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Explosion/expansion isn't there a period of acceleration

  1. May 15, 2010 #1
    Forgive me if this is a stupid question.

    In any explosion/expansion isn't there a period of acceleration before slowing? Could the universe still be in this accelerating period? What would that do to the size of the big bang?
     
  2. jcsd
  3. May 15, 2010 #2

    nicksauce

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    Re: Expansion

    Lots of misconceptions here:

    -The big bang wasn't an explosion
    -The acceleration only started recently. The universe has been decelerating for most of its life.
    -The big bang doesn't have a "size"
     
  4. May 15, 2010 #3

    russ_watters

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    Re: Expansion

    4. In an explosion, the acceleration is only for the first few micro(nano?)seconds. Tom Clancy gave a pretty neat full-chapter explanation of the workings of an atom bomb going off in one of his books: essentially, the explosion is finished before the casing of the bomb is even touched!
     
  5. May 15, 2010 #4
    Re: Expansion

    I was referring to size as something like megatonnage, or was that understood?
     
  6. May 15, 2010 #5

    nicksauce

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    Re: Expansion

    Well, then that goes hand in hand with the big bang not being an explosion.
     
  7. May 16, 2010 #6

    Nabeshin

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    Re: Expansion

    In times like these, Fred Hoyle must be rolling around laughing in his grave.
     
  8. May 16, 2010 #7

    Chronos

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    Re: Expansion

    The supernova legacy study found that acceleration took over around 5 billion years ago. The universe was apparently deaccelerating prior to that time [due to gravity]. The initial acceleration lasted only a brief amount of time [probably a matter of seconds or less] under current theory.
     
  9. May 16, 2010 #8

    AWA

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    Re: Expansion

    There seems to be contradiction between the answer of nicksause (no explosion) and chronos and russ waters answer (implicitly talking about explosion). so what' happening then?
    There was no explosion, and then ,what other phenomenon causes such an initial accelaration, or there was explosion and you are left to explain the ultimate cause of the expansion.

    I know this isn't easy to explain but another question is how do you cope with a universe that had a decelerated expansion and suddenly 5 billion years ago it began to accelarate. Odd, isn't it?
     
  10. May 16, 2010 #9

    apeiron

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    Re: Expansion

    Rather than an explosion, think instead of a phase transition. The universe was born the moment it started to cool via expansion. So the pre-bang realm was already hot - a weight of energy in suspension. Then it "poured out" when it had the dimensionality to give it a direction in which to expand and spread-out, and so cool.

    The initial expansion rate was always decelerating because the energetic contents of our universe also had gravity. (Though the effect was weaker while the contents were just a bath of radiation, stronger once matter condensed out with sufficient cooling).

    The acceleration of dark energy could have been there all along from the beginning as some other kind of effect, but would have only showed 5 bya because the bigger story, the decelerating expansion, had slowed sufficiently for this other remaining small background creepage to become observable.
     
  11. May 16, 2010 #10

    AWA

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    Re: Expansion

    Apeiron , don't you think this explanation is a little convoluted, specially from the second paragraph. That is to say, with so far-fetched arguments just about anything can be explained.
    If I understand you ,first we have the cooling that initiates the expansion from a previous state that I find hard to explain thermodinamically, but nevermind that, then we have deceleration from gravity that was strong initially in spite you admit gravity must have been weaker at first and stronger later. At the same time we have the accelaration from dark energy also from the beginning but that is only detectable when the decelaration has slowed down enough for dark energy to cath up. Hmmm... maybe , who knows, but I wouldn't bet on it. But again as I said this isn't easy at all to come up with something one can be comfortable with.
     
  12. May 17, 2010 #11

    Ich

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    Re: Expansion

    Whether you see it as an explosion or not, there is (at least presumably) a period of acceleration before slowing, called inflation. The end of inflation set the initial conditions for the universe we observe. The state before inflation is unobservable, wiped out by the extreme expansion.
     
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