Big Bang, Explosion or what?

  • #1
MathematicalPhysicist
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Main Question or Discussion Point

So from what I understnad the model, once the universe was hotter and densier, but I heard also the saying that at the instance of it occurring that spacetime started as well, so it's not really a big bang, as in an explosion; and they also describe it as not having a centre to it (obviously if you still don't have spacetime).

So is it an expolsion as the name suggests or what?
 

Answers and Replies

  • #2
mathman
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The theory of the big bang is fairly well established after time 0. However how it started exactly is a matter of speculation. Probably it should not be called an explosion.
 
  • #3
bapowell
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As mathman says, the nature of the Big Bang itself is a matter of speculation. The Big Bang was not a localized explosion, as originally exemplified by Lemaitre's 'primeval atom'. In fact, the term 'Big Bang' was coined derisively by Fred Hoyle, whose competing steady state model predicted an eternally evolving universe.

The Big Bang is best thought of as having occurred everywhere at once.
 
  • #4
Fredrik
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Definitely not an explosion. That would be a localized event somewhere in spacetime.

The original big bang theory is the claim that the large-scale behavior of the universe is described approximately by a FLRW solution. In this context, the "big bang" is just a property of those solutions that can be characterized in many different ways, one of them being that the coordinate distance (in the "default" coordinate system used with these solutions) between any two timelike geodesics goes to zero as t (the time time coordinate of the same coordinate system) goes to zero.

Every event in these spacetimes has t>0, so there is no t=0, and therefore no specific event that we can call "the big bang".

In other big bang theories, such as theories involving inflation, the "big bang" is something different, something that happened everywhere in space at some specific time. I haven't studied such theories myself, so I won't try to elaborate. The reason I mention these theories is that they actually describe the big bang as something that "happened" in spacetime, but still not as an explosion, because it happened "everywhere" at roughly the same time. (Probably not at every point of the entire universe, but at least at every point of a much larger region of the universe than the part we can see).
 
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  • #5
PhilKravitz
Explosion is not that bad a phrase. The universe at some time close to t=0 is small and hot as time goes on it is big and cooler just like the plasma of a chemical explosion.
 
  • #6
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Fredrik said: because it happened "everywhere" at roughly the same time.

If you multiply that volume with the Planck density you will have a huge numbers of Universes.
I think the Universe started from a volume: Mass of the Universe divided by Planck density; a very small volume
 
  • #7
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The universe at some time close to t=0 is small
Incorrect, if Universe is infinite now (very likely), it was infinite at any time. It never was 'small'.
 
  • #8
bapowell
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Fredrik said: because it happened "everywhere" at roughly the same time.

If you multiply that volume with the Planck density you will have a huge numbers of Universes.
I think the Universe started from a volume: Mass of the Universe divided by Planck density; a very small volume
The universe could well have always been infinite. If you multiply a large volume by the Planck density you get a large energy, not many universes -- not sure how that follows. But there's nothing wrong with having huge numbers of separate causal patches -- after all there is an important distinction between *the* universe and *our* universe.

What if the mass of the universe is infinite? Then you get an infinite volume, not a very small volume as you suggest. The global properties of the universe are not known, but current observations support a homogeneous and isotropic spacetime and there's little reason to believe that our observable universe is all there is.
 
  • #9
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PhilKravitz said:
Explosion is not that bad a phrase. The universe at some time close to t=0 is small and hot as time goes on it is big and cooler just like the plasma of a chemical explosion.
This seems at least somewhat reasonable, assuming that our universe is spatially and temporally finite. It could, presumably, be modelled as an expanding wave shell in some preexisting medium with the wavefront determining its spatial extent, a finite kinetic energy (imparted via initial disturbance and decreasing with time evolution) powering the expansion (accounting for 'dark energy'), with periodic anomalous accelerations/decelerations determined by topological irregularities in the preexisting medium. The original extent of our universe in such a picture would necessarily be smaller than it's current, and ultimate, extent, but it could involve all points in, say, a 10 or 100 or 1000 light year diameter volume (assuming, for simplicity, at least a roughly spherical original shape and evolution).

But I don't know if this sort of picture is compatible with current mainstream cosmological models, and since the OP seems to be referring to the relationship between current mainstream models and the term 'big bang', and insofar as the terms 'big bang', as referring to an explosive initial event, and, therefore, 'explosion' are incompatible with current mainstream models, then the answers generally given here at PF wrt questions akin to "Big Bang, explosion or what?" would seem to be correct.

Dmitry67 said:
... if Universe is infinite now (very likely), it was infinite at any time. It never was 'small'.
Ok, except for the "very likely" part, but I don't think PhilKravitz was assuming that our universe is, in any sense, infinite.

In another thread you stated:
Dmitry67 said:
Observational data proves that universe is the same in any direction. No center, no edges.
Maybe that's what's generally inferred from the current mainstream models. Observational data, afaik, indicates only that there's an observational boundary. As for no center, well, that would depend on how things are modelled.

The fact that our universe appears roughly the same in all directions doesn't, by itself, rule out the notion that it might be an expanding disturbance in a preexisting medium. In which case it would have 'edges', and there would be some volume within its boundary corresponding to its center.

In that other thread ("Why was the big bang not an explosion?") you also stated, as requirements for an 'explosion':
Dmitry67 said:
1. Pre-existing space/time (before the explosion).
2. High pressure at the center and empty space around
3. Pressure accelerated matter outside.
All that is not correct for the BB.
I'll have to take your word for it that the term 'big bang' (and the 'explosion' that that term would seem to imply) doesn't really apply to current mainstream cosmological models. It's just a misnomer -- one of many in the popularization of physics.

Of course, the possibility remains that those models are wrong in some way or other. It doesn't seem unreasonable to suppose that GR is a simplification of a more fundamental wave mechanics (with much of this wave activity being undetectable via our senses, and, to large extent, via the instruments that we employ to augment our senses), that our universe might be an expanding disturbance in a preexisting medium, that the more or less isotropic expansion is powered by energy imparted via an initial disturbance (a 'big bang' as it were), that the dominant force is therefore the expansion, and that the archetypal wave dynamic, evident wrt all particulate media at all scales, is the dynamic of the boundary of our universe.

And while that view might also be wrong, it does solve a lot of 'conceptual' problems that the current mainstream views don't, with the added plus for, say, PF that it might reduce the number of threads asking whether the origin of the universe was an explosion of sorts (because, in that view, it actually would be an explosion, of sorts). Of course, if a model based on that sort of conceptual approach is ever developed and accepted into the mainstream, then it would probably not be called a 'big bang' model -- thereby increasing the confusion associated with this topic (and, along with that, the number of 'big bang and explosion' threads at PF).
 
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  • #10
bapowell
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And while that view might also be wrong, it does solve a lot of 'conceptual' problems that the current mainstream views don't
Which 'conceptual' problem do feel are adequately addressed by modern cosmology?
 
  • #11
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Which 'conceptual' problem do feel are adequately addressed by modern cosmology?
I'm not sure it adequately addresses any conceptual problems. What do you think?
 
  • #12
bapowell
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I'm not sure it adequately addresses any conceptual problems. What do you think?
Sorry..that was supposed to read, "which conceptually problems do you believe it does *not* adequately address?"
 
  • #13
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Sorry..that was supposed to read, "which conceptually problems do you believe it does *not* adequately address?"
We're getting off topic with this. Insofar as this can get sort of complicated, it would be better to start another thread on it.

As for the OP's question, it's been asked and aswered adequately many times before at PF. Big bang is just a popular misnomer wrt current mainstream cosmological models.

Edit: At the risk of this post getting clipped, here's a recent paper:
http://arxiv.org/PS_cache/arxiv/pdf/1006/1006.3348v2.pdf
 
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