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Where/when are we in a yo-yo universe?

  1. Apr 15, 2010 #1
    Here's a novice question...

    Assuming we accept the yo-yo model of the continually expanding and contracting universe, can we know whether the universe is currently expanding or contracting?

    Maybe I've missed the point, but I assume that when a contraction occurs, time (along with space) moves backwards?

    Surely if it were contracting, we would only see it contracting by viewing it from the perception of someone moving FORWARD in time? I mean wouldn't we see a contracting universe just as we see one moving forward? Only spacetime would be contracting rather than expanding?

    Maybe I'm talking utter drivel but if someone could enlighten me a bit on this whole affair I'd be very grateful!

    Cheers! :smile:
  2. jcsd
  3. Apr 15, 2010 #2


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    "yo-yo model" ?:confused:?
  4. Apr 15, 2010 #3
    Sorry, the Cyclic Model. Bang/Crunch/Bang/Crunch...
  5. Apr 15, 2010 #4


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    Okay daveshorty, and I missed to say – Welcome to PF! :smile:
    First, there’s (currently) absolutely no way to know if the universe will end in a 'recycling' "Big Bounce", and even if we could know – there is absolutely nothing that points toward a 'recycled carbon copy' of current universe. In fact, the initial conditions in Big Bang are so special it creates a lot of work for the cosmologists to work the 'mystery' out... (not finished yet).

    Second, we are 100% sure that the universe is expanding. In fact, it’s not only an expansion – it’s currently an accelerating expansion!

    In 1998 two teams, the http://en.wikipedia.org/wiki/Supernova_Cosmology_Project" [Broken], separately proved that this is a fact.

    Third, the http://en.wikipedia.org/wiki/CMB" [Broken] (CMB) radiation shows beyond any doubt that the universe has expanded, and cooled down, since the Big Bang. The CMB is the 'afterglow' of the Big Bang, everywhere.
    Last edited by a moderator: May 4, 2017
  6. Apr 15, 2010 #5
    Thanks for that.
    Yeah, I guess my question was just, for the sake of interest, based in that hypothetical cyclic universe. If that were the case, is there any way to know how we might be perceiving the crunch in terms of the flow of time?
  7. Apr 15, 2010 #6


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    This is a difficult question dave, and depends on the exact implementation. During the contracting phase of a cyclic universe, time is still trudging along in the forward direction. In some models, the scale factor of the universe, a(t), decreases as time moves on -- hence the contraction. However, in some implementations (the actual 'cyclic model' of Steinhardt and Turok), a(t) is always increasing according to observers in the universe -- what's cyclic is the entropy density. Cyclic models in general have a difficult time with the actual "crunch" part -- when the universe has contracted to zero size. This is a singularity and according to general relativity, space and time end here. However, it is widely believed that quantum gravitational effects become important before the universe crunches to zero size, so that the singularity is never reached. Of course, what time means in this quantum theory is still unknown.

    But in most cyclic models, nothing funny is happening with time from one cycle to the next. The flow of time does not reverse direction in the contracting phase.
  8. Apr 15, 2010 #7
    Cheers for that response! Very helpful :smile:
  9. Apr 15, 2010 #8


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    bapowell explained it very well, and I agree. It’s very hard to see an un-boiled egg popping out of the saucepan, cold into my hand, making its way to the hen – just because some galaxy cluster, 13 billion light years away, is 'getting closer'...!? :biggrin:

    It’s hard to know – but my feeling is that a "Big Crunch" will not be "The Movie of Big Bang" in reverse...

    Last edited by a moderator: Apr 25, 2017
  10. Apr 15, 2010 #9
    Cool, thanks DevilsAvocado!
  11. Apr 15, 2010 #10


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    You are welcome!
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