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GKZ cutoff confirmed by Auger

  1. Jul 23, 2007 #1


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    GZK cutoff confirmed by Auger


    If cosmic rays are mostly highly-accelerated protons, then
    above a certain energy they should react with the pervasive photons of the CMB
    and get wiped out.

    this is known as the GZK (Greisen, Zatsepin and Kuzmin) cutoff

    In other words, space (being full of CMB photons) should not be transparent to any proton that is above about 1018 eV.
    And we should see comparatively little cosmic rays above that energy.

    there was some doubt about this. But Auger confirmed it.

    the blog Backreaction, which should be a household word by now, has discussion and more links
    Last edited: Jul 23, 2007
  2. jcsd
  3. Jul 23, 2007 #2
    wasn't the absence of the expected GZK cut off taken as a possible indication
    for an energy dependent speed of light (or in other words a violation of Lorentz invariance), which would in turn give a boost
    to DSR models (double special relativity). This is at least, what I remember from Smolin's book "The trouble with physics".
    I think, Smolin was also seeing possible connections to loop quantum gravity,
    which would at least in 2+1d spacetime predict such effects, whereas in 3+1d
    the situation still was unclear. (Don't beat me, if I get something wrong.)
    If so, it would very interesting, how the "leading heads" of LQG react.
    (Smolin, Rovelli, etc.) after they haved analyzed the data.

    More generally I think, if lorentz invariance prooves right through all
    accessable energy scales, another place to look for effect of quantum gravity seems
    gone. And so we have to rely even more on the LHC to see something beyond the standard model.
    Last edited: Jul 23, 2007
  4. Jul 23, 2007 #3


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    Hi Micha,
    my recollection is that Smolin was looking to see the GZK cutoff confirmed right where it was found.

    But I could be wrong. If you found something in TWP book, please tell me the page. I have the book and can look up to see what you are talking about.

    I have not heard much for a year or so about energy-dependent speed of light. People in the LQG community have been saying that the models they are working on do NOT predict dispersion.

    (Smolin would be an exception, and even he does not make a definite prediction based on a definite model)

    So if GLAST does not see energydependent speed the LQG people will presumably be happy.

    This is certainly a change from the way things sounded in 2003-2005!

    When you ask about PREDICTION related to LQ COSMOLOGY, you get a different picture. then there are the recent papers by Magueijo-Singh (2007) and Bojowald (2007, Dark Side...)

    What I expect to see is predictions coming out of Loop Cosmology---to check against Planck observations of CMB, observations of largescale structure, and supernova data. Bojowalds LQG mechanism to explain Dark Energy would result in a substantially different history of expansion acceleration than what you get with LCDM, as I understand it. So it should be testable by further supernova observations at higher z.

    The "leading heads" who are closest to observational data must be Bojowald and Ashtekar now. If you are interested you should pay close attention to what they say and what they are writing about. Especially Bojowald now, I think.

    Roy Maartens is also a key player. He is an observational cosmologist who is interested in extracting testable predictions from Loop Cosmology.

    I am not aware that Rovelli or Smolin are currently involved in research that is near testability. I don't think they are much concerned with that side now.
    You are right, it seems the 2005 results in 2+1d could not be extended as Freidel hoped. So the big effort related to DSR got dropped (AFAICS) in 2006.
    Things move very fast. In the whole of the Loops '07 conference, there was no talk by a proponent of DSR. Only one person (a DSR skeptic) mentioned it.

    So at least in very crude terms one can say that the focus is now on cosmology.

    Let me know if you want links to recent papers.
    Last edited: Jul 23, 2007
  5. Jul 24, 2007 #4
    I have lent away my copy of Smolin's TWP, so unfortunately I can not come up with a page number.

    But I think there is only one passage in the book in the alternatives to string theory section, where Smolin refers to the Auger experiment, introduces DSR
    approaches and then goes on to say, that he wishes, that by the time, the experimental data come in, they would already have a prediction in place for those data derived from LQG. He then discusses the prospects for this to come true. And this is the place, I am referring to. This section then ends, that no matter if this prediction then turns out to be right or wrong: "In any case, we would be doing real physics."

    I also remember Smolin saying in a discussion about the book, that the only thing string theory would have to say to this, is, that it claims the validity of Lorentz invariance up to the Planck scale.

    Lubos Motl discussing experimental testability of string theory once turned this argument around stating, that if Lorentz invariance was proven right in higher energy ranges, this would be experimental confirmation for string theory. Of course, as somebody else pointed out in a blog discussion, by finding Lorentz invariance you can only confirm special relativity and not string theory. I am unable to come up with the exact source for this point, but this link and the links given by Lubos there give you a good flavour of the discussion:

    What I am curious about after reading your reply is, why you need to go on from LQG to LQC to derive a dispersion relation.

    Papers: If you have two or three links easily at hand, I could give them a try...
  6. Jul 24, 2007 #5


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    Micha, you asked about Loop COSMOLOGY. That is probably more apt to be generating PREDICTIONS, but they will NOT have to do with gammaray dispersion.
    You expressed some interest in the recent LQC papers I was talking about, so I will get the links
    this is very new work which should lead to a test of LQC using supernova data. (no dispersion involved here) just an idea for how LQC explains accelerated expansion, which is either false, or (if true) predicts a different history of expansion from what the standard LCDM model predicts. My impression is that it is highly vulnerable to falsification by probing higher redshift supernovae---to distinguish a distinctively different earlier expansion history. My distinction between Loop Cosmology and the full LQG is somewhat artificial here but I'm not able to draw the line usefully in this case.

    this illustrates a new research thrust in which LQC is challenging INFLATON-driven scenarios---and offering alternative resolution to puzzles such as "horizon problem" "flatness and structure problems". So far inflation-scenarios have not been much challenged because they seemed the only way to resolve e.g. the horizon problem.
    But LQC models have a certain amount of inflation that does not require exotic matter, like an inflaton, but arises naturally from quantum corrections to the dynamics with any kind of matter at very high density. this natural inflation, together with a bounce, now seems to some people* to offer an alternative expanation for how different parts of the sky might be in thermal equilibrium.

    The paper to which I gave the above link goes beyond this "horizon problem" discussion and addresses large scale STRUCTURE FORMATION. It asks if there is some that the scaleindependent fluctuation spectrum of the CMB might have arisen WITHOUT exotic-matter-driven inflation?

    *for example Chiou and Vandersloot, we have a PF thread about their recent paper.

    Micha, you also asked about DISPERSION, modified Lorentz invariance, and all that. My feeling is that even tho it got into Smolin's book, which was probably in final draft over a year ago and could in that respect be a bit out of date, dispersion is not going anywhere. I could easily be wrong of course.

    I remember Smolin saying much about the test of modified Lorentz invariance (dispersion, DSR etc) represented by GLAST. I've never understood the relevance of Auger and tend to think that the Auger part was not important.

    By contrast, GLAST offers a clear decisive test of energy-dependent speed of light----it detects gammaray bursts and is sensitive enough to notice if the higher-energy photons come in a split-second earlier.

    Smolin has consistently said that on general grounds he think any QG model should predict this kind of dispersion of gammaray photons. However he has not made a concrete prediction regarding some specific model (say some definite spinfoam.) And other people have stopped trying to derive the desired prediction from specific models.

    GLAST will probably provide some data in 2008. It now looks as if we have missed getting a firm dispersion prediction, so that if gammaray dispersion not seen it will unfortunately not falsify any particular model. Indeed if dispersion is seen, I would judge that many people in the QG community will be thrown into confusion because they are not expecting it.
    Last edited: Jul 24, 2007
  7. Jul 24, 2007 #6
    thanks for all the information.

    Probably you are right, that Smolin was laying his hopes mainly on GLAST in the book, not on Auger, because GLAST can test dispersion more directly. It is a little disappointing to hear though, that no specific predictions from LQG models yet exist.

    Regarding Cosmology: I secretly think by myself, that cosmology in itself is such a complicated and speculative topic, that I prefer to see any theory standing on its own grounds before being applied to cosmology.
    But probably, only having the whole universe as a laboratory available at the energy scales we are interested in, there is not much choice.

    For example, in Road to Reality Penrose argues in my opinion quite convincingly, that inflation, instead of having solved the horizon problem, has only made it worse. The main argument is, that what really is to explain, is not the horizon problem, but the low entropy state of the universe at the big bang. And if you buy the universe some time to come to thermal equilibrium, then this will only increase the entropy, so it makes the original problem of the low entropy state even bigger.
  8. Jul 24, 2007 #7


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    Yes I would like it so much better if I knew of some definite case! But also keep in mind that I am just an observer from the sidelines and I don't speak for the researchers. All I can tell you is what I see by watching attentively and I can easily be wrong.
    (That said, I will continue to tell you what I believe as if it were expert opinion:wink:)
    Yes! This is right! You could say that Loop Cosmology---that does wonderful things like exhibiting a "bounce", and a brief natural inflationary expansion, and several interesting things---is really not yet a theory but rather a scenario in the sense that Inflation Scenarios based on exotic matter are that.

    In an inflation scenario with exotic matter, it violates energy conditions hugely, and you cannot see the exotic scalar field in the laboratory. So there are drawbacks, but if you can't think of any other way to explain things then :confused: what can you do?

    The rules of the game seem to be that any theory (or call it "scenario") that describes the early universe must be able to explain the blotchy pattern of the CMB with its peculiar "scale-invariant" power spectrum----there is as much little blotch business as there is big blotch business---everybody gets a fair share of the action. And also the theory (or sc****io) must explain the remarkable fact of the temperature being nearly the same in all directions.

    But each person must decide how he feels about the different theories. So far nothing comes up in a laboratory and FORCES you to accept one or another explanation. As a matter of taste, I tend to prefer Loop Cosmology explanations because of minimality. There is not much there besides what you have to have to quantize the basic cosmology equations---actually not much more than John Wheeler and Bryce DeWitt set forth in 1970: just changes in minor detail that make it work instead of blow up. So that minimal style appeals to my taste and if it can explain things, I like it.

    Now about Penrose argument, I differ with you about that. I share many of your attitudes and viewpoint to an extent, but I am not as convinced by Penrose.

    I think that even to define entropy, you need an observer who has a map of phasespace showing the macrostates that arise from what the observer is able to measure. I think that entropy is not absolute (there is no One Official Observer who defines the Map and who sees everything). I think entropy is relative and depends on the observer and his point of view.

    So in the case of a Loop cosmology bounce, the observer Before will see his universe collapsing and a huge increase in entropy as everything goes down the toilet. And the observer After will see a beautiful low entropy Dawn of Creation. This is not a contradiction and NOBODY ever gets to see the second law being broken. But Penrose seems to think it is some kind of contradiction.

    However this is just a minor difference----I see your general viewpoint about many of these other things.
    Last edited: Jul 24, 2007
  9. Jul 25, 2007 #8
    regarding entropy and Penrose:
    You are right, I was implicitly thinking of entropy as an objective concept. And you are also right, identifying different macro state in phase space involves some subjectivity. So perhaps the idea you described, will work. At least it sounds original.

    The reason, I was bringing up Penrose, was, that one of the main motivations for inflation, the horizon problem, might stand on shaky grounds.
    As far as I understand, the story goes, that the largely isotropic CMB shows an early universy in thermal equilibrium. But how can this be true, if in standard cosmology without inflation big parts of the visible universe have never been in causal contact. According to Penrose, this is not the right question to ask, because the universe was simply showing this large uniformity or in other words, a very low entropy state from the big bang on. So it is this low entropy state at the big bang, which you have to explain. (Low entropy and uniformity may sound like a contradiction, but as Penrose explains with much care, for the force of gravity uniformity means low entropy unlike for all other forces, where uniformity means high entropy.)
    Last edited: Jul 25, 2007
  10. Jul 27, 2007 #9


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    I believe Hossi is our resident expert on DSR. Be interesting to hear her read on this [and elaborate on double v deformed special relativity].
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