offer to educate me, but I find it difficult enough to appreciate the complex logic of mainstream cosmology without delving into alternatives to GR, which I understand only dimly, despite the help of experts like Pervect.

    The main point of my post was to suggest that maybe it is the nuclear physics of element synthesis has somehow gone awry, rather than GR ... and that the angst I believe many feel about dark matter/energy might be misplaced. Perhaps I should have asked:

    Has it been conclusively established that the physics which determined present light-element abundances, i.e. physics at temperatures of 10^9 K, is flawlessly understood? There may be more mysteries about physics in the simple state of the early universe other than the pedominance of matter over anti-matter. Recognition of this possibility could save lots of hoop-jumping of the kind you refer to:

    I liked this also. Did you know that the cat referred to is not domestic, but the fearsome Cat O'Nine Tails? Lending bite to my definition of "space".
     
  18. Apr 22, 2007 #17
    Thanks for your clear summary of the situation, which I agree with, except for a few niggles. It's true that "exotic non-baryonic dark matter" wouldn't "interact(s) with electromagnetic radiation", but what evidence (apart from elemental abundances, that is) is there that the "dark matter component" is indeed exotic?

    I suspect that this evidence is the very impressive bullet-cluster observations, which are certainly evidence of huge quantities of dark matter that is not charged plasma. But does such dark matter have to be exotic stuff? This seems to me as fanciful as claiming it to be discarded dirty cell phones of an exotic civilisation, which I think would also fit the bill. Or would it?
     
  19. Apr 22, 2007 #18

    Garth

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    The only evidence that DM of 23% critical density is exotic non-baryonic 'stuff' is the limitation put on baryonic density by the standard BBN of 4% critical density.

    If that density limit could be lifted to 20 -30% then the need to invent a new breed of 'stuff', or 'entity', to fit the data, or 'save the appearances', falls away. None interacting DM is also useful to begin the formation of large scale structure before the epoch of last scattering of the CMB.

    Of course from the particle physics point of view candidates for such exotic particles abound.

    The problem is none have yet been found.

    Garth
     
    Last edited: Apr 22, 2007
  20. Apr 22, 2007 #19

    Wallace

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    I think that even ignoring BBN there are no plausible theories as to how there could be so much baryonic matter it a state that is so invisible. For instance I remember reading that while brown dwarves are so faint are hard to detect, if DM was entirely explained by brown dwarves the diffuse glow from them all would be very easily seen. There is just so much DM needed that if it really is just baryons, then the baryons making up the DM must be is such an exotic state that the discovery of such would be every bit as exotic as non-baryonic DM!
     
  21. Apr 22, 2007 #20

    Garth

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    Good point, but have you tried a combination of IMBHs and WHIM?

    These might be the products of a PopIII epoch

    Intergalactic cold H2 is pretty difficult to observe as well.

    Garth
     
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