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

  1. Jul 18, 2017 #1
    The ubiquity of the big bang holds no matter how big the universe is or even whether it is finite or infinite in size. How are we so confident that the part of the universe that is unobservable also falls under the Big Bang model?
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  3. Jul 18, 2017 #2


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    The bit of the universe we can see is well modelled by FLRW spacetime. We have no reason to believe the rest of the universe would be any different

    So are we confident? I don't think that's really a question we can answer. We have no experience of the rest of the universe, or of other universes, on which we could base an estimate of confidence. All we can say is that, so far, everything we've seen is consistent with the non-visible parts of the universe being the same as the visible bits. But we have no idea how we might go about testing that.
  4. Jul 18, 2017 #3


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    Generally, we aren't. The Big Bang model probably holds for some distance outside the observable, but there's no way to say how far.
  5. Jul 19, 2017 #4


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    I'd agree with the other posters who say that it would be a misrepresentation of the current state of knowledge to say that we are confident about the behavior of the unobservable parts of the universe.

    I would say that the general viewpoint is that because the unobservable part of the universe are unobservable, questions about these parts of the universe cannot be addressed by the scientific method. This means answering questions about the unobservable parts of the universe gets into the realm of philosophy (or possibly even religion), so to address those aspects, one would need to find a forum that talks about philosophy and/or religion. That forum isn't Physics Forum, however. Religious discussions were never allowed on PF , and philosophical discussions, which were initially limited to their own sub-forum (the Philosphy forum), became problematical in terms of academic quality and their tendency to spread to sections of the forum that were not dedicated to philosophy, so eventually the philosophy section of the forums was closed.

    We can study scientifically what the big bang has to say about the observable part of the universe, and there has been quite a bit of rather impressive work done in the area. A key feature of the big bang model is that it predicts cosmic microwave background radiation (CMB) which we actually observe. Other cosmological models do not predict this radiation, and have fallen by the wayside as a result of this failure. I believe there have been at least two Nobel prizes awarded for such studies, the initial discovery of the cosmic microwave background radiation (CMB) with the prize awarded to Penzias and Wilson, and the COBE results, the award going to Smoot and Mather. The work did not stop with these Nobel-winning results, even more sophiticated studies (such as WMAP) have been done. The very short management level summary of all this work would be to say that the detailed predictions of our cosmological models are consistent with these experimental observations. So we must be doing something right.

    Something else that can probably be talked about here is that the current scientific conception of the big bang is not that of a single event at a single point. This gets to be potnetially off-topic, and rather long, and I probably haven't worded this precisely enough. I believe Ned Wright discusses this somewhere in his cosmology tutorial, but I don't have an exact quote on this point.
  6. Jul 21, 2017 #5


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    Generally speaking unobservable regions of the universe are veiled by some sort of horizon that causally separates them from the observable universe. We do not have a scientific basis to understand the laws of physics that apply to causally disconnected regions.
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