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Why was the big bang not an explosion?

  1. Dec 13, 2009 #1
    Hi, Just a quick question.

    Does anyone know the original reason for discarding the idea of the big bang as an explosion?
  2. jcsd
  3. Dec 13, 2009 #2
    Big Bang - is a solution to Einstein equations, and it does not have any properties of an explosion. Nobody never was thinking about the 'explosion'. The image of 'something exploding into empty space' comes from very bad popularisations, created later.

    So originally there was a correct idea, then came a crowd of journalists, asking stupid questions like 'where was the center of Big Bang'?
  4. Dec 13, 2009 #3
    Just so we're on the same page here. What would you say are the properties of an explosion?
  5. Dec 13, 2009 #4
    1. Pre-existing space/time (before the exposion).
    2. High pressure at the center and empty space around
    3. Pressure accelerated matter outside.
    All that is not correct for the BB.
  6. Dec 13, 2009 #5
    And how exactly are those things discounted in the BB theory?
  7. Dec 13, 2009 #6


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    An explosion is a pop sci description of the origin of the universe. Would you characterize the unfolding of a blanket as an 'explosion'? The initial state of the universe was not unlike the unfolding of many blankets. An explosion is descriptive of a blast wave on earth - a pressure wave propogated by the atmosphere. Space has no atmosphere, so the analogy is invalid.
  8. Dec 13, 2009 #7
    For example, average density is the same everywhere. So pressure did not play any role. Even more, it slowed the expansion. Observational data proves that universe is the same in any direction. No center, no edges.

    But if we look at General Relativity, it is even easier than that. There are no solutions where there is empty space for some time, then something 'explodes' in that space, creating matter.
  9. Dec 13, 2009 #8

    D H

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    The Hubble expansion.

    If you must insist on thinking of the big bang as an explosion, it is far better to think of it as an explosion of space rather than an explosion in space.
  10. Dec 13, 2009 #9
    And yet you can have an explosion in space, say nuclear, as long as it doesn't require a component of our atmosphere in the reaction.
  11. Dec 13, 2009 #10
    This just proves that the pressure didn't come from a central distinguishable source within whatever exploded. However there still could have been pressure from every part of the 'mass' acting on every other part.

    I thought gravity slowed the expansion.

    Hate to disagree but observational data only proves that the universe is the same for 12 billion ly or so in all direction. Any speculation on centers or edges is just that, speculation.
  12. Dec 13, 2009 #11
    what provided the driving force for this expansion? surely the force of gravity for a universe, say a second old, would be overwhelming.
  13. Dec 13, 2009 #12
    Astroscott, nobody can't tell you exactly what BB was, except that it was the beginning of time as we know it, and that things were much more dense than today.
    It was an event 13.7 By ago, which marked the transition from unknown state, not describable in our language or physics, to universe that we can study and talk about.

    But I guess that scientific community is to blame for common misconceptions, not pop science. If you call something "Big Bang" then you should expect questions about explosion. "Big Birth" would be more appropriate, at least acronym could stay.
    Last edited: Dec 13, 2009
  14. Dec 13, 2009 #13
    1 Pressure increases gravity, and gravity slows the expansion.

    2 If you agree that there is no center, then what you "simple question" is about? Do you have any doubts, and if yes, what exactly?
  15. Dec 14, 2009 #14


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    The point is that we have no evidence that any point in the observable Universe is special, to be specific that any point is the centre of some original explosion. It is possible (though hard to explain theoretically) that the Universe suddenly becomes very different just beyond the reaches of the observable Universe in such a way as to imply some kind of special point somewhere.

    The point of the 'BB was not an explosion' catch-cry is to counter to common misconceptions that modern cosmology has evidence that the Universe started with an explosion at some initial point. This is a misunderstanding of the theory we have based on the evidence we have. The evidence is consistent with there not being a special point that is the origin of all material. It may not completely rule out the possibility, but it does not in any way suggest it, which is the what is commonly misinferred.

    I tell students ( and anyone else that will listen!) that it is okay to think of the BB as an explosion, as long as your mental picture has the original exploding material having infinite extent, or at least bigger than the observable universe, and uniform. For instance if your mental picture of the BB imagine a nuclear bomb in space blowing up and throwing material into the void, then this is incorrect. However if you imagine that the bomb itself is infinite in size, or at least bigger than the observable universe, then if you think through what happens when it goes off then you'll get a reasonable picture of the BB.

    Note that on a technical level this explanation is not correct, but the reasons for that are somewhat subtle and if you need to resort to mental pictures rather than understanding GR then you won't need to worry about them.
  16. Dec 14, 2009 #15


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    The reason that we have such a bad name for the Big Bang is that it was actually Fred Hoyle, a BB critic, who first coined the phrase. He intended it to be a silly name for what he thought was a bad theory, but somehow that was the one that stuck!

    The one other major misconception I'd like to counter is this idea that 'the Big Bang was the beginning of time'. This is a bit more subtle and not so bad, but still a misconception. We have a set of theories (GR, QM etc) that tell us with a good degree of certainty what happened going back around 13-14 Billion years from now. At some point, which corresponds to when we think the Universe was above a certain average energy density, our theories stop being meaningful. That doesn't mean that the Universe stopped being meaningful, just that we can't say what happened before this time. Suggesting that 'time began' at this point is a possible interpretation, but its not the most reasonable or probable.

    The FRW model of the universe has an important function a(t) (loosely speaking the size of the Universe in a comparative sense) in which a->0 when t->0, but that's the result of extrapolating the maths beyond the point where the physics makes sense. We all want to know what happened before the furthest point we can currently discuss with any certainty, and progress is being made on it, but for now we don't really know. Suggesting that 'time began' is only one possibility, with no supporting evidence.
  17. Dec 14, 2009 #16
    Ok, but if you are suggesting that time in any way existed before t=0, than BB was not birth of the universe, but (loosely speaking) some kind of 'phase transition'. If universe began at BB then time also did so.
    I agree that 'time began at BB' is my preferred way, but only because I can't see it any other way.
    What are your thoughts on imaginary time? Is that concept of any use?
  18. Dec 14, 2009 #17
    It does not suggest that something existed at t<=0 either.
    Regarding "I can't see it any other way" I can give you 2 examples if you want.
  19. Dec 14, 2009 #18
    Of course Dmitry.
  20. Dec 14, 2009 #19
    1. So t>0. What if t (what we observe) is an operator. The underlying, more basic concept, is another time, lets call it Q

    Say Q = t - 1/t

    While t->0 Q-> -inf
    t->+inf Q-> +inf

    So we just mapped ]0,+inf[ to ]-inf,+inf[

    So Universe had infinite history and existed forever, but the time operator 'underestimates' time intervals close to BB.

    2. Big Bounce with 'even' solution:

    StateofTheUniverse(t) = StateOfTheUniverse(-t)

    like cos(x)

    So negative times are just equivalent to positive ones.
  21. Dec 14, 2009 #20
    I guess my original question is based on what I like to think of as the common sense way of looking at things. To me it seems that if you can follow events backward and find a time when everything was in a compressed state it makes sense to assume it came from some sort of explosion. Now I understand that you can't always rely on common sense, but to completely discount it should require a sound reason. I am yet to find that reason and was hoping someone here could provide it.

    Are you talking here about the idea that if it were an explosion then there would be no way to account for Inflation due to the speed limit that matter from the explosion could travel at?

    I read somewhere recently that Hoyle has said that he didn't mean it like that, though I'm not sure whether that's true or he was just trying to backtrack.

    I can only agree with this. While I can assume that we probably do not have the same view of the way it all happened, It's refreshing to read this as it's so different from what I'm used to reading.
    I'll have to leave it there for now as I have to go to work now.
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