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Please explain the statement "the big bang happened everywhere at once"

  1. Nov 11, 2014 #1
    Obviously, I can understand the literal meaning of the phrase "the big bang happened everywhere at once" But I have never read a satisfactory explanation that eloquently helped me understand this concept........
  2. jcsd
  3. Nov 11, 2014 #2
    Space-time mathematically starts at a point. There is no space or time outside of the singularity at the start of the Big Bang. There was no time or space at that point. Then things change
    and time and space are created at that point; they are linked together and define each other.

    The Big Bang theory is an abstract construct created through math and geometry. What happened before the Big Bang in my view a distraction.

    "Where is the center of expansion in the universe today" a student asked.

    "All the points in the universe today are the center of expansion of the universe today because all points where the same point in the beginning" the professor replied.

    Yes - the BB happened at a point in time not in space. Because; to me, the time differential logically preceded the space differential. So for the Big Bang model there is no space outside of space time.

    Just remember 95% of the universe is covered with "Dark energy and matter" which is saying we know nothing about 95% of the observed universe. On a human scale we know a lot but on a cosmic scale we know little, but we have good leads.
  4. Nov 11, 2014 #3


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    Imagine a volume of space. It may be either infinite in extent, or finite in a way that curves on itself, so that going straight in any one direction makes you come back to where you started.

    This space is filled with some sort of energy at high density and high temperature.

    Pick any number of points in that space, and measure how far away they are spaced.

    At some point in time, all the distanced you had measured begin to increase simultaineously. As a result, the temperature and density of all the energy filling the space drops. Where once there was only radiation, particles and antiparticles begin to pop up and manage to stay around longer and longer without annihilating. Finally it gets cold enough for the electrons and protons to combine which in turn allows light to move freely. That light is the CMBR. The distances keep on growing, and the stuff filling the space keeps on cooling and getting on average less dense (although local clumps of matter coalesce to eventually form galaxies).

    That's pretty much the Big Bang theory. It states that the universe as we see it today expanded from an earlier hot and dense state. Nothing more. That's why a question of "where did it happen" is not really applicable. It happened in the universe, or it happened everywhere are the best answers one can come up with, but the real answer is that the question is just not very sensible - the only reason it keeps getting asked is that BB is often, and misleadingly, described as an explosion.

    A natural follow-up question to ask is: but what happened earlier than that? Where did all that energy come from?
    If the BB were a person, it'd say: "Don't know, don't care. Not my jurisdiction".

    Another source of confusion is the distinction between the BB theory and BB singularity. The singularity is what you get if you try and extrapolate the expansion backwards in time until you reach infinite densities and infinite temperatures at t=0. This is still a statement about the totality of space, and the reason people tend to confate it with a point in space is probably due to the fact that they first tend to come in contact with the concept of singularity in the context of black holes, whose singularities are of the spatial variety. It's important to remember that singularity is just a region where mathematics breaks down. The function ##y=1/x## has got a singularity at x=0. It's just an indication that the function is undefined for a certain value of the variable.
    Last edited: Nov 11, 2014
  5. Nov 11, 2014 #4


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    It's almost a matter of philosophy. The answer could be ... In the beginning, there was nothing - and it became everything: or, if you prefer; In the beginning, there was everything - and we still have most of it left.
  6. Nov 11, 2014 #5
    That phrase is used to keep people aware that there is no center of the universe and that popular artistic depictions of the Big Bang (you know, the little bright light emanating things) are not correct. Like they explained before, if the universe is infinite or it's curved so that is finite but there are no boundaries (think of the balloon analogy), then all points started separating from each other, thus "it happened everywhere at once".
  7. Nov 11, 2014 #6


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    I agree w/ your post but this opening statement promotes the mistaken belief that it started at a point in SPACE, which it did not (as you clearly point out later) since if it had, there would be a center to the universe and there would consequently be a preferred direction to the motion of the expansion. There is neither.

    I realize I'm not telling you anything you don't already know, I'm just trying to help others who read this thread to not get caught up in that fallacy.
  8. Nov 11, 2014 #7


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    Actually, strictly speaking, it doesn't, because the point singularity is not actually part of spacetime--it can't be, because the values of physical invariants (like the curvature scalars) are infinite there. The point singularity is really a limit of spacelike hypersurfaces that have a smaller and smaller scale factor as you get closer and closer to the limit. Each such hypersurface represents an instant of cosmological time, so you are correct that the Big Bang is best viewed as a moment of time, not a place in space.
  9. Nov 12, 2014 #8
    Supposing that the universe is infinite, then it is infinite for all times, there's just a change in a scale factor. What bugs me is how some universal happening can happen everywhere at once. We marvel at synchronized swimmers doing their act in sync. It seems that all of infinite space had to do something on cue to get things started, and this contradicts my intuition as to how this is possible.
  10. Nov 12, 2014 #9
    That depends on what you mean by "big bang". As you can see below, you can redefine "big bang" in order to push it further and further back in time. [If you find it practicable. I sincerely don't know why people insist on doing this. It is my failing of imagination.]


    Inflation removes the singularity that used to be attached to the start of the Hot Big Bang. As I understand it (not having studied general relativity) the removal is inherent:

    "Hence general relativity cannot be used to show a singularity.

    Penrose's theorem is more restricted, it only holds when matter obeys a stronger energy condition, called the dominant energy condition, which means that the energy is bigger than the pressure. All ordinary matter, with the exception of a vacuum expectation value of a scalar field, obeys this condition. During inflation, the universe violates the stronger dominant energy condition (but not the weak energy condition), and inflationary cosmologies avoid the initial big-bang singularity, rounding them out to a smooth beginning."

    [ http://en.wikipedia.org/wiki/Penrose–Hawking_singularity_theorems ]

    Maybe something that could be said to break down spacetime reasserts itself "later" if we manage to look further back in time. (I assume then as a result of exotic physics.)

    In the meantime, it is much more generic to identify various definitions of big bang to a certain time, meaning the space volume experienced it "everywhere at once". (Whatever "it" means for you.)
    Last edited: Nov 12, 2014
  11. Nov 12, 2014 #10
    So far something like "all of ... space had to do something on cue" (or contra-intuitively, "infinite") has not been a necessary description.

    The scale factor can expand space faster than the universal speed limit of anything within space, and the inflationary mechanism that solves the horizon problem so it appears everything happened on cue involves having a finite volume described as the observable universe..

    The same appearance goes for the Hot Big Bang events, the only difference is that they happen over a larger volume (the local universe) intersected by the observable universe.
    Last edited: Nov 12, 2014
  12. Nov 12, 2014 #11


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    The Big Bang is not a "happening" in this sense. In the simple model with an initial singularity (i.e., without inflation and the refinements that leads to, which Torbjorn_L described in his post), the Big Bang just refers to that initial singularity, which is a boundary of spacetime, not a "happening". In inflation models where there is no initial singularity, the Big Bang just refers to the fact that, when the inflation period ends, the universe is filled with matter and radiation in an extremely hot, dense state, and is expanding very fast. This is not a single "happening", it's just a global description of the state of the universe at that instant of cosmological time; the "happenings" are the individual bits of matter and radiation in each little volume of the universe.
  13. Nov 12, 2014 #12
    Do cosmologist who believe in an original singularity believe that infinity can originate out of point? Or does a singularity necessarily imply a finite universe? Or conversely, would an infinite universe negate the singularity belief?
  14. Nov 12, 2014 #13


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    "Singularity" just means "the place where our math model breaks down and we don't know WHAT the hell was/is going on". ("was" in the case of the BB, "is" in the case of a black hole). It iS NOT a point.
  15. Nov 12, 2014 #14


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    No, they believe that the singularity is not part of spacetime. See post #7.
  16. Nov 12, 2014 #15
    Guywithdoubts said: That phrase is used to keep people aware that there is no center of the universe and that popular artistic depictions of the Big Bang (you know, the little bright light emanating things) are not correct. Like they explained before, if the universe is infinite or it's curved so that is finite but there are no boundaries (think of the balloon analogy), then all points started separating from each other, thus "it happened everywhere at once".

    Thanks for all the replies. Very good information. However, I need to ask another question. Am I mistaken in saying I think the phrase "the big bang happened everywhere at once" could be rephrased as "inflation happened everywhere at once." Having asked that, even I dont think that is correct because, if I remember correctly, inflation began a micro-fraction of a second AFTER the big bang.
    Yet alot of you seem to be referrencing inflation in your answers.
    Am I making it harder to understand than it is? Is my "eloquent" answer simply that space time started at a "point" (I know this is not correct but I dont know how to phrase it right) and then inflation expanded space/time to "everywhere at once"?
    I also want to sk about the universe having to center. If the universe is infinite, that is understandable. But if the universe is finite, it is harder to understand. How can any finite geometric shape have no center?
    P.S. I am very happy to have found a place to enrich my understanding of life, the universe, and everything. ;)
    Last edited: Nov 12, 2014
  17. Nov 12, 2014 #16


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    You are getting confused because of the two radically different ways the term "big bang" is used. One way, the only way in my opinion that is meaningful or helpful, is the "Big Bang Theory" which is a very (but not completely well understood description of how the universe evolved starting at about one Planck Time after the "singularity" and saying NOTHING about anything before one Planck Time (other than "don't know WHAT was going on back then") and the other is just a reference to the singularity (more properly called the Big Bang Singularity) which is just a place where the math model breaks down and something we can't really say anything meaningful about (other than "don't know WHAT was going on back then")

    Also, I would suggest that you refrain from saying that anything started out at a point and just say that it started out at a TIME and we don't know HOW it started out but we know a lot about what happened since then.
  18. Nov 12, 2014 #17
    I find this to be a very interesting question. Could somebody please give a thoughtful reply?
  19. Nov 12, 2014 #18
    For an infinite universe, I don't believe that shrinking the scale factor will every result in a point. A point is finite. An infinite universe does not extrapolate back to a point. It never resembles a point at any stage, ever. A point-singularity is absurd.

    But an infinite singularity ( sounds oxymoronic ) also is confounding. How can time start everywhere at once. It's as if we had an infinite number of marathon runners poised on an infinitely long starting line and God says "on your mark, get set go" and he fires a track pistol. Or how could inflation begin or end everywhere at once, as if a memo were distributed "inflation will begin in one second" and the whole infinite universe is going to get the word.
  20. Nov 12, 2014 #19


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    The point is not part of spacetime (see my previous posts), so it's not the result of "shrinking the scale factor" the way you appear to be thinking of it.

    However, it is true that the limit of the scale factor as the singularity is approached is zero. This could be described (sloppily) as "shrinking to a point". But it's not a "shrinking" in any physical sense; it's just a mathematical statement about a limit.

    Time doesn't have to "start". It's just a coordinate, a way of labeing events. Or, if you like, a way of slicing up spacetime into an infinite sequence of spacelike slices, each of which is labeled with a "time", and each of which represents "space at an instant of time". If the universe is spatially infinite, then each of those spacelike slices is infinite.

    We actually don't know that it did, and inflationary models don't require it to. They only require that inflation began and ended homogeneously within our observable universe--which, since our observable universe was much, much, much, much smaller when inflation ended (smaller than an atomic nucleus, IIRC), does not present any problem.
  21. Nov 12, 2014 #20
    Actually that's how they describe and imagine singularity -- infinitesimal scale/density and the direction of the pure equation. Unless some dynamic is introduced. It can either bounce, tunnel or both depending on how we model reality. We really don't know. IMO It is more intuitive to think that our universe is in a state of transformations/transitions and identify each extent/event in a temporal fashion than weak origins and creation.
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