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B How can the Big Bang model apply to the entire universe?

  1. Sep 7, 2017 #21


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    Another link of possible interest:


    About megastructures which seem to be in conflict with the cosm.principle.
  2. Sep 7, 2017 #22


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    The main argument I'm making with respect to this is that you can't reasonably apply Occam's Razor to an approximation in this manner, not when we know that there's a more detailed underlying model that is more precise.

    One thing I find interesting is that this model does predict a breakdown of the simple notion of homogeneity applied to the universe as a whole, but you can still recover it if you're willing to broaden your definition of homogeneity a bit.

    In the context of cosmic inflation with a simple scalar spectral index, the primordial perturbations will, at any given time, lead to very large differences in density in different Hubble volumes. However, inflation will always end at the exact same energy density, and the subsequent expansion of the universe will continue in pretty much exactly the same way in all subsequent volumes. Thus you could say that as long as something more exotic isn't going on, the universe at 14 billion years from the local end of inflation will look pretty much exactly the same to all observers in all locations that are stationary with respect to the local Hubble flow. Depending upon the model of inflation, inflation might end everywhere within a single second, or there might be billions, trillions, or any number of years between the end of inflation in different locations (in eternal inflation models). But regardless of that timing, all of the subsequent evolution will be effectively identical, just with a different pattern of matter with the same statistical properties.

    This expanded notion of homogeneity is only broken if we consider more exotic possibilities, such as spontaneous symmetry breaking leading to different low-energy laws of physics. Given that spontaneous symmetry breaking is a component of the Standard Model of particle physics, I don't think we can discount these possibilities as unlikely through the use of Occam's Razor (because you'd have to add additional assumptions to the Standard Model to discount them). If you define it so that regions with different symmetry breaking are different universes, then that's fine.
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