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Cosmologists, astrophysicists, astronomers: loop quantum cosmology or string cosmolog

  1. Apr 2, 2007 #1
    There's no debate which post-SM approach the vast majority of HEP/particle physicists prefer: string theory.

    While I don't know what the vast majority of post-SM, post-GR approach cosmologists, astrophysicists, astronomers prefer, loop quantum cosmology or string cosmology, in solving a variety of outstanding issues in cosmology and astrophysics and astronomy, at least one credentialed phD in astrophysics, Chrstine Dantas, used to devote her blog to BI, which is more sympathetic to LQC than string cosmology. (I am not speaking for her, just inferring from her website http://christinedantas.blogspot.com/)

    Both approaches attempt to wrestle with the Big bang, the big bounce in LQC and ekpyrotic scenario and brane cosmologies promoted by string theorists like Lisa Randall.

    Obviously if you're a string theorists, you will say that string theory is the most promosing approach to the outstanding issues of cosmology: the flatness problem, dark matter, dark energy, inflation, singularities, etc,

    but if you are a credentialed research-oriented cosmologists, astrophysicist, or astronomer, and you've looked at both models (i.e LCQ by the Penn group of Ashketar and Singh) and string brane cosmology, which do you think is more promising?
  2. jcsd
  3. Apr 2, 2007 #2


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    We are a long, long way from knowing the answer to that question. In general what you see in the literature are 'inspired' models that have some similarity to a gross feature of one of the microphysics theories. But beyond that, a lot of parameters are still arbitrary and the form of the model only holds some passing aesthetic quality inhereted from the intellectual father. It wouldn't surprise many of us if one turned out to be true and was inspired by say one theory, but turned out to be derivable and more fundamental from another.

    Far more often, the models have no obvious relation to quantum gravity theories and are semiclassical.

    The space of models for say inflation and big bang cosmology is enormous, far greater even than particle physics phenomonology, and the constraints we know off limit them only passingly. Its still a far cry from beyond standard model constraints, both theoretically and experimentally.
  4. Apr 3, 2007 #3


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    Judging by recent discussions in the community at large, string is a hooker preaching morality to a nun. Neither side is willing to yield.
  5. Apr 3, 2007 #4

    I would say that astrophysicists with a stronger background in general relativity than in high energy theoretical physics would get a more immediate interest in non-perturbative approaches like LQG than in string theory and vice-versa. But I would not make that a rule, since other "stochastic" factors may came into play.

    For instance, my background is much stronger in GR than in particle physics. But I was first interested in string theory/braneworlds and attended the M-Theory Cosmology conference in Cambridge, UK, in 2001, willing to understand how these ideas would have a contact with cosmology (at least with "the cosmology" I knew), as the conference title suggested. Unfortunately I couldn't relate my previous knowledge in cosmology with what I saw there. The mathematics was above my head, and at that time I did not know about LQG (and most certainly I would have attended a LQG conference at that time if I learned about it).

    I liked the enthusiasm of the string theory people there, I was somewhat impressed with their technical abilities, and so on, but I was at the same time a little confused and disapointed. The theorists seemed too theoretical, too mathematical. When I eventually found out about LQG, it picked my interest because of the background independent issue -- something I could directly relate with my interest in Mach's Principle and relational theories (I even wrote a short paper on Mach's principle and braneworlds when I was already aware of LQG, but I had the intention to move on to eventually write something about Mach's Principle and non-pertubative QG in a second paper. That now will have to wait, though). The mathematical sophistication of LQG is high as well, I'm still struggling with it, but at least it seems more comprehensible to me and highly attractive, as I said, because of the background independence issue. This is an elegant issue, but only Nature can be the final judge...

    In summary, what theory/approach an astrophysicist will find more interesting is a highly subjective issue, apart perhaps for some natural bias coming from their background knowledge, as I highlighted in the beginning of this post, or some other external influence. In any case, astrophysicists are extremely observational/experimentally driven, so they naturally attempt to look for quantum gravity models that they can relate to observations. A theory which cannot be tested will have a hard time to get the attention of an astrophysicist, and will be seen as some kind of speculation, whatever its mathematical sophistication.

    Back in 2001, I gave a seminar about M-Theory to the Astrophysics dept. where I was postdoc here in Brazil, presenting only the major/basic ideas. The audience knew next to nothing about string theory, and they ended up not really convinced at all about an 11D universe. Well, since I was not convinced myself (although I thought at the time there seemed to be interesting issues like braneworlds), perhaps I was not the best promoter of string theory (well, I *did* my best :grumpy: ). It ended up that they felt the ideas were too speculative, almost to the point of some sci-fi tale. It does look like that, if you do not go deep into the maths. (But if I were some Brian Greene, perhaps I could have made an impression to them. :tongue2: ).

    Well, since then, much has changed, and although I no longer work at that institute, I visit it sometimes and get lunch with people there. Recently I have talked to some of them who told me they read Lisa Randall's book and were starting to get interest on these issues. So perhaps it's time for me to give a talk there on LQG, right? :wink:

    Now seriously, it is not that I think string theory is useless knowledge and wrong, etc. I do have a lot of admiration for the highly sophisticated maths involved. It might end up to be interesting for a quantum gravity theory, which is yet to come. And LQG might end up to be completely wrong (although the field is experiencing a boom these past few years!). Nobody knows. So I agree 100% with Lee Smolin that, given the present, open scenario to what quantum gravity really is, diversity in research should be incentivized. :cool:

    Now one thing that is really, really very impressive to me is the endless battlefield on String X Loops... Science is not to be played on a battlefield, but on a vast stage where the human intellect is free to reason and the only confrontation is with Nature herself.

    Best wishes,
  6. Apr 3, 2007 #5
    I was waiting to hear from you and I'm glad you did reply :)
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