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How did the quantum fluctuations in the inflationary universe turned classical?

  1. Jul 23, 2008 #1
    Im trying to understand inflation, specially the part of structure formation, so im following Mukhanov's Book. There he does an analysis on quantum fluctuations, more specifically he quantize the newtonian potential and finds the power spectrum and turns out to be scale invariant. I kind of understand that, but what i dont understand at all is how this supossedly quantum fluctuation of the FRW metric turned classical and gave birth to all the inhomogenities and anisotropies in our universe. Can anyone help me on this
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  3. Jul 23, 2008 #2

    George Jones

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    This a question that I find fascinating. Viatcheslav Mukhanov, Steven Weinberg, and Roger Penrose all agree that this is an example of the measurement problem in standard quantum mechanics. Both Mukhanov and Weinberg believe that decoherence plays plays a role, and that decherence (as we now understand it) alone is not sufficient.

    As I understand things, the state (pun intended) of the measurement problem in quantum mechanice is roughly this. Decoherence takes us from a quantum superposition to a mixture of classical states, but does not (yet) show in general how to pick out one of the classical states.

    Mukhanov believes that, in this cosmological context, the measurement problem forces us into the many-worlds interpretation of quantum mechanics. From pages 348, 349 of Physical Foundations of Cosmology by Mukhanov:
    For Weinberg, this is no more or less puzzling than getting a classical output from our apparatus when we measure the quantum mechanical spin of a particle. From page 476 of the new book, Cosmology, by Weinberg:
    Penrose points out that a quantum superposition of field configurations corresponds to a quantum superposition of spacetime geometries, since different field configurations have different stress-energy tensors, and thus correspond to different solutions of Einstein's equation. From page 862 (hardcover edition) of The Road to Reality by Penrose (U = unitary,; R = reduction)
    As I said, fascinating stuff!
  4. Jul 23, 2008 #3
    Thanks for your reply, but now i have more questions

    This is exactly where i am confused, as far as i know, when we measure the quantum mechanical spin of a particle, we need a time of measurement, a basis, and a measuring device. In the cosmological context i dont see these, so my questions are:

    1) What is performing the measurement?
    2) Precisely, what is the set of quantum observables that is being measured? and what determines them?
    3) When is this measurement taking place?

    In the case of the quantum mechanical spin of a particle i can answer this with a Stern-Gerlach type experiment, but in the cosmological scenario i dont see this. The Many-World interpretation to me seems to be in one on one correspondence with these issues. (time of measurement, basis, measuring device).

    I still have more questions, specially with decoherence but i think these are the ones which confuses me the most. Thanks again for your help.
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