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Ways to accommodate modern cosmology

  1. Dec 10, 2008 #1

    marcus

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    People find their own ways of making sense of modern cosmology. Unless they are utterly unable to get comfortable and reject it whole hog (as some do quite vehemently :biggrin:). If you have a way of making your peace with cosmology, feel free to describe how you do it. Or try this recipe and see if it works for you.

    PF Cosmo functions to some extent as a sort of lab where we encounter the problems people have had in adjusting to the emerging picture of the universe, so this recipe derives a lot from our common experience.

    You won't believe how simple. Here's the recipe:

    1. There's an idea of rest.

    2. It's about geometry.

    3. A revolution is occurring.

    ===========================
    I owe you some explanation.

    1. 1905 Special and 1915 General don't have a preferred idea of rest. But Cosmology does. An observer can be at rest with respect to background. In other words, with respect to the matter of the early universe. If you move too fast wrt the light coming from the matter of the early universe your nose will be roasted by the doppler hotspot ahead of you and your tail will be chilled by the doppler coldspot behind you. Or words to that effect. :biggrin: It's not an especially precise idea of rest because the background has fraction of a percent temperature fluctuation bumps, but it is a practical one.

    2. When in doubt, throw space out. It is said that the Greek philosopher Aristotle did not believe that space had objective physical existence, but simply consisted of all the spatial relationships among things. If someone had asked him he might have said that spacetime doesn't exist either, but is merely the sum total of relationships among events. In other words don't worry about space or spacetime, think about geometry.

    1915 GR is a proposed law to govern the evolution of geometry. We have no right to assume the angles of a triangle will sum to 180, or that distances between stationary objects will always remain the same. Geometry is dynamic and we affect it whenever we move massive objects around. Only very slightly, one must admit.

    The practical notion of rest (point 1.) gives us an approximate but workable idea of simultaneity. Cosmology has a succession of present moments. So there is a collective pattern to how geometry has evolved thru time. Hubble law. A rough pattern of increasing distances.

    The Friedmann metric, with a scale factor that evolves in time according to the two Friedmann equations.

    3. Comes the revolution. It makes a difference that modern cosmology is in the midst of a revolution. I actually think you can't understand it without appreciating how hot the field is, and how fast it's changing. One thing this means is that alternative ideas are getting considered and discarded at a rapid pace. Inevitably there are disconsolate theorists whose pet proposals have fallen by the wayside. During any given 3 or 4 year period one will be able to discern a main direction in which research is going, or two or three major contending directions.

    Yesterday I saw a preprint of a 4-page paper which I am confident will be important. It was about a completely new approach to early universe cosmology (this one based on Alain Connes noncommutative geometry version of the standard particle model.) The preprint appeared to have an unusual number of typos, as if the authors, whom we can call William and Mairi, were in reckless haste to post. The arxiv number is
    http://arxiv.org/abs/0812.1657
    (which could be remembered as December 1657)

    One way to give a sense of the rapid change in cosmology is to quote from their introductory paragraphs. I'll do that in a separate post. Meanwhile, if you don't like my recipe, or want clarification, or prefer a different way of making sense of modern cosmology please feel free.
     
    Last edited: Dec 10, 2008
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  3. Dec 10, 2008 #2

    marcus

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    Here's what the authors, William Nelson and Mairi Sakellariadou had to say in the introduction of their paper:

    ==quote 0812.1657==
    Theoretical early universe cosmology is gaining a constantly increasing interest from the scientific community. The predictions of the theoretical models can now be compared with a plethora of astrophysical data, in particular the measurements of the Cosmic Microwave Background temperature anisotropies; all having a surprising good accuracy.

    Moreover, present high energy experiments, in particular the Large Hadron Collider, will test some of the theoretical pillars of the cosmological models.

    Despite this golden era of cosmology, a number of questions, like

    the explanation of space-time dimensionality [1],

    the origin of dark energy [2]

    and dark matter [3],

    the search for the natural and well-motivated successful inflationary model,


    are still awaiting a definite answer.

    The main theoretical approaches upon which the cosmological models have been built are either string theory or quantum gravity. Here we will consider another one, which up to now has, surprisingly enough, gained only a limited interest, namely NonCommutative Geometry (NCG) [4]. More precisely, we will study cosmological consequences of the NCG approach to the Standard Model (SM) [5], an approach which leads to all the detailed structure of the SM, while it also implies a number of physical predictions at the unification scale...
    ==endquote==

    For the purposes of this thread, the sketchy overview they give of the situation in their field is more important than the results of this particular paper. But in case anyone is curious about the paper itself, here is the abstract:
    ==quote 0812.1657==
    We study cosmological consequences of the noncommutative approach to the standard model. Neglecting the nonminimal coupling of the Higgs field to the curvature, noncommutative corrections to Einstein’s equations are present only for inhomogeneous and anisotropic space-times. Considering the nonminimal coupling however, we obtain corrections even for background cosmologies. A link with dilatonic gravity as well as chameleon cosmology are briefly discussed, and potential experimental consequences are mentioned.
    ==endquote==
     
    Last edited: Dec 10, 2008
  4. Dec 10, 2008 #3

    atyy

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  5. Dec 10, 2008 #4

    wolram

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    I am sure modern cosmology will be fine when they find an engine to pull the train, and a train for the engine to pull.
     
  6. Dec 10, 2008 #5

    marcus

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    Yes that's one of the great things about it! :biggrin:
    What other explanations of the standard model of particle theory do you know that have an actual falsification to their credit?
     
  7. Dec 10, 2008 #6

    atyy

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    Yes, that is something to its credit! To be honest, I haven't the foggiest idea what non-commutative geometry is, it means as much to me as Santa Claus predicting the Higgs mass. So I have some questions:

    1. Has the non-commutative model been verified to be internally consistent?
    2. Was its prediction of the Higgs mass from its assumptions correct, ie. did they make any arithmetic errors?
    3. Can the model be easily extended to include another Higgs mass, or was it a very tight parameter free prediction?
    3. With what certainty has the Higgs mass been excluded to be 170 GeV? Actually, this is a question I have in general about the LHC. Suppose the Higgs is not found in its energy range, this would be very exciting for physics. But how do we know they really didn't find it? Do they exclude masses with 2,3 or 5 standard deviations from what they would observe if it were at a particular energy?
     
  8. Dec 10, 2008 #7

    marcus

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    We could start a thread in Beyond forum asking questions like these. Arivero, who sometimes posts in Beyond, has studied NCG and was one of the participants in the thread you linked to. Besides Arivero's, there was the post of Fedele Lizzi.
    http://noncommutativegeometry.blogs...howComment=1219054200000#c3470177482379448548
    I think Lizzi's post gave some clues as to what to expect.
    Connes has recovered momentum and mounted a new initiative, with an improved NCG model. He has this paper in preparation:
    [18] A. Chamseddine and A. Connes, Uncovering the noncommutative geometry of spacetime:
    a user manual for physicists,
    to appear.
    This is reference [18] in the paper he just came out with a week or so ago:
    http://arxiv.org/abs/0812.0165
    You can see on pages 7-9 of this paper what he now says about the Higgs mass.
     
    Last edited: Dec 10, 2008
  9. Dec 11, 2008 #8

    atyy

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    Intriguing.
     
  10. Dec 11, 2008 #9

    mysearch

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    This is one of the most succinct descriptions of the present status that I have come across. Do you mind if I use this as a “quote” at some future date?
     
  11. Dec 11, 2008 #10

    marcus

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    I agree. Connes new paper posted on 1 December.
    And it said why they think the initial prediction of Higgs mass was wrong, and what this means. And they went forward with the development of the NCG standard model, bringing in gravity effects. After some three months quiet, Connes is coming back looking like a strong contender.

    So what happens? Ten days later, Nelson and Sakellariadou bring out the first NCG cosmology paper. Where they didn't bother to spell check or even make sure the notation was consistent. That's fine, you can see what they are saying well enough and stuff like that can be fixed later. It just gives the paper a slightly wild unkempt appearance. They were in a hurry to get it out there. I like it.

    A nice thing about William Nelson is his scope. He has written some string/brane cosmology papers. And more recently quite a bunch of loop cosmology papers. He is always looking for observable consequences---possible ways of testing. And now he has turned his attention to NCG cosmology.

    The early universe will be a prime test-bed for new fundamental physics. You can see the various theory gambits (string, loop, and now NCG) as rivals---each with a chance to explain details of the observations which will be coming in.
     
    Last edited: Dec 11, 2008
  12. Dec 11, 2008 #11

    marcus

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    heh heh, that's one way to look at it---a very funny quip, Wolram. I see it differently. As far as I can see, cosmology is fine, and in fact it is the engine pulling a good deal of the rest of physics behind it.

    The field is on a roll---it predicted dark matter, and now dark matter has been seen and mapped as irregular contours around galaxy clusters. Attempts to identify it motivates a lot of physics research. Ideas of what its decay/annihilation products might be motivates experimentalists' efforts to detect that kind of evidence, and so on. It's hard to imagine how this could have gone better than it has.

    The vitality of a field of science can be gauged by the questions it raises based on the volume of new data it's taking in. Cosmology has taken a leading roll in physics in part because of the quantity and quality of data---the large amounts of new kinds of data being absorbed. And in part because of its coming up with immensely stimulating questions--questions that continue to draw in theoretical and experimental physicists from other areas.

    We can take a look at that list of Nelson's that I blued earlier:

    the explanation of space-time dimensionality,

    the origin of dark energy

    and dark matter,

    the search for the natural and well-motivated successful inflationary model,

    are still awaiting a definite answer.

    Intensely interesting questions. That, in my view, is how it ought to look in a research field that is on a roll. Have to go out for part of the afternoon. Back later.
     
  13. Dec 11, 2008 #12

    atyy

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    So while waiting for Chamseddine and Connes dummies guide, (which I doubt will be dummy enough for me), I came across this review which has accessible remarks on non-commutative geometry at the end:

    Early History of Gauge Theories and Kaluza-Klein Theories, with a Glance at Recent Developments
    Lochlain O'Raifeartaigh, Norbert Straumann
    http://arxiv.org/abs/hep-ph/9810524
     
  14. Dec 11, 2008 #13

    atyy

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    Noncommutative Geometry Year 2000
    Alain Connes
    http://arxiv.org/abs/math/0011193

    "Noncommutative measure spaces evolve with time!" ... But everything after that might as well be Sumerian to me. Incidentally, back to the Rovelli and Smolin time you brought up in another thread, I still can't understand Smolin's argument, but there are several suggestions that time really exists. In GR, there is causality from the light cone structure, which CDT takes seriously. Then most (all?) real world GR solutions seem to be splittable into space and time, just not uniquely. It seems the condensed matter people can even get special relativity out from universes with an absolute time. So I need to figure out what the Connes's Sumerian means ...
     
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