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I Q re Inflation and Gaussian CBR pertubations

  1. Jan 19, 2017 #1
    I have been exploring the paper:
    Statistical Tests for the Gaussian Nature of Primordial Fluctuations Through CBR Experiments
    http://lss.fnal.gov/archive/1993/pub/Pub-93-294-A.pdf .
    Here is a quote.
    Cosmic inflation [1], on one hand, provides a natural way to generate Gaussian initial perturbations [2]; spontaneous symmetry breaking, on the other hand, will lead to the formation of topological defects [4] via Kibble mechanism [3], and the perturbations generated by topological defects can be characterized as non-Gaussian.​
    The four references are:
    [1] A. Guth, Whys. Rev. D 23, 347 (1981); A. Linde, Phys. Left. B 108, 389 (1982); A. Albrecht and P. J. Steinhardt, Phys. Rev. Left. 48, 1220 (1982).
    [2] J. M. Bardeen, P. J. Steinhardt, and M. S. Turner, Phys. Rev. D 28, 679 (1983); A. Guth and S-Y Pi, Phys. Rev. Lett. 49, 1110 (1982); S. Hawking, Phys. Lett. B 115, 295 (1982); A. A. Starobinskii, Phys. Lett. E 117, 175 (1982).
    [3] T.W.B. Kibble, J. Phys. A9, 1387 (1976).
    [4] For a review, see A. Vilenkin, Phys. Rep. 121, 263 (1955); N. Turok, Phys. Rev. Zett. 66 262.​

    I confess I have reservations about inflation being a theoretically necessary era of the early universe expansion. The arguments I have seen supporting inflation seem to all involve it's ability to provide explanations of phenomena that seem to me to not necessarily require an explanation. One example: it explains why scientists so far are unable to discover evidence for any of the hypothetical magnetic monopole particles.

    I am wondering if the article cited above is just another of this kind of explanation, or if it is instead a theoretically well accepted phenomenon that without inflation only non-Gaussian CBR perturbations would be observed. I understand that I might possibly find the answer to this in the cited references, but I have several problems.
    1. I do not know how to interpret the citations to find the references.
    2. I think it likely that i may not have any convenient access to the references.
    3. I think it likely that i won't be able to understand the content of these references.​

    I am hoping someone here at the PFs will be able to explain to me the rationale for the hypothesis that:
    without inflation only non-Gaussian CBR perturbations would be observed.​

    I would much appreciate any help anyone can offer.

  2. jcsd
  3. Jan 23, 2017 #2


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    I don't see how a statement like this could possibly be true in general. Without inflation, I can come up with any number of alternative processes in the early universe that generate Gaussian fluctuations (a homogeneous gas in equilibrium exhibits Gaussian fluctuations, for one). If the argument is that topological defects formed through spontaneous symmetry breaking will inevitably generate non-Gaussian perturbations on top of these, maybe, but such perturbations are already ruled out by CMB data (perturbations are dominantly isocurvature, there is generically a large vector component, there are no correlations on superhorizon scales, etc.)
  4. Jan 24, 2017 #3
    Hi @bapowell:
    Thank you for confirming my reservations.

    Can you help me find additional examples of processes that would generate Gaussian fluctuations, and cite some references that I (hopefully) I might be able to understand?

  5. Jan 24, 2017 #4


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    I believe that string gas cosmology, a well-known alternative to inflation (with, however, far fewer advocates) can produce them as simple thermal fluctuations. I mean, even if all you had was a plasma in equilibrium sitting in the early universe doing nothing, you'd get Gaussian fluctuations. So the presence of Gaussian fluctuations alone is not strong evidence for inflation: it's Gaussian fluctuations along with all the other things it gets right: near scale invariant spectrum of adiabatic perturbations, superhorizon polarization correlations, flatness, and so on.
  6. Jan 24, 2017 #5
    Hi @bapowell:
    Thanks for the list. I will try to find some references, and perhaps I will be able to understand them.

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