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Duration of the Big Bang

  1. Oct 30, 2015 #1
    Do we actually know if the Big Bang was an instantaneous event, that is an event not measurable in time?
    Or, would it be possible that the Big Bang had a duration, a length in time?
    In other words, was the Big Bang more like, say, an explosion or more like a volcanic eruption, and how do we know that?
     
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  3. Oct 30, 2015 #2

    phinds

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    There really are two meaning to the phrase "big bang". The first, the one you mean, is more properly referred to as the "big bang singularity" and the second is "the cosmology theory that describes the evolution of the universe after, and not including, the singularity".

    The word singularity in this context does not at all mean "point", it means "the place where our model gives nonphysical results and we don't know WHAT was happening". The big bang singularity happened everywhere at once and it's size and duration are unknown. It might have been infinite or finite. Mathematically it is taken as an instant in time but that's just the math. We have no idea, really.

    EDIT: Oh, and it was NOT an "explosion". Those happen at a point. The BB singularity did not.
     
  4. Oct 30, 2015 #3
    Thanks for your input, but allow me to go further.
    The Big Bang was a release of energy, lots of it.
    What I mean by "instantaneous event" is that all the energy available in our universe today was released at once.
    On the other hand, if the Big Bang was, or is, a "continuing event", the energy could have been released incrementaly.
    There comes my analogy with a volcanic eruption.
    First, under enormous pressure, a huge amount of energy is released, together with a toxic cloud and so on.
    Then, things get quieter and the lava keeps flowing along the sides of the volcano...the volcano is still erupting, but it not quite the same thing as the initial explosion.
    Back to the Big Bang, there could have been an initial massive release of energy, but then things didn't stop, they only got quieter...and invisible, yet detectable.
    Under the initial conditions, matter was created and together with it the observable universe.
    Then, after the initial burst, much more energy was released, but it didn't meet the required conditions to be turned into matter.
    Instead, part of this energy became what we call dark matter and the rest remained as what we call dark energy.
     
  5. Oct 31, 2015 #4

    PeterDonis

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    This is true if by "Big Bang" you mean what happened at the end of the inflation era, when the matter and energy in our current universe was first formed in a very hot, very dense, rapidly expanding state. But this event is not an "initial singularity" despite what many pop science presentations say. See below.

    This is not quite correct. The matter and energy that appeared at the end of inflation wasn't created out of nothing; it came from the energy contained in the field that caused inflation. Whether we say our observable universe was "created" at the end of inflation depends on what form of inflation theory we use.

    The end of inflation also did not happen everywhere in the universe at the same instant; it happened over a period of time (a very short one by our current standards, but still a period of time).

    I'm not aware of any cosmological theory that says this. All the theories I'm aware of agree that ordinary matter and radiation, dark matter, and dark energy have all existed since the end of inflation.
     
  6. Oct 31, 2015 #5
    Thanks for your extensive reply and explanations.

    Since you mention time...very short periods of it...I must say this is also something that makes me wonder.
    My question is: did time really exist from the very beginning, or said differently did the clock start ticking with the Big Bang?
    It is not because time, or at least a measurement of it, is required for our mathematical models that it existed in the absolute.
    Time appears to be rather elastic, stretching from nothing if you are a photon to the passage of time as we experiment it.
    Yet, it seems that time is somehow related to mass, since there is no time for massless objects travelling at light speed.
    At the very beginning of the universe, there was no mass, so how was there time...at least from the point of view of the observed, the early universe, not from our human point of view?
     
  7. Oct 31, 2015 #6
    If the universe is infinite, then it contains an infinite amount of energy.
    But how could a Big Bang happening in a finite and extremely short amount of time release an infinite amount of energy?
    In other words, for the universe to be infinite, isn't it necessary that the Big Bang is also infinite in time?
    Going further, if at some point, during its early expansion phase, the universe had the size, say, of a golf ball, that means it was finite in size.
    But then, how could something finite in size evolve into something infinite in size?
     
  8. Oct 31, 2015 #7

    Bandersnatch

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    All you need, is to consider the universe at BB, whatever it was, to be infinite as well, so that in every finite unit volume there is always finite energy content.
    The 'golf ball', or any other size approximation for the early universe you might see or hear about describes only the observable universe at some specified time in its evolution. It should never be taken to mean the entirety of the universe, as its extent is unknown. Both infinite and finite (but very large) extent is possible, and whichever of the two is true of the universe now, was likely true of the universe always.
     
  9. Oct 31, 2015 #8
    Thanks for your explanations.
    Regarding the Big Bang, isn't there some kind of contradiction in saying that the BB happened in an already infinite universe?
    I agree that in order to be infinite, the universe had to be infinite from the get go, but the BB is at the origin of the universe, thus it predates the universe and cannot happen inside it.
    Or am I wrong, or not understanding what you mean?
    Regarding the golf ball, things get even more complicated, at least for me.
    What you say would mean that there is an observable and finite universe expanding inside an infinite universe.
    Things could be explained differently if time, the fourth dimension, didn't exist from the very beginning of the universe, but that the clock only started ticking when matter and mass appeared.
    What would be the meaning of time before that, other than for mathematical purpose that is?
    So, if time was not turned on in the early stages of the universe, because everything was moving at the speed of light (no mass), the universe was able to expand from a point to infinity...instantaneously, even if from our human point of view, we measure things differently.
     
  10. Oct 31, 2015 #9

    PeterDonis

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    There is no way to answer this question. "Time" is not an absolute thing, and there is no one "clock" that applies to the entire universe.
     
  11. Oct 31, 2015 #10
    I am following the discussion with interest.

    Could someone please clarify what we mean by “infinite” in this context? Is it not so, that the universe is everything, in its broadest possible meaning, disregarding multiverses? In this case, the universe must be infinite and was always infinite. The problem seems to be that we can’t imagine infinity, so we are always imagining a finite sphere or other shape, which is wrong. Otherwise I am not understanding what “universe” means.

    Could someone clarify the terms please?
     
  12. Oct 31, 2015 #11

    Bandersnatch

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    You're probably thinking of the BB as a point in pre-existing space, right? A place from which all matter emerged to fill the universe? Is that the correct representation of your perception of BB?
     
  13. Oct 31, 2015 #12

    Bandersnatch

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    No, it doesn't have to be infinite.
    We're talking about its spatial extent. If it is infinite, it means that the metric space of the universe is unbounded. That is, you can go in at least one direction infinitely, and never get back to the starting point*. Or, equivalently, there is no volume large enough to contain all of the space in the universe.
    The space of the universe could be of a closed shape (e.g. 4-dimensional torus or sphere), making it finite (i.e., bounded, but without a boundary). That is, in such a universe you could draw a sphere of volume large enough to contain all of space, or move in any direction and eventually get back to your starting point.

    So, to put in yet another way, the everything in the broadest possible sense might turn out to be finite.

    *in an expanding universe this is true also of a bounded space, due to the limitations of the travel speed (c). What I mean here, is a 'magical' travel with arbitrary speed, or travel in a 'frozen' universe where you don't have to worry about cosmological horizons.
     
  14. Oct 31, 2015 #13

    The idea of getting back to the starting point seems to be without foundation. It is not possible because the starting point would cease to exist and would not be discoverable. I am saying that I can go in one direction infinitely because I will never find a boundary. If the universe has no boundary, then it goes on forever, and that is why I wrote that it is infinite. Similarly, I think that if I did this shortly after the BB, the result would be the same.


    I don’t understand what you mean by a “closed” shape, nor what you mean by “draw a sphere of volume large enough to contain all of space”. This is impossible. Calling on geometrical shapes does not solve the problem. It even makes it worse. How can you have a universe with a “shape” when the universe is everything?


    I don’t mean to hijack the thread, I am only wanting to clarify the size of the BB issue.
     
  15. Oct 31, 2015 #14

    Bandersnatch

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    Let's take it one dimension down, and imagine a 2D universe. This is to make the shape easy to imagine for us 3-dimensional beings.

    If the shape of the 2D universe is that of a flat plane, it is infinite. It has no boundaries, since you can't point to a place that is not on the 2D plane (remember, the 3rd dimension doesn't exist as part of the universe in this analogy). It is not bounded, since no matter how large a circle you draw on that plane, there will always be points not encompassed by this circle.

    If the shape of this 2D universe was that of a sphere (again, we're talking only about the 2D part of the sphere, i.e. its surface), then the universe would have no boundaries, but be bounded. That is, by the definition of a bounded set, you can draw a circle on the surface of a sphere of radius large enough to encompass all the points on the surface of this sphere. In this case, the radius needs to be equal to half the circumference of the sphere.

    Going back to where you started on a surface of a sphere should be also easy to imagine.

    An important point to note here, is that while the third dimension in which we embedded the sphere in this analogy, to help with visualisation, is not necessary for the 2D surface to have a well-defined shape. Even without embedding, if your 2D surface has certain properties, like the sum of angles in all triangles adding up to more than 180 degrees, and all parallel lines eventually intersecting, this 2D surface will have the shape of a sphere.

    This works in exactly the same way for any number of higher dimensions.
     
  16. Oct 31, 2015 #15
    Thank you for the analogy, which has been quoted before. Some students have remarked that the 2D space analogy does not help them, and I also belong to this group.

    I prefer to discuss the 4D universe which we seem to live in. The OP asked about the timing of the BB and inflation and he later added a question about the spatial aspect.

    I see it like this, based on my limited knowledge. I am grateful for your expert comments.

    BB theory and its timing depends heavily on our observation of the increasing expansion of the visible universe. We extrapolate backwards and arrive at the BB about 13.8 bn years ago. Possible criticisms of the calculation include that the sample of galaxies is a long way off 100% and that we don’t have much data of previous rates of expansion. I am aware of research that indicates a slower rate of expansion in the past. Nevertheless the calculation of 13.8 bn years is widely accepted.

    Decoupling occurred at about 380 million years after the BB. During this period matter was created, photons were confined and then released as CMBR.

    So the first question which comes up is, how do we know that the formation of the universe until decoupling took 380 million years and if it did, what does this mean? PeterDonis already remarked, “"Time" is not an absolute thing, and there is no one "clock" that applies to the entire universe.” So is it not also true that there is no one “clock* that applies to the universe at all times in its history? If there is not, can we conclude from this that we have little idea about the timing of the early formation of the universe in terms of today’s earthly clock. Does the 380 million years really mean anything in today’s terms?

    The second question relates to the spatial dimensions of the universe, which I already commented in part. My understanding is that during the inflationary period, the universe was a hot, dense, crowded-with-matter-and-energy place. Further, that the inflation happened everywhere, meaning in the whole universe. Is it not the case, that the universe was everything then and is still everything now? So everywhere you travel and for however long you travel, you will remain in the universe, because that is all there is (ignoring multiverses). It doesn’t matter what imagined shape or size the universe has. Since the universe is expanding faster than the speed of light, you can travel infinitely. If you somehow run out of matter to overtake, you will expand the universe yourself.

    Putting it all together, I don’t see how we can say that the universe could be finite. Furthermore, if we were able to imagine a spatially finite universe now, then we would have to imagine a spatially finite early universe in its formation too. I think that this goes against current theory, or at least against current opinion. It comes back to your question to the OP, “You're probably thinking of the BB as a point in pre-existing space, right?”
     
  17. Oct 31, 2015 #16
    Not at all.
    Space was created from the BB, so the BB could not take place in pre-existing space.
    Yet the BB could either start from a point, or happen everywhere, not in, but with the infinite universe.
    If time, the fourth dimension, did not exist at the BB and until there was matter and mass, then the universe could start from a point and expand, or inflate, infinitely, in no time!
    Thus, it was both point sized and infinite, at the same time...so to speak.
     
  18. Oct 31, 2015 #17
    I agree with you, but yet science assumes, mostly for mathematical purposes, that time existed from the get go, and maybe such assumption leads to wrong conclusions.
    According to the models, at the very beginning, inflation seems to happen in a fraction of a fraction of a second, which is mindboggling.
    But maybe it is not, because if the clock is not ticking, inflation has all the time in the world, so to speak, in order to take place.
    With our mostly mathematical tools, we try to observe something that happened at a time, so to speak, when there were no observers, and the conditions were not met to harbor observers anyway.
    I am not convinced that this is possoble..
     
  19. Oct 31, 2015 #18
    You are not hijacking the thread and I agree with you regarding the measure of time beyond a certain point.
    I just posted a couple of answers about this issue.
    Yet, you make me raise another question which is: what is the meaning of an expanding, yet infinite universe?
    How can something infinite expand?
    Isn't that contradictory?
     
  20. Oct 31, 2015 #19

    phinds

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    Uh ... I'm thinking you'd better find a different way to speak. That doesn't really make sense.
     
  21. Oct 31, 2015 #20

    phinds

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    Yes, it certainly is.
    No, when we say it takes a fraction of a second we are using the current definition of a second. That's what it takes a fraction of.
    The universe never cares what we think, it just does what it does.
     
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