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A The Big Bang model only deals with the known universe - True or False?

  1. Jul 16, 2017 #1
    True or False? A clear answer would clear up years of confusion. Thank you.
    Edit: I mean observable universe, obviously. I always thought the Big Bang model somehow also accounts for the part of the universe beyond the observable limit, since it is always advertised as the beginning of "the" universe.
    Last edited: Jul 16, 2017
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  3. Jul 16, 2017 #2


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    What else? If something is unknown, how can we say something sensible about it?

    Or do you mean 'observable'? In that case it's the same dtory.
  4. Jul 16, 2017 #3
    Thank you for the response. Please see my edit in the original post.
  5. Jul 16, 2017 #4

    Vanadium 50

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    Please don't do this. Add something to clarify - but don't leave us with answers to questions that aren't there anymore.
  6. Jul 16, 2017 #5


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    It's a bit like asking if the theory describing the Sun raising and setting every day deals only with the known (=past) observations.

    The model is built on available data, but we can extrapolate beyond that. We don't expect the universe to be much different just beyond our observable patch - in the same way as we don't expect the behaviour of the Sun to change much in the near future.
    Sure it's possible that the laws of physics just outside what we see (in time or space) might suddenly change, but based on how they haven't for the past billions of years (and light-years), it's rather unlikely.

    Like with all extrapolations, it requires some Bayesian thinking.

    On the other hand, we are aware of limitations of such extrapolations - given enough time, we do expect the Sun to stop raising and setting due to some other effects (because it will have become a red giant and evaporated the Earth), and given sufficiently far removed reaches of the larger universe we have reasons suspect that it would be different enough out there not to be described by the Big Bang theory (because it's still inflating*, and the BB hasn't yet happened there).

    *with the caveat that the theory of stellar evolution is on a more robust footing than the inflationary theory.

    So the answer is not a simple t/f, but more along the lines of: 'BB is very likely to describe more than just our observable universe, but perhaps not all of it.'

    As far as I can tell, he did not delete anything - he just added the clarifying bit after the 'edit'. The question is in the thread title.
  7. Jul 16, 2017 #6


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    In science it is customary to test a model by applying it to new observations. If new observations are consistent with the model it confers the model additional credibility. In this sense no model is ever considered complete because there is an unlimited potential for new observations [which explains why statistical confidence is a vital component of science].
  8. Jul 17, 2017 #7


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    As others have noted, the only thing that we can say for certain is that the Big Bang theory accurately describes the observable universe. This theory probably describes the universe for some distance beyond the observable, for the reason that we would expect some sort of change in the properties close to any sort of transition, and no such change is apparent. There's just no way to say how far, though: it's impossible to quantify just how much certainty our observations give us about the universe beyond our observations. Those models certainly break down at some point, but we have no way of knowing precisely where and how those models do break down.

    Our theories built upon observations within the observable universe can give us clues, can suggest which types of ideas of what goes beyond the observable are more or less likely. But there's probably no way to ever be sure.

    You can, in principle, come up with a testable model that relies upon many experiments and observations done within our observable universe. That model may say, very precisely, what must happen outside our observable universe. But there will always be the question, "But what if there's something we haven't thought of yet that matches both the observable universe and whatever lies beyond the observable, and is very different from this model?" Chances are the most we'll ever be able to say with confidence about the universe beyond our observations will be some relatively vague generalizations (such as "The Big Bang theory holds for some unknown distance outside our horizon.").
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