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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! :)
  22. Dec 28, 2015 #21
    It's Dark Energy that results in expansion, not Dark Matter :)
    But the jury is still out on if there's any direct connection between DE and the Big Bang or Inflation.

    Well, the obvious question is how did the something the size of a galaxy get to be there and get to be the size of a galaxy?
    When considering any concept of the early universe's size, one must consider the 'Horizon Problem' - that there is a seeming connection or "synchronicity" in the energy density of a point on any "side" to its "antipode" - therefore, the upper limit on any size must have been that which allows for light (as causality) to cross from one side to the other.
    Then, there is a minimal size, too, requiring the quantum uncertainty to be stretched through the expansion to result in the variations measured in the CMB today.

    These two problems must be both addressed by any coherent theory of the universe origins.
  23. Dec 29, 2015 #22
    I often here in documentaries about the big bang, that in the first planck time, the universe was smaller than an atom, then a few planck times later it's the size of a grapefruit. This really irks me. What they mean is that the OBSERVABLE universe fit in such a size, the universe itself is much larger than what we can observe, and could be infinite.
  24. Dec 29, 2015 #23


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    A personal observation FWIW. Maybe somebody else will find it helpful.
    I try to avoid using the term "Big Bang" both in writing and in my own thoughts. It is used so vaguely in so many different ways that it really has no meaning.

    Instead (see if this seems reasonable) I talk about "the start of expansion". We see distances expanding according to GR--if we follow it back it seems it must have begun somehow. So "the start of expansion" has some simple concrete meaning--at least to me.

    Different theories or models represent the start differently, so they would give different answers about conditions (density, size of what became the currently observable region, matter content if any, evolution of the Hubble expansion rate i.e. inflation or no-inflation, etc etc).

    IOW there is no one "Big Bang". There are different versions of the start of expansion. So asking questions about the "Big Bang" (as if it meant something definite) tends to lead to confusion.

    For me the easiest version of the start of expansion to imagine, and read research papers about, is the LQC bounce. There you have estimates of the density at bounce. You have computer models run under a variety of conditions---and a solvable equation model. You have definite predictions that can be tested by further observations of the CMB. For example see the recent paper by Ivan Agullo
    Just google "agullo lqc predictions" and you should get
    Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
    We provide an exhaustive numerical exploration of the predictions of loop quantum cosmology (LQC) with a post-bounce phase of inflation for the primordial power spectrum of scalar and tensor perturbations. We extend previous analysis by characterizing the phenomenologically relevant parameter space and by constraining it using observations. Furthermore, we characterize the shape of LQC-corrections to observable quantities across this parameter space. Our analysis provides a framework to contrast more accurately the theory with forthcoming polarization data, and it also paves the road for the computation of other observables beyond the power spectra, such as non-Gaussianity.
    24 pages, 5 figures

    Or if you don't recall Agullo's name, just google "lqc detailed predictions".
    That also brings up the paper as the first hit.
    Last edited: Dec 29, 2015
  25. Dec 29, 2015 #24


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    I share Marcus's discomfort with BB terminology. The way I see it, the BB is a causality based attempt to explain the origin of the universe - which assumes the universe actually had an origin at some finite time in the past. I feel it is perfectly reasonable to dispense with the classical notions of causality, where all events are the determinstic [or quantum probabilistic] consequences of a definable preexisting set of conditions. The obvious question here is what could have preceeded the universe, which by definition unconditionally includes all of physical reality - bearing in mind the 'observable universe' is merely a subset of the totality of physical reality just like integers are merely a subset of rational numbers. A precursor state would, afterall, still meet the definition of a universe. This line of reasoning merely pushes the question of origins back to the origin of the state that led to the BB. It's obviously a classic chicken and egg paradox. Scientists and philosophers are obviously no dummies, so this is less a 'Eureka' moment than an indictment of the limited ability of the human mind to comprehend reality on the grandest of scales. Most scientists are content to say 'we don't know', which is perfectly legitimate yet somehow unsatisfying. At some point we must relax our cherished notions of causality to achieve a deeper understanding of the universe. An interesting try is the universe from nothing idea. Nothing is famously resistant to definitions, the best obvious try is nothing is merely the absence of something and reminiscent of a mathematical proof that 0=0. In the end it becomes little more than an intriguing diversion strugglling under the weight of axioms. As mathematicians well know, proving an axiom is the road to insanity.
  26. Dec 31, 2015 #25
    A couple of the responses in this thread were probably the most helpful things I've read to explain my misunderstanding of the Big Bang (or they just made me feel significantly more confident about a misunderstanding!)

    Can someone help me make sure I understand the model correctly?
    1. ~13.8B years ago, the observable universe was something smaller than an atom (probably the size of the Planck constant, maybe a singularity, etc. -- some debate).
    2. This "universe that was smaller than an atom" was really inside of a much larger (potentially infinitely large) universe. (let's call this "the Larger Universe")
    3. The spacial fabric of the universe expanded enormously fast for a very short period of time (an explosion of space) like a balloon. And then the matter moved away from each other.

    Is that right? And if so:
    1. Do we think the rest of The Larger Universe experienced the same inflation?
    2. Is the same inflation that caused the Big Bang still the cause of continued expansion, or is that something else? (If something else, then what?)
    3. Here's where I'm particularly confused. According to Wiki, here's what caused the Big Bang:
    If the observable universe was really inside a larger universe, does this explanation really make sense?
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