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A double-cone-shaped universe?

  1. Nov 6, 2009 #1
    I've been reading Julian Barbour's excellent book 'The End of Time' which is based on his work on foundational issues in Physics (specifically time and space). I like his ideas a lot, however I'm sure I read in it a description of the big bang which was in reference to Mach, I think, that showed the reversibility of direction of time. So you could imagine a universe in which the big bang is not the 'beginning' but the middle as it were. To put it in very flaky terms, imagine that on the 'other side' of the big bang event is a mirror universe with time running in the other direction. I've imagined it like two cones touching at their apexes. I might have hacked it together from more than one place though.

    I like the idea because that it gets rid of the question 'what was before the big bang' because you can imagine an infinite universe temporally (ie heat death at either end, well you know what I mean) but with a creation event in the middle.

    Is this a possible scenario? If so where can I read more about it?
  2. jcsd
  3. Nov 6, 2009 #2
    Hmmm. Does sound rather novel. I think we need more details before saying whether it's viable or not. Usually reverse time Universes have a Big Bang/Big Crunch symmetry with exact time-reversal between the two 'sides'.
  4. Nov 7, 2009 #3


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    Well, from what I understand, this picture is incredibly implausible, because it completely fails to explain the extreme low entropy of that middle point between the two universes.
  5. Nov 7, 2009 #4
    Low entropy is low at the BB - it is a part of the initial conditions.
    Entropy increases symmetrically on the both sides.
    On both sides time arrow points away from the BB and observers in both parts of that universe interpret it as 'expansion' with BB in the past.

    This is much more beautiful then the Big Bounce scenario, with infinitely complex initial conditions at -inf, and the mysterious decrease of entropy just before the Big Bounce.
  6. Nov 7, 2009 #5


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    Yes, but why? The thing is, it's vastly more likely to spontaneously generate a universe created just as it is today (as today our universe has much higher entropy), than it is to go through the trouble of making a big bang.

    Any theory of origins, for it to be plausible, must make it so that it is more likely for a big bang to happen than for a universe like the one we see to spontaneously pop out of a thermal bath.

    Uh, it's actually the same thing. A decrease in entropy is the same thing as having the arrow of time run in the other direction.
  7. Nov 7, 2009 #6
    1 I believe initial conditions fix the state of the omnium at some point or region. So it is just one another equation in the theory. Then the initial conditions are NULL, and there are just the equations

    As MWIer, I see this picture as even more beautiful: as on both sides of BB evolution is the same, then the state of the omnuim is symmetric at t=0. So both sides of the BB are identical

    To my surprise, Big Bounce people believe that before the BB entropy INCREASED, so BB was in the future. Then there was a sudden and mysterious drop of entropy few plank times before the BB. Universes before and after BB in that theory look totally different.
  8. Nov 7, 2009 #7


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    Yes, but this hypothesis fails when compared against Boltzmann Brains: if it is true, then there are vastly more Boltzmann Brains than real observers, which is counter to our observation that we are real observers (evidenced by the fact that inference works).

    It does depend upon which bounce scenario you are talking about. The fully-symmetric bounce is the most simple, and least likely.
  9. Nov 7, 2009 #8
    We are entering very slippery territory: we are talking about different mathematical universes. Can we use the 'probability' argument there?

    For example, say, there is only one universe with null initial conditions
    1000 universes with number N<1000 as initial condition
    1000'000 universes with 2 number N,M <1000 as initial conditions
    1000'000'000 universes with 3 numbers etc.

    So there are 1'000'000'000 such universes per only one universe without initial conditions. Does it mean that there are 'more' such universes and they are 'more likely'? I think the opposite is true.
  10. Nov 7, 2009 #9


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    The point is that this idea (a universe symmetric in time) predicts that observers should observe things very, very differently then what we see. It is, therefore, almost certainly wrong.
  11. Nov 7, 2009 #10
    Please clarify
    What would be different?
  12. Nov 7, 2009 #11


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    As I said, it's the Boltzmann brain argument. Basically, it's a reductio ad absurdum. The argument goes like this:

    Imagine that the universe could be born out of a thermal bath. Small fluctuations out of a thermal bath are quite common, while large fluctuations are exceedingly rare. Our universe could just be a rare fluctuation. But then, if our universe's distant past is a rare fluctuation, then the higher-entropy later universe would be more common. And single brains fluctuating out of nothing would be more common still. Thus, if this low-entropy early universe fluctuated out of a thermal bath, then the prediction is that the vast majority of observers would be those that fluctuated out of a vacuum, and those will, overwhelmingly, not see an ordered universe (generally their observations will be highly disjointed and disordered, making no sense). Because we do observe an ordered universe, one in which inference works, we can be pretty confident that that is not us.

    Which, in turn, makes us quite confident that any theory which predicts that the most common sort of observer is a Boltzmann Brain just can't be correct. That is why the simplest bounce scenario, the symmetric time one, just doesn't make sense.
  13. Nov 7, 2009 #12
    Imagine that the universe could be born out of a thermal bath. - as we know, it is false, and the 2nd law exists because of the initial conditions at the Big Bang (check Loschmidt's paradox). And if entropy at BB was low then arrow of time always points away from the BB (in our case, symmetrically in different directions of both sides)

    So I dont understand your argument. You begin from a false statement... so what?
  14. Nov 7, 2009 #13
    Do you mean to say that Time might be a line running in two directions?
  15. Nov 7, 2009 #14
    Yes. Like the number line with 0 in the middle. From the point of view of observers on either side of the BB there appears to be a beginning to the universe but there is no such thing. It would simply be an artefact of the arrow of time on each side.
  16. Nov 7, 2009 #15
    Hi Dmity67. It's like the North Pole analogy: every direction from the NP is south. Likewise every time from a BB is the 'future' but there are only two paths to take.

    I'm not a physicist so I can only think philosophically about it. But I'd like to know more.
  17. Nov 7, 2009 #16
  18. Nov 7, 2009 #17
    One really weird idea is to see how Boltzman's brain arguments would work if a random thermal fluctuation tripped over some phase transition (i.e. inflation). It would be cool, if one could show that if you end up hitting some phase transition, then this resolves Boltzman's brain issues, hence any coherent universe must have inflation.
  19. Nov 7, 2009 #18
    Trying to figure out what the universe is like using abstract philosophy rarely work very well. Also beautiful elegant theories generally fall apart when you hit messy reality, so that if someone comes up with a beautiful, elegant theory in an area with no data, then I tend to distrust it.

    The way physics works is

    (assume something) -> (figure out the consequences of that assumption) -> (see it doesn't work)

    And after eliminating things that don't work, you end up with stuff that is standing. The problem with a double cone-shaped universe is that it's not obvious how you prove that it's wrong.
  20. Nov 8, 2009 #19


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    My gripe with bounces is not a single quark is lost or gained in the process. The whole idea otherwise collapses [or falls apart] after a finite number of bounces.
  21. Nov 8, 2009 #20
    Thanks for the lesson in the scientific method but I understand that already ;)

    I was indirectly asking what physics would falsify it, whether is a case of being 'not even wrong', rather than philosophical objections.

    [There are lots of other hypotheses that deal with why the BB occurred that originate with semi-philosophical ideas (colliding branes, multiverse etc), there's nothing wrong with that. Inflation is an example of a conceptual what-if hypothesis which then drives the physics. Likewise relativity began as an idea, Einstein didn't come across it by directly manipulating equations. Maxwell's another example too. Science is creative.

    String theory isn't exactly teeming with testable predictions and seems to be based on intuition and a mathematical sense of 'beauty'.

    As I understand it both approaches (mathematical and philosophical) attempt to find correlations between currently unconnected areas of knowledge to create new structure. You then test that new structure against experiment.
    (physics is predicated on the 'abstract philosophical' idea that you can represent real things as symbols in mathematics. Mathematics has its own rich structure independent of reality, its the task of Physics to find which mathematical patterns match reality).]

    Back to the question then. I can think of two reasons why its not a good idea.
    1.All information is destroyed at the BB so no causal connection between both sides. You might as well hypothesise a single BB event as we do now. Unless its feasible for each side to develop independently.
    2.It might remove the 'why' question for the BB as a creation event of the entire universe, but you're still left with 'why' a time-symmetrical universe existed at all.
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