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LQG predicts inflation and solves horizon problem on its own

  1. Jul 19, 2004 #1


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    Papers by a number of authors have shown that Loop Quantum Cosmology removes the big bang singularity.

    Loop Quantum Cosmology development leads that of the full LQG theory because in cosmology one is essentially quantizing the prevailing (Friedman) cosmological model, which is a deal simpler than the full theory of General Relativity.

    Papers by several authors have also shown that LQC implies inflation.
    It is something that happens of its own accord around the time of the ex-singularity (which has become a bounce). In LQC one does not have to "rig up" an Inflation Scenario with special contrivances (or at least not quite so much)---it happens comparatively spontaneously: comes with the theory.

    Two fairly new authors, Date and Hossain, have posted a new paper on this.

    "Genericity of inflation in isotropic loop quantum cosmology"

    they have also just posted a paper on the LQC hamiltonian, but this is an aside.

    One of the points in the new paper is that LQC does not need inflation in order to solve the horizon problem. The bounce takes care of that. When inflation was invented by Alan Guth and others one of the key motivations was the puzzling thermal uniformity of the Microwave Background. It looked like opposite sides of the observable universe had been in contact and had achieved thermal equilibrium. Otherwise how
    would the two sides know to be the same temperature?

    this is well explained in Lineweaver's cosmology tutorial
    "Inflation and the Cosmic Microwave Background"

    In effect, condensing details discussed by Lineweaver, the Friedman model did not have enough expansion prior to the moment the CMB originated to give an opportunity for the different parts of the observable universe to get in touch and arrive at equilibrium. Most of the expansion predicted by the vanilla (non-inflation) model was after the CMB was already on its way. So Guth imagined a scenario or mechanism to get expansion briefly into overdrive before anything else happened. In the tweaked model most of the expansion came before the origin (or time of last scatter) of the CMB.

    So inflation may initially have seemed a rather tortuous kludge to explain the nice even temperature of the CMB. Of course people came up with other reasons to like it---it helped explain spatial flatness and other mysteries.

    Now it turns out that LQC has no horizon problem and Date and Hossain talk about this in their new paper.

    Judging by Date and Hossain, it's fine to have no horizon problem. It means one less thing that inflation has to fix and fewer headaches----less constraint the inflation scenario. As they point out, LQC gets inflation for free anyway, whether or not it's needed to solve that particular problem.

    This is a 4-page paper. I should probably at least copy the abstract to give an idea of the technical context and results

    they have an bibliography which gives arxiv links to other papers about such things as bigbang singularity removal and inflation arising spontaneously in LQC.


    "Non-perturbative corrections from loop quantum cosmology (LQC) to the scalar matter sector is already known to imply inflation. We prove that the LQC modified scalar field generates exponential inflation in the small scale factor regime, for all positive definite potentials, independent of initial conditions and independent of ambiguity parameters. For positive semi-definite potentials it is always possible to choose, without fine tuning, a value of one of the ambiguity parameters such that exponential inflation results, provided zeros of the potential are approached at most as a power law in the scale factor. In conjunction with generic occurrence of bounce at small volumes, particle horizon is absent thus eliminating the horizon problem of the standard Big Bang model."
    Last edited: Jul 19, 2004
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