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I Problems with the theory of inflation

  1. Feb 23, 2017 #1
    what is the current consensus on inflation, as recent article in SciAM


    the 3 authors are highly respected physicists in the field

    they conclusion
    inflation is internally inconsistent and inconsistent with best measurements from PLANCK CMB

    inflation predicts primordial gravitational waves that should be observed. none has been.

    if inflation is rejected what are implications

    those 3 propose a big bounce should replace inflation

    something LQG/LQC has predicted
    Last edited: Feb 23, 2017
  2. jcsd
  3. Feb 23, 2017 #2


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    The bounce model suffers the same malady as every other cyclical model: it ultimately runs into the thermodynamic wall. Some bounce models evade this issue with only a single bounce, but, this fails to resolve the age old 'chicken or egg' paradox. It is progress only in the sense the clock can be run further back without any apparent need to invoke supernatural intervention. But, entropy is the relentless, invincible enemy of any solution in perpetuity. For further disuscussion, this paper may be of interest; https://arxiv.org/abs/1501.03054, Cyclic Entropy: An Alternative to Inflationary Cosmology.
  4. Feb 23, 2017 #3
    There is one (semi)cyclical model which doesn't run into the entropy problem per se, namely Penrose' Conformal Cyclic Cosmology (CCC). In fact, the entropy problem is solved in the form of the Weyl Curvature Hypothesis, explaining the origin of the 2nd Law, in conjunction with an inverse Higgs mechanism. This is coupled to a phase space prediction that unitarity is violated in black holes as Hawking originally predicted, and doing so offers a possible self-consistent resolution of the measurement problem in QM.
  5. Feb 24, 2017 #4
    This 2017 article is based on a 2013 paper which e.g. one particle physicist described as "very silly". It produced a response coauthored by another founder of inflation (Guth), the original authors produced their reply, but basically no-one cares. They admit, in that 2013 paper: "The usual test for a theory is whether experiment agrees with model predictions. Obviously, inflationary plateau-like models pass this test." Their criticisms are all meta stuff like "what came before inflation" or "how can probability mean anything in an infinite universe". So model builders like the authors of SMASH keep making models that include inflation, and the ambitious people try to solve the meta issues. Something other than inflation, like bounce cosmology, might be true, and Steinhardt and friends are welcome to work on such alternatives, but no-one who works on inflation thinks that the meta issues alone are a reason to work on the alternatives.
  6. Feb 24, 2017 #5
    Steinhardt and friends claims inflation has no satisfactory plateau issue. that inflation as a particle physics is implausible, there is no inflation primordial gravitational waves all these difficulties is enough to debunk inflation as a theory of cosmology
  7. Feb 25, 2017 #6
    Well, I'm no expert but those all seem to be bad arguments. The fact is, inflationary models make predictions, and there are simple models that are consistent with the basic data. See Planck 2015 review.
  8. Feb 25, 2017 #7


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    But there are other reasons to work on alternatives.

    Testable science requires alternative models for the standard model to be testable against.

    Although that is difficult when the standard model is as flexible as Inflation.

    A less flexible alternative that also passes observational tests would be nice....

  9. Feb 25, 2017 #8
    I think the clearest way to evaluate the arguments from Steinhardt et al, would be to see how they look when directed against a specific inflationary model, one that is actually consistent with the data. They make all these generalizations about inflation in the abstract, let's see if those generalizations have any justice when confronted with a specific model.
  10. Feb 26, 2017 #9


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    I don't think this is generally accepted. Some very specific models of inflation don't seem to agree with the Planck data, but there are many inflation models. Inflation is considered pretty widely to be a viable class of models for the early universe.

    There's a wide range of parameter space in many inflation models that are quite consistent with current data on primordial gravity waves.
  11. Feb 26, 2017 #10
    the authors if you read it anticipate this and say that inflation is too flexible, and use the word "patch" by making inflation more arcane it loses explanatory power
  12. Feb 27, 2017 #11


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  13. Feb 27, 2017 #12


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    Then they should have made that their principle argument rather than saying it's been countered by observations. Because it hasn't been. Saying it has is simply a lie.

    Suggesting that inflation is "too flexible" is a much better argument in that it's actually true that inflation models are diverse and have rather different predictions for these particular sorts of observables. But I still don't think it means anything. If we got to a point where all of the simple inflation models failed to match the evidence, and we had to come up with very complicated models to do so, then they'd have a point. We're not there yet.

    Bounce cosmologies have their own theoretical problems, not least of which is the fact that they assume a collapsing phase of the universe that isn't explained. There are also potential issues with entropy at the bounce, and they rely upon a theory of gravity which is still speculative. Whether you feel the theoretical difficulties with bounce cosmologies are more concerning than with inflation is largely a matter of taste, not a matter of empirical evidence at this time. I believe a majority of cosmologists still side with inflation (though fairly mildly), as it is consistent with a wider range of possible physics beyond the standard model.
    Last edited: Feb 27, 2017
  14. Feb 27, 2017 #13

    you can read pdf here


    loeb is a harvard astronomy department chair

    steinhard is princeton advance institute cosmology astrophysics

    illjas is princeton advance astrophysics and cosmology

    loop cosmology predicts bounce.

    perhaps loop cosmology bounce - no inflation, no inflaton field is better theory

    perhaps loop cosmology bounce plus higgs inflation since higgs field exists

    since higgs field exists, perhaps higgs inflation plus bounce
    no evidence of inflaton
  15. Feb 27, 2017 #14


    Staff: Mentor

    You already posted this in the OP of the thread. The response has not changed.

    Thread closed.
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