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How big was the big bang?

  1. Dec 27, 2015 #1
    i haven't even got any idea how what to search in order to find such an answer, but whenever i watch a documentary about physics, astrophysics, astrobiology,.etc and they do thr token intro of this is how the universe began... this question always niggles.... because i don't reckon it would of been all that "big" of a bang actually nanoscopic or some such scale in all probability but the big probably meant to capture the significance of the moment... however my question mark still remains firmly over my head? Remember i mean in literal terms when i ask how big... i mean in terms of however they measure neutron mergers and supernovas and that type of stuff?
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  3. Dec 27, 2015 #2


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    The question of size when applied to the Big Bang doesn't even make much sense, since the event (BB is an event in time, not an explosion in space) marks the early stages of the whole universe. Since it happened everywhere in the universe, its size was 'as big as the universe', including the possible case of 'infinitely large'.
  4. Dec 27, 2015 #3

    Are there many theories pre-bang possibilities?
  5. Dec 27, 2015 #4

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    The question doesn't have an answer. It's the same question as "How big is 2001?"
  6. Dec 27, 2015 #5
    From a popular science point of view It occurred at the same time throughout the Universe and the amount of mass energy released was the mass energy of the Universe - you can't get any bigger than that in either respect.
  7. Dec 28, 2015 #6


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    As far as I'm aware, no real theories - all propositions remain firmly in the realm of hypotheses that still need to develop testable predictions. Google: 'cyclic cosmology' to learn more.
  8. Dec 28, 2015 #7
    How can you state this?
    Aside from cyclic or possibly certain multiverse theories (that have notions fo an overarching 'pproper time'), the very concept of time as a causal parametric is intimately connected to the "fabric of the universe" itself (i.e. spaceTIME - The initiation of which was only resultant from the Big Bang (or the process which constructed the Big Bang) - without the Big Bang, there is no temporal arena, no means to determine any such properties, therefore, no 'time' - there is no external observer from which a frame of reference can be ascertained.

    I am really struggling to understand how you mean occurring 'IN TIME', please can you help?
  9. Dec 28, 2015 #8


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    This confusion probably comes from the two ways Big Bang is used in literature - it can mean either 'the hot, dense state in the early universe', or 'the big bang singularity'.
    The former is just what it says on the tin: a period and a state the universe was some time before now, that extrapolating the observed expansion backwards in time predicts. The latter is what you get if you keep on extrapolating until the model ends up in a singularity.
    The thing is, the BB singularity is not thought to be an actual physical feature in the history of the universe, but more of an indication of limitations of the model that gives raise to it. I.e., the BB theory, strictly speaking, doesn't have anything to say about the genesis, which is what focusing on the singularity might suggest.
    That's why it is preferred to use BB in the former sense, as it describes the actual, physical state in the history of the universe, and is a valid domain of the theory.
  10. Dec 28, 2015 #9

    But it might make sense. Nobody really knows.

    It's not just that we have no testable theory, we don't even have a generally agreed upon theory past about 10-32 seconds after the 'big bang'. There is no real framework to answer the question.

    Wikipedia has a good descriptive passage here:

    "Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinitedensity and temperature at a finite time in the past.[18] This singularity signals the breakdown of general relativity and thus, all the laws of physics. How closely this can be extrapolated toward the singularity is debated—certainly no closer than the end of the Planck epoch. .."

    I'm not sure "all the laws of physics" really break down. I'd prefer to say we may need to transition from the large scale laws of general relativity to inflation theory [which has been done] and then back to the small scale laws of quantum physics, but no one yet knows how to do that or if it can be done.

    Also, click on the Wikipedia illustration under "Introduction" for a visual representation.
  11. Dec 28, 2015 #10


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    No, it doesn't make sense. We're not talking about anything with spatial extent as a property.
  12. Dec 28, 2015 #11
    "I'm not sure "all the laws of physics" really break down."

    Oh they do - largely due to the irreconcilability you mention between GR and QM.
    At a singularity, spatial and temporal) extent becomes 0, so density becomes infinite - QM would require that the universe seeps out from this 0 spatial extent, but SR would forbid this. A point has fewer degrees of freedom, therefore higher potential for symmetry, but the this would not allow for a Big Bang, since entropy tends to increase...
    There are lots and lots of physical principles that simply do not work with a singularity. Our current physics, whilst acceptably accurate for their respective domains (GR for the large, QM for the small) fail to provide meaningful results when applied to these extremes.
  13. Dec 28, 2015 #12
    Since there is no theory, I can't disagree!
  14. Dec 28, 2015 #13
    Nature seems to get along splendidly; it's our current best models that are not up to the job.
  15. Dec 28, 2015 #14
    The actual term "Big Bang" was meant disparagingly, but light-humoured. At the time of Hubble's discovery that the universe consisted of other galaxies and these all expressed motion away from each other etc. the only other theories around consisted of 'steady state' theories in which stars and such were somewhat constant in nature and that the universe existed for infinite duration, or variations whereby new stars would be created and die out striking an overall balance and of course, the religious impositions on ideas too.
    On Hubble's discovery, the implication was apparent that, if everything was moving apart, then in the past, extrapolation logically deduced that everything must have been closer together. This became the basis for the idea that the universe 'exploded' (in terms of perceived motion, not necessarily of any pyrotechnic event) from a pointlike region
    Fred Hoyle, who opposed this notion, referred to it in something along the lines of "I reject the idea that the universe started with a Big Bang" and thus, the theory gained a popular - if rather unimaginative name.
  16. Dec 28, 2015 #15
    Precisely. And our 'laws of physics' are those models. Which break down.
  17. Dec 28, 2015 #16
    Max Tegmark writes ( from the perspective of Inflationary Theory), that the Big Bang occurred within "a region of space...but much smaller than an atom" that it was not at all points, but a defined region of spatial extent.

    The catch here, is that within the theory, regions of a bulk universe can (and necessarily do) undergo inflationary periods, some continuing indefinitely, others ending with an actual Big Bang event and nucleosynthesis. Though our 'universe' is then but a bubble in a larger expanse of spacetime, it takes no more actual volume of that spacetime, therefore this is largely regarded as a 'mutiverse' where each bubble is a separate universe, rather than the traditional all-encompassing universe.

    What of "spectral tilt" within the CBR as predicted by the basic Steinhardt/Turok cyclic/epkaryotic universe?

    Google Cyclic Cosmology to learn more.
  18. Dec 28, 2015 #17
    hmmm since the bb was considered the beginning of the universe it was the size of,.. well,.....the universe (at the time).
  19. Dec 28, 2015 #18


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    Fair enough, they're testable. I should probably have said 'none give predictions allowing to distinguish them from inflationary models with our current observations'. As far as I remember a recent discussion here on PF, the 2015 Planck results were still inconclusive in this respect.

    Heh. Thanks. It wasn't my intention to sound patronising.
  20. Dec 28, 2015 #19
    :) 'll admit, it was lucky I only came across this recently:


    But it's quite interesting (though I feel it's a little more shaping the lock to fit a key).
  21. Dec 28, 2015 #20
    i dont think the entire universe as we see it was compacted in the size of an electron .... does anyone..... it all mathematically works out until we try to explain why things are still accelerating? :) We have all these complex formulas and calculations that all go to pot, and we just interject the biggest variable possible.. a variable greater than the subject matter, that of " Dark Matter", and say that it is causing things to accelerate and not decelerate as we would first guess.
    so many holes, but humans with all the ego, dont want to accept something that can t be explained. there might have been a big bang event, but i dont think everything exploded from something the size of an electron, do you?? im thinking something the size of a galaxy, maybe! :)
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