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B Multiverse theory question

  1. Aug 17, 2017 #1
    In the multiverse theory, I keep on hearing that our observable universe is one of many universes. Is that saying that our universe ends at the point we can no longer see it - at the end of the observable universe. That would mean that the earth is the literal centre of our universe.

    Am I missing something here? How can our universe be the same size of the observable universe?
     
    Last edited: Aug 17, 2017
  2. jcsd
  3. Aug 17, 2017 #2
    The universe and the observable universe aren't exactly the same thing. One of them is literally everything in existence, even what we can't see, which is the universe, the observable universe is the universe in which light since the start of the universe travelled through to make it "observable" to us.

    What I like to think is the universe is an infinetly large dark room and the observable universe is the the area where a light bulb is turned on in the room (and light is ever expanding to light up the room)
     
  4. Aug 17, 2017 #3

    jedishrfu

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  5. Aug 17, 2017 #4

    But why is everyone assuming that all matter in the universe is in our observable universe? That is my question. The multiverse theory, described in some programs I have watched, have said that there are more expanding light bulbs in that dark room, and our expanding bulb (or our universe), is the same size as the observable universe. Therefore our universe ends at the point where we can't see it, making earth the centre of our universe.
     
    Last edited: Aug 17, 2017
  6. Aug 17, 2017 #5
    thank you for the link, I find it useful, however my question lies in some programs I have watched about the multiverse theory. They are probably just over simplified pop science, but I wonder about how they have described our observable universe as that separate bubble of universe. It doesn't make sense to be. Is it just over simplified, or a different interpretation of the multiverse theory?
     
  7. Aug 17, 2017 #6

    Bandersnatch

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    Nobody is assuming that. You must have misunderstood.

    There is more than one way in which (cosmological) multiverse can be understood, so there's potential for confusion.

    In terms of standard BB theory:
    - it's the collection of observable patches of some larger whole, where each patch is sufficiently distant from any other patch so as to ensure that they'll never be in causal contact. At the present epoch these patches are a bit over 60 billion light-years in radius. Everything over that distance is functionally another universe (but essentially identical to ours, and coming from the same Big Bang).

    If we add inflationary theory to the mix, we get:
    - it's the collection of larger than observable patches, in which observable universes are embedded (so, each is the 'larger whole' or the 'universe' as understood above). Each of those patches has come to existence when the inflationary field locally decayed, and was causally disconnected from any other patch by inflation still happening elsewhere. Here, each of those patches have underwent their own Big Bangs and may have
    a) different parameters, or
    b) different physics.
    There's still a larger whole, consisting of the eternally expanding inflationary field, with universes embedded in it.

    The reasoning in the first case is that anything causally disconnected from us can be thought of as another universe (it's forever separate). The case with inflation calls another universe that which underwent it its own Big Bang.
     
  8. Aug 17, 2017 #7
    that really clears things up thank you.
    I think I misunderstood the standard BB theory.
    Thanks :)
     
  9. Aug 17, 2017 #8
    The Earth is the centre of the observable universe, not the universe. Our vision of the universe is in the shape of a sphere and we are the centre of the sphere. That is the observable universe, the universe itself isn't fully explored for us yet. And remember the multiverse theory is still just a theory, there's no telling how we would prove this theory without having to do numerous crazy things requiring more advanced technology than we have today. But why worry about other universes when our universe hasn't even been explored by us, not even our galaxy has been explored by us yet. Making theories about the multiverse even if proven to be true won't do us anything really, we're far from being a universal traveling species.
     
  10. Aug 17, 2017 #9
    I'm not sure if the multiverse proposition even qualifies as a theory.
    It does offer an explanation for a number of conundrums both in cosmology and in quantum physics, so the idea is a plausible hypothesis.
    However it offers no testable prediction, and no possibility of investigating, (since any one local universe would be causally disconnected from the rest)
    In a sense it's like offering the tooth fairy as an explanation for missing teeth and mysterious rewards associated with them.
    In reality there is a much better explanation than the fairy, but nobody has thought of it just yet.
     
  11. Aug 17, 2017 #10
    I am myself a bit skeptical of the theory but admire its explanation for quantum superposition.
    I do prefer to call it the many worlds interpretation as it actually isn't a theory as such, however people often call it the multiverse theory.
     
  12. Aug 17, 2017 #11

    Bandersnatch

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    Cosmological multiverse is not a theory, nor is it a hypothesis. It's a prediction. Of a particular class of Big Bang models in the first of the two senses given in post #6, and of inflation in the second.
    Furthermore, it has nothing to do with the many worlds interpretation of quantum mechanics (which also isn't a theory).
     
  13. Aug 17, 2017 #12

    kimbyd

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    A lot of the terminology here is seriously confusing. "Universe" can, in different contexts, mean everything that exists everywhere, or it can only mean the observable universe, or it can mean the part of the universe that is connected to our observable universe. Unfortunately you have to use context to figure out what a person means by the term (or ask them what they mean).
     
  14. Aug 17, 2017 #13
    I think I got confused by all the multiple multiverse theories and contexts of universe like you said. I understand it now.
    Thanks.
     
  15. Aug 18, 2017 #14

    Chronos

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    It's easily confusing trying to understand what constitutes an 'alternative' to our observable universe. The observable universe includes all the space and matter that has ever existed. That provokes an obvious question - What could possibly lay beyond all the space that has ever existed? The answer rests upon the assumption space is some unbounded, if not infinite, preexisting container within which our universe originated. Many cosmologists would probably object to that assumption. Another popular description is alternative universes are regions causally disconnected from our universe. If you really focus on what that means you quickly realize this means independent, objective [IOW scientific] proof of the existence of any such realm is impossible, since nothing in or about it could leave any clues to its existence within our universe. If neither of these options are altogether appealing, you are in good company.
     
  16. Aug 18, 2017 #15

    Bandersnatch

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    Well, that's patently untrue. You just have to look at the CMBR tomorrow and see that it's still there to disprove it.
     
  17. Aug 18, 2017 #16
    All the space and matter which we know of.

    That probably works better.
     
  18. Aug 19, 2017 #17

    Chronos

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    I assume it is safe to say we all agree the CMBR qualifies as something known to have existed in the history of the observable universe. I cannot offhand think of any features beyond our knowledge that merit serious consideration.
     
  19. Aug 19, 2017 #18
    all the space and matter that we could theoretically observe - on paper.
    That is how I think of it.
     
  20. Aug 29, 2017 #19

    PeterDonis

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    Thread closed for moderation.
     
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