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B Is the center of the universe empty?

  1. Feb 20, 2017 #1
    If the big bang through matter in every direction away from itself (the center) shouldn't there be a huge void there?
     
  2. jcsd
  3. Feb 20, 2017 #2
    See https://www.physicsforums.com/threads/center-of-the-universe-what-again.878741/
    See https://www.physicsforums.com/threads/there-must-be-a-center-of-the-universe.845032/
    Short answer: There is no center of the universe.
    Take the example of a balloon and blow it up. Even though it is getting bigger you cannot find a center. However from the point of view of an observer (you for instance) the furthest you can see in all directions is what is called the observable universe and from this point of view you are indeed in the center. The idea of a big bang explosion going into all directions is not what really happened, it is popular science that made that appear as true.
     
  4. Feb 20, 2017 #3

    Drakkith

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    The big bang was not an explosion in the sense that everything is moving away from an initial location. The big bang refers to the rapid expansion of space that occurred in the early universe. Expansion means that the distance between all unbound objects increases over time. Unbound means that the force of gravity (or other forces) is not strong enough to keep two or more objects together against expansion. We usually talk about galaxies and galaxy clusters, since they are typically the minimum scale at which expansion overpowers gravity. One of the key points is that for any observer, anywhere in the universe, distant galaxies will appear to recede directly away from them.

    So here in the Milky Way, all galaxies other than the ones in our own galaxy cluster and supercluster are redshifted and moving away from us over time. Yet, at the same time, an observer 1 billion light-years away would look up into their sky with their telescopes and see distant galaxies (including our own) redshifted and receding directly away from them.

    That is what the expansion of space means. Not expansion away from a single point, but the increase in distance between all unbound objects.
     
  5. Feb 20, 2017 #4

    phinds

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  6. Feb 21, 2017 #5

    Chronos

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    The notion of the BB as the mother of all explosions has persisted despite the efforts of cosmologists to clarify the meaning of observational evidence. Consider this: if the observable universe began as a Planck size point that flew apart in every direction to become what is now the entire observable universe, then we are forced to accept a shocking truth - we are the most distant and ancient point in the observable universe relative to the BB. Due to the finite speed of light, everything we see in the distant universe must be younger, and consequently less distant, relative to the BB point of origin. No matter where you are now in the observable universe, you are the victim of this same illusion simply because you, and all of those other 'places' you see in the distance, were once a part of the same tiny point in the very beginning.

    It is, however, equally valid and logical to assume the universe started out infinite and remains so today. If we could see that the most distant visible object in the universe was, say, a billion light years distant in one direction, and 2 billion light years distant in the opposite direction, the idea the universe sprang from a specific point within the observable universe would be viable. Even if we cannot see the entirety of the universe in any one direction, it would still hint at being finite if it expanded a little faster in one direction than in the opposite direction. That is not, however, what we see. As we build bigger and better telescopes we see even more distant objects in every direction and the rate of expansion also appears to be the same in every direction. These two facts lead to the inescapable conclusion the universe must either be infinite or far vaster than we can observe in order to exhibit no discernible difference in its matter distribution, or rate of expansion, regardless of which direction we look. Thus, an infinite universe is the popular choice among cosmologists. A universe that is infinite now must always have been infinite, because a finite entity cannot grow to infinite size. This sums up why we believe the universe did NOT originate at any specific point within the current observable universe.
     
  7. Feb 21, 2017 #6
    If the universe is in fact infinite and the cosmological principle holds true, there should be an infinite amount of matter in the universe. Does this mean that an infinite amount of matter was packed into the size of a pinpoint prior to the big bang?

    That doesn't seem to make any sense. What am I missing here?
     
  8. Feb 21, 2017 #7
    Well, thank you everyone. Even though most of it is over my head I did learn somethings. I have no formal education in physics but watch every science program on cable including Brian Cox's recent series. He loves saying billionths of billionths.......of a second after the start of the universe everything was say 100 feet wide which reinforces the idea of a center.
    What this forum could use is a 'For Dummies' section that explains these ideas to folks with no physics background.
    Thanks again,
    Ed.
     
  9. Feb 21, 2017 #8

    phinds

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    You are missing that it was NOT "packed into the size of a pinpoint prior to the big bang", if it is infinite now, it was infinite then, exactly as Chronos said.
     
  10. Feb 21, 2017 #9
    "It is, however, equally valid and logical to assume the universe started out infinite and remains so today."

    I interpret Chronos' comment to mean that the Big Bang ‒ although not an explosion from a single point outward as often portrayed in popular science ‒ marked the beginning of the universe and that the universe has been infinite since its inception. Prior to the Big Bang, however, there was a singularity that is believed to have had infinite density. I would like to know if this singularity contained an infinite amount of matter.
     
  11. Feb 21, 2017 #10

    phinds

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    "Singularity" in this context does NOT, as you seem to believe, mean "a point in space". What is means is "the place where the math model gives unphysical answers and we don't have any idea WHAT was really going on". This is perhaps the single most common misconception in cosmology and has been commented on here on PF approximately 14,000 times.
     
  12. Feb 21, 2017 #11
    Singularity means that the model we are discussing breaks down. It's nothing physical, so actually your question doesn't make sense.
     
  13. Feb 21, 2017 #12

    phinds

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    Exactly.
     
  14. Feb 21, 2017 #13

    Drakkith

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    No, it just means that the density of the (possibly infinite) universe was extraordinarily high. Much higher than we can recreate here on Earth. If all of the matter of the visible universe was packed into a volume of space the size of a pinpoint, then there may have been an infinite number of these pinpoint-sized volumes of matter filling the entire universe.
     
  15. Feb 21, 2017 #14

    Chronos

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    The notion the universe existed in some form [like a singularity] before it came into existence is pretty weird, if you think about it. A analogous question would be, 'Did you exist before you were conceived?' Most people would instinctively reply, 'Of course not, that doesn't even make sense.'
     
  16. Feb 21, 2017 #15
    Sorry, I'm still a bit confused about this even after reading some of the other threads on this topic. Are these statements correct?
    A.) Before the Big Bang, the universe didn't even exist at all so it was not infinite then.
    B.) In the very instant the Big Bang took place, the universe that was created immediately had an infinite amount of space.
    C.) There is an infinite amount of matter in the universe.
    D.) It's impossible to know what was before the Big Bang and where all the matter in the universe came from.
     
  17. Feb 21, 2017 #16

    Drakkith

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    We can't speak of anything prior to the big bang. The universe either didn't exist or it existed in an unknown state.

    Unknown. The universe could be infinite or it could be finite.

    The first is correct, but the latter may not be. It's possible that all matter in the universe originated from an inflation field, but then the question is pushed back to "where did this field come from?", for which we again have no answer.
     
  18. Feb 21, 2017 #17

    Bandersnatch

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    @lifeonmercury you're going at it backwards.
    Start with the present day, when you look at galaxies and CMBR and see expansion in a universe that might be infinite (or not, it doesn't matter!). Then extrapolate the expansion backwards in time, which makes all distances become shorter. This shortening of distances works regardless of the scale you pick, so either finite or inifinite works fine.
    At some point, as your distances go to zero, the process gives you unreasonable results: densities and temperatures trendining to infiinity. This is an indication that the extrapolation you were using enters a domain it's no longer applicable in. So you just stop using it. You don't take the infinities (i.e. the singularity) as a valid prediction of a physical state - you just accept that your theory doesn't work beyond some time in the past.
    The point here is that it doesn't make sense to ask what the BB theory says about what was before what it describes.
     
  19. Feb 21, 2017 #18
    I'm fond of saying that the BB was simply an evolution of space and time [henceforth known as spacetime] as we know them. The observable Universe we see today, arose from that.
     
  20. Feb 21, 2017 #19

    Bandersnatch

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    I don't think I understand what that means.
     
  21. Feb 21, 2017 #20
    Simply referring to space and time, as the accepted 4 dimensional background against which the laws of physics and GR operate and in which we exist and within which it is possible to locate events and describe the relationships between.
     
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