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QC test idea-Physical Review D

  1. Jun 11, 2007 #1


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    QC test idea--Physical Review D

    This paper is to appear in Phys. Rev. D. I don't know when, just saw the announcement yesterday.

    Thermal fluctuations in loop cosmology
    Joao Magueijo1,2,3, Parampreet Singh4,1

    1 Perimeter Institute for Theoretical Physics, 31 Caroline St N, Waterloo N2L 2Y5, Canada
    2 Canadian Institute for Theoretical Astrophysics, 60 St George St, Toronto M5S 3H8, Canada
    3 Theoretical Physics Group, Imperial College, Prince Consort Road, London SW7 2BZ, England
    4 Institute for Gravitational Physics and Geometry, The Pennsylvania State University, University Park, PA 16802, USA

    "Quantum gravitational effects in loop quantum cosmology lead to a resolution of the initial singularity and have the potential to solve the horizon problem and generate a quasi scale-invariant spectrum of density fluctuations. We consider loop modifications to the behavior of the inverse scale factor below a critical scale in closed models and assume a purely thermal origin for the fluctuations.

    ... to more fully work out this complex aspect of loop cosmology, since the full picture would not only fix the free parameters of the theory, but also provide a model for a non-inflationary, thermal origin for the structures of the Universe."

    they are talking about an alternative solution to the horizon probem and structure problems that does not require imagining inflation scenarios and invoking an 'inflaton' scalar field. Another way to refer to an initial inflation scenario phase is as a "deSitter phase". Here is an exerpt from the introduction:
    "The possibility that primordial thermal fluctuations might seed the structure of our Universe is an intriguing alternative to quantum fluctuations in a deSitter phase...

    Unfortunately a number of obstacles present themselves to such an enterprise. Firstly any thermal scenario should necessarily be based on a [non-inflationary] solution of the horizon problem. This is so that the assumption of thermalization itself makes sense: ... A number of solutions to the horizon problem have been proposed... and in this paper we use in effect a combination of two..."
    Last edited: Jun 11, 2007
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  3. Jun 11, 2007 #2


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    One way to deal with the inflation idea is to test alternative ways of resolving the puzzles that inflationry was invented to address.

    what has tended to convince people about inflation scenarios is that they can't think of any other way cross-sky thermal equilibrium could have been achieved (horizon problem) or how the observed spectrum of density fluctuations could have been imprinted in the early universe providing the seeds of clumping and clustering (structure formation)

    unfortunately that is also how primitive creation myths get established. How was the world made? An "inflaton" did it. One disposes of several puzzles by inventing an imaginary something, a unicorn or whatever.

    Well maybe there was a unicorn, since that provides a neat solution to horizon problem etc. But until we actually see a unicorn we should make a determined effort to come up with alternative solutions to the puzzle.

    And that is what Joao and Parampreet are doing.

    What I find significant is that this paper is going to be published in Physical Review D----a top peer review journal. This is a far out paper. In today's world you do not lightly challenge the necessity of assuming a magical scalar field episode.
  4. Jun 12, 2007 #3


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    I agree, I hate it when science invents things to solve problems. Like that Newton guy. We can see that massive bodies attract each other so he made up this thing called 'gravity' to explain why this happened. Crazy huh!

    Until we actually see some gravity we should keep an open mind about other possible explanations. Maybe http://www.theonion.com/content/node/39512" could do the trick?

    Or that Maxwell chap, he couldn't explain electricity without inventing 'electric and magnetic fields' pfft! All those fancy diagrams with field lines on them, but has anyone every seen an electric field? I think not!

    Those darns unicorns just keep appearing everywhere in science :uhh:
    Last edited by a moderator: Apr 22, 2017
  5. Jun 12, 2007 #4


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    A convincing explanation about the origin of inflation is missing in the current cosmological model. Assume that (i) there existed a scalar field and (ii) its self-interacting potential was flat enough in order to guarantee sufficient amount of inflation. Then, there is no clear reason for the scalar field to have started at a high potential energy, leading actually to an inflationary phase. Quantum cosmology models try to explain this. For example, this was the goal of models such as Vilenkin's tunneling proposal or Hartle and Hawking's no-boundary proposal. Assuming a scalar field with an appropriate self-interacting potential they "predicted" a high probability of the scalar field having a high potential energy in the very early universe. In a similar way, recent models such as loop quantum cosmology or string cosmology have to explain the origins of the inflationary phase, but as a bonus it may turn out that the scalar field with self-interacting potential is not needed, making unnecessary both assumptions stated above.
    Last edited: Jun 12, 2007
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