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B Where is the void?

  1. Oct 5, 2016 #1
    I read that the universe is almost 14 billion years old expanding out from the point of the big bang. This implies the universe is an expanding egg with a thin shell. It also implys there should be a void 28 billion years across? I have never heard this ever mentioned. All the galaxies should exist in the thin layer of this shell. moving away from this central point. Where is the void.
     
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  3. Oct 5, 2016 #2

    phinds

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    Undoubtedly read in a pop-sci source, since it is not correct. The universe is 13.7 billion years old but the big bang is not a point, it happened everywhere. There are, to date, approximately 7496 threads on this forum explaining this.

    Well, it would if it were true but since it isn't there is no such implicaiton.

    No, the current Observable Universe is about 96Billion light years in diameter. Again, tons of threads here explaining that.

    As pointed out above, this is nonsense.
     
  4. Oct 5, 2016 #3
    Then, what is all the talk of "a singularity?" A point in space with infinite mass. If there is not point, "it happened everywhere" then it would be infinite before it started? Doesn't "everywhere" imply infinite? So, tons of threads here explain all this. Unfortunately, not all of us have tons of time to explore infinite threads, sadly.

    It was just a question after all. I guess the context of infinite understanding and conscienceless is also an infinite singularity. Seems, the great and powerful singularity of OZ has spoken.
     
  5. Oct 5, 2016 #4

    russ_watters

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    The universe may or may not have started in a singularity (and it would not have been infinite mass), but that may cause confusion since it means every point was contained in one point. I'd start from what we know, just after the Big Bang, when the universe was small.

    Remember, a ruler that's a foot long contains an infinite number of points even though it is a finite length. If it is rubber, you can stretch it. How many points does it contain then? Still an infinite number of points. But still finite in length (and mass).
     
  6. Oct 5, 2016 #5

    phinds

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    You misunderstand. "Singularity" does not mean "point in space" it means "the place where the math model breaks down and we don't know what's going on". The misconception that it means "point in space" is a pop-science fantasy.
     
  7. Oct 5, 2016 #6

    russ_watters

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    While that's true in general, in this particular case "the math model breaks down" because the mathematical model reaches a divide by zero error in the universe having zero volume -- occupying only a point. You may have been referring to one of several other misconceptions in the post (mass vs density, "in space" vs "of space"), but I wanted to be sure the "single point" part was left intact.
     
  8. Oct 5, 2016 #7

    phinds

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    Russ, you are contradicting everything I've ever read here on PF, where all threads have said that singularity is what I say it is, that the big bang singularity was not a point if space but a point in space-time of indeterminant spacial dimensions, possibly infinite, but NOT a point in space, so I meant exactly what I said. If I'm wrong, I'll have to relearn this, but so will a lot of other people here.
     
  9. Oct 5, 2016 #8

    russ_watters

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    Possibly infinite or possibly zero? According to Steven Hawking, it was zero:
    http://www.hawking.org.uk/the-beginning-of-time.html

    This follows logically from a universe that gets smaller and smaller as you travel back in time toward the big bang: smaller and smaller, until reaching a spatial size of zero.

    Here's another that says the universe probably didn't begin with a singularity, but nevertheless describes what that singularity is in the same way:
    https://profmattstrassler.com/2014/03/21/did-the-universe-begin-with-a-singularity/

    Indeed, my reading of the etymology of the term is that "single" refers to a single point or dimension of zero:
    "point at which a function takes an infinite value"
    http://www.etymonline.com/index.php?term=singularity

    Such as 1/x where x=0

    This is more explicit:
    https://en.wikipedia.org/wiki/Gravitational_singularity
     
    Last edited: Oct 5, 2016
  10. Oct 5, 2016 #9

    phinds

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    see Marcus, bolded, below

    Yes, I agree that it approaches zero as a limit but it is my understanding that that is WHY we call it a singularity meaning a breakdown in the model, not a point in space.

    Again, I'm regurgitating what I have heard hear on PF. A quick search turned up these and I'm very confident I can turn up a bunch more. All are from Science Advisors and/or moderators. I have flagged each of them in case they feel that I have taken their comments out of context or misunderstood them.

    @Simon Bridge https://www.physicsforums.com/threads/big-bang-singularity-or-singularities.767069/#post-4829639
    @marcus in the same thread:
    @PeterDonis in https://www.physicsforums.com/threads/black-hole-singularity.816682/#post-5126320
    @marcus in https://www.physicsforums.com/threads/big-bang-singularity.803795/
    and in the same thread (also Marcus, bolding is mine)
    @Bandersnatch in https://www.physicsforums.com/threads/big-bang-singularity.669357/
    @bapowell in [oops ... forgot to copy the link]
    @nicksauce in https://www.physicsforums.com/threads/the-big-bang-singularity.557874/
     
    Last edited: Oct 5, 2016
  11. Oct 5, 2016 #10

    DaveC426913

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    I'm afraid I'm with phinds on this.

    The early universe started off as very dense, very hot and extremely small but there is no indication it was a point. The singularity refers to the limit beyond which our models do not apply.

    A conflation of those two concepts often leads to a simplistic and (in my understanding) erroneous explanation that the Big Bang was a point.

    In deference to Russ, I confess this is simply my understanding, and I do not claim to be an authority.
     
  12. Oct 5, 2016 #11
    The void is the earth, as it defines the center of our observable universe.

    Infinitum actu non datur.
     
  13. Oct 5, 2016 #12

    phinds

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    Hard to tell if you are being silly or serious. Yes, the Earth is the center of the OU but if you are serious then I have to say, that's not really relevant to the OP. The Earth is not billions of light years across which is the "void" the OP is asking about
     
  14. Oct 6, 2016 #13

    Chronos

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    The homogenity of the universe [meaning it is of similar matter density across all volumes of space at sufficiently large scales] is an observationally well established fact. So, your putative void does not exist in reality. There are plenty other examples of singulariies that exst mathematically, but, do not exist in nature. Nature abhors infinities.
     
  15. Oct 6, 2016 #14
    I don't have any idea what the OP means when he asks about a "void" in the universe. It seems he was comparing the universe to an egg, whereby the matter existed on the ever-expanding surface and the middle was nothingness.
     
  16. Oct 6, 2016 #15

    phinds

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    Yes, that is my interpretation also of his complete misunderstanding of the universe, but he does describe the inside of the egg as "a void 28 billion years across" so he clearly does not mean the Earth.
     
  17. Oct 6, 2016 #16
    I tried to interpret the reasoning behind his question. The 14 billion year reference seems to refer to the number of light years it took for the stars at the farthest reaches of the observable universe to reach the Earth, which would create a diameter of 28 billion light years across. The best that I could extrapolate was a question pondering the center of the big bang relative to our observable universe, which constituted the inner part of the "egg" analogy.

    In other words, I was trying to show him the underlying nature of his question and the best fit answer.
     
  18. Oct 6, 2016 #17

    phinds

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    Very reasonable, and of course I agree w/ you about how he came up w/ 28billion. That's a very common misconception. I do think though that your original answer was too terse to get across the point which I now understand you were intending to make.
     
  19. Oct 6, 2016 #18
    You are correct. I will try and do better in the future.
     
  20. Oct 6, 2016 #19

    russ_watters

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    Ok, first I want to say I'm ok with being wrong, but if I am I'll be very disappointed in Stephen Hawking because that will mean he said something that just plain isn't true (not just an oversimplification) and I can't see why.

    Next, let's make sure we're addressing the right issue, because most of the "answers" there were to the wrong questions. Here's the three related questions:

    1. What, in general, is a singularity? (a: a mathematical discontinuity, such as a divide by zero error)
    2. What, specifically, is the nature of the Big Bang singularity? (My answer: r=0 @ t=0)
    3. Do scientists believe the universe really started with a singularity? (a: no)

    The question we're addressing here is #2 and if my answer is not correct, I'd like to know what the correct answer is. To be specific, I'd like to know what the relative diameter of the universe was at t=0 as implied by the equations....or, as I've seen it expressed, the average distance between galaxies or density.

    Because I've seen over and over again it stated by sources that look credible that at the singularity, the universe's density and temperature would have been infinite. That's an awfully specific thing to be flat wrong and can't see what it could be an oversimplification of (that could be explained too...).
    If the quibble here is on the difference between being zero and approaching zero (with the density and temperature approaching infinity instead of being infinite), I think that would be an awfully petty quibble, since I think even most laypeople understand that "infinity" is not itself a number.
    That's specific in the initial statement, but very non-specific with the explanation, so it doesn't satisfy my question. Specifically: what infinities do you get as you approach the start of the expansion? Density....? And does this answer apply to a finite universe?
     
  21. Oct 6, 2016 #20

    russ_watters

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    I'm aware that scientists reject the results of their models when they rewind back to t=0, but that isn't the question. The question is: what is the parameter that they are looking at (and its final value or limit) that causes them to reject the model as erroneous? Is it density = infinite? That's what Hawking said.

    I know this is simplified (let's call it Russ's Model), but as an example, you could describe an expanding chunk of the universe like this:
    r=kt
    Where
    r = radius
    k is a proportionality constant
    t = time

    Thus, at t=0; r=0
    And given a finite mass m, the density of my universe is:
    d=m / (4/3 pi (kt)^3))

    And oops -- as t approaches 0, d approaches infinity, with a discontinuity or singularity at t=0. Unless it's beyond my mathematical grasp, I'd be curious to see the actual equations that produce this graph (mine appear to me to produce the blue line) and I'd like to know specifically what fails when you plug in t=0:

    glossary_critical_density.jpg
     
  22. Oct 6, 2016 #21

    phinds

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    As am I. As I have said, I'm merely regurgitating what I have heard here and, like you, I am much more interesting in getting it right than in just THINKING I have it right.
    I understand that as well, although I was under the impression from Marcus's post that this was Hawking writing for laypeople, not Hawking being wrong.

    I think THIS is the heart of the matter, as I understand it. My belief is that it is NOT at t=0 where the problem starts, it is at some extremely small amount of time PRIOR to t=0 where we stop understanding what is going on. If I am wrong about that then I'm way less confident in my interpretation of what others have said. Sadly, Marcus is not with us to defend his statement, and I was hesitant to include it for that very reason, but I always held him in great esteem so included it anyway.

    I hope that some of the other people I quoted will weigh in. I'm not really the person to defend this concept.
     
    Last edited: Oct 7, 2016
  23. Oct 6, 2016 #22

    phinds

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    Last edited: Oct 7, 2016
  24. Oct 7, 2016 #23

    Simon Bridge

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    iirc the current cosmological physical models tend to need fixing down around the scale of the plank length ... I'd have to revisit since I'm probably out of date.
    For OP's question it is not all that relevant - OP is confusing the pop sci language used to describe black holes with the pop-sci language used to describe the big bang. A nice reference for @lhedrick should cover the points well enough:
    http://map.gsfc.nasa.gov/universe/bb_concepts.html

    We also need to be careful about terms like "relative size of the Universe" when considering something that seems infinite and flat.
    Also the nature of the singularity and what exactly is being referred to as "the big bang" tends to depend on the model referred to.
    In accord with PF policy: the standard cosmological model would be the ##\Lambda##CDM model ...
    https://en.wikipedia.org/wiki/Lambda-CDM_model (fwiw)

    The "size" may be related to the "scale factor" "a". The usual anayltic solution ##a(t)=(\Omega/\Omega_\Lambda)\sinh^{2/3}(t/t_\Lambda)## tells us that a=0 when t=0. Which would be the singularity for this theory... however, there are a lot of ideas about what happens at very small times, and not everyone uses "the big bang" to mean the same thing.

    "The big bang" would most properly apply to the rapid expansion that started sometime before "t=1 plank time" ... that is, time started before then too, but maybe not exactly t=0 ... that is just where this model predicts infinite density (just like absolute zero temperature for a gas). That could give the Universe a non-zero "a" when time becomes a thing. So we have a difference between t=0 from the ##\Lambda##CDM model and t=0 in the sense that time started to mean something (other than this other distance you could move into) then. Aaaaand this is where I need a stiff drink.

    All the while I am aware of being somewhat simplistic too - and I am not accounting for the wealth of work in this field.
    I'm also probably a bit out of date.
     
  25. Oct 7, 2016 #24

    Bandersnatch

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    @russ_watters @phinds
    Saying that the cosmological singularity is spatially a point has a number of issues.

    Does it mean it's got a defined location somewhere in a higher-dimensional space? No. It contains all the space, so it's everywhere.

    Does it mean it's 1-dimensional object? No. At no point in describing the evolution of the universe do we get to change from a 3D space to a lower dimensional one. This is especially apparent when you look at an evolution graph drawn in comoving coordinates:
    lightcone com vs proper time.PNG
    Where dotted lines marked with redshifts represent comoving points in the universe. At scale factor a(t)=0 you do get, just as Hawking wrote, everything on top of each other, but you still have all the points in space as separate. At no point does the model predict them to fuse into a single entity. They just get to have their separation shrink to 0.

    Can we even talk about the spatial extent (radius, volume, whatever) at the singularity as being 0? Well, maybe if the universe is finite you could get away with it. But if it isn't, then you need to contend with yet another batch of infinities as you try to shrink an infinite distance to 0, with infinite contraction velocities. This is in addition to all the other infinites that you get at a(t)=0: infinite density, pressure, temperature.

    So, does it mean that the cosmological singularity does not conform to the definition russ posted earlier? That it is a point at which a function reaches infinite values? No, because that point is in the temporal dimension. It is at the value of t for which a(t)=0 that you get all the infinities in the evolution model. It's a point in time, not in space. It's not correct to say that the spatial extent at the singularity is zero, in the same way as it isn't to say that temperature is infinite - the model, and all of its predictions, is simply undefined at this point (in time).
     
  26. Oct 11, 2016 #25
    . Where is the void.[/QUOTE]
    This sounds like a "boundary" question. I have trouble following the definition of terms concerning around what seems to be different types of boundaries. As far as I can tell, which I suppose you can tell isn't far, there is no reason to believe in the embedding in a higher dimension; but there is also no definitive answers on where or if there are boundaries.
     
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