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B Early state of Universe and the "size" of Universe

  1. Nov 23, 2016 #1
    Hey guys,

    According to most pop science sources, "before" the Universe started to exist, there were no time, which can be understand easily as time is only possible if there are at least two existing object (e.g. two quarks, two atoms, etc.).

    Time as I understand it is a measurement of movement, and motion is relative. But when it comes to space, I can't imagine there is no space "before" the Universe started to exist, I also can't imagine that space has some kind of geometry (e.g. closed topology, finite but unbounded, etc.).

    If the Universe indeed shaped like a sphere, finite but unbounded, then the "space" inside it can be described as the volume of the Universe. If I'm going to describe a "space" with closed geometry, it is not actually space but a really large spherical object with massive volume and given that it is something with size, then it is always be finite and limited despite a composite object within this spherical object known as Universe keep passing its starting point.

    Can someone explain these without using mathematics, in a way that someone can understand it much easier?
    Last edited by a moderator: Nov 23, 2016
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  3. Nov 23, 2016 #2


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    Learning science from pop science sources is a path fraught with failure. But you know that.

    Without mathematics, the best picture I can come up with is the balloon analogy. A closed, finite three dimensional universe evolving over time can be compared to the surface of sphere that is expanding over time. The difference is that the real universe has three dimensions of space and the "balloon" model has only two dimensions of space on the surface of the balloon.
  4. Nov 23, 2016 #3


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    It isn't.
  5. Nov 23, 2016 #4


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    I don't understand this comment. Space time obviously must have some kind of geometry.
  6. Nov 23, 2016 #5


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    The problem here is that there are a lot of ideas of how this works, but no clear evidence.

    The earliest picture of our universe we have is the Cosmic Microwave Background, which was emitted when our universe was a few hundred thousand years old. Our understanding of how the universe behaved before then comes from indirect evidence. Because of that, our understanding gets worse and worse the earlier you go (though interestingly, we still have a pretty good picture back to as early as a few minutes after our universe began). The evidence completely falls apart when we go all the way back: there are lots of ideas, and nobody knows which (if any) are right. So when it comes to questions about the overall shape of the universe or its total size, or what happened before our universe, we just don't know.
    Last edited: Nov 24, 2016
  7. Nov 23, 2016 #6
    The difference between analogies and equations is like a shadow and the object that casts it, it only goes so far. I am just starting the process of learning the math behind physics so I can really understand the concepts astronomy shows use analogies to explain, I suggest you do the same.
  8. Nov 24, 2016 #7
    According to the latest experimental data from various, independent sources (WMAP, BOOMERanG and Planck for example) confirm that the observable universe is flat with only a 0.4% margin of error. That means Universe is infinite. If I'm gonna describe the Universe without using math, the amount of matter (objects) and time (which is relative to objects) is finite and limited. But I'm not really sure about space.

    Some people said space is synonymous with distance. We measure distance with a measuring tool, for example a ruler. Assuming we are measuring the distance or space between two books with different location each, but what we are measuring is not actually the distance of space, we actually measure the size or extension of the ruler. I always thought of space as absolutely nothing and space is a "where", a place, not a "what" such as something with properties. Thus, space is unbounded, boundless and without limit.

    People regularly said "before" the Big Bang event, the Universe started as a singularity but others said the Universe started as a hot, dense state. I'm confused, which one is the correct statement of Big Bang theory? The Universe is thought to be expanding until today, what is it expanding to? Other space? If there is no space, Universe can't be expanded because dynamic action (e.g. motion, change, expand, etc.) is possible only if there is space "beyond".

  9. Nov 24, 2016 #8
    My layman thoughts say the existence of space is relative to the presence of energy and matter. The universe is either expanding or collapsing: it is not static or infinite.
  10. Nov 24, 2016 #9


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    The putative singularity is merely a mathematical artifact that mocks our inability to correctly model the universe. The model is actually very good back to the point when it was in a hot dense state. Not so much when you attempt to push beyond its realm of applicability. A proper theory of quantum gravity is believed to be the key to resolving this enigma, but, I'm not so sure about that. I suspect a deeper mystery will will arise and something beyond our concept of logic is necessary to comprehend the universe.
  11. Nov 24, 2016 #10
    finite but unbounded universe.png Alright, I will try to imagine the Universe without applying mathematics. Here I'm imagining a finite but unbounded universe. A universe with closed geometry. One can also imagine this lone sphere as the "initial state" of the Universe "before" the Big Bang event. It seems there is space "outside" the finite but unbounded Universe. If we mean the space (or "volume") within the Universe is finite and limited, then everyone can understand it. But if we mean the entire space, I mean, including "something" "beyond" the Universe, is finite then some people will have trouble understand it. Including me.
  12. Nov 24, 2016 #11


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    No, it means we cannot tell if the universe is finite or infinite within the margin of error.

    There is a geometrical relationship between the ends of the ruler. Or between the books. Or between two points. These geometrical relationships are what we call spacetime. It doesn't matter if you attribute it to the ruler, to the books, or to the space between the books. It is all geometry.
  13. Nov 24, 2016 #12


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    It may seem that way, but there is not and can not be any proof of such. Nor do we have good reason to conclude that our universe is finite but unbounded. It is a possibility only.

    Just because you cannot imagine an unbounded universe without something outside its [non-existent] boundaries does not make the concept of something outside the universe sensible.

    One can imagine a non-Euclidean geometry by "embedding" that geometry in a higher dimensional space. For instance, the surface of a sphere is a two-dimensional space embedded in a three dimensional space. But there is nothing in the mathematics that requires that the three dimensional space exists. One can describe all of the properties of a two-dimensional finite-but-unbounded space without ever requiring that that space be embedded in, for instance, Euclidean 3-space.

    A good read is https://www.amazon.com/Sphereland-Fantasy-Curved-Expanding-Universe/dp/0064635740
  14. Nov 24, 2016 #13
    Unfortunately, you haven't quite followed what the pop-sci sites say. When they talk of the universe being "like" a sphere, that is a 3D analogy to a 4D model. The 3D volume of our real universe is equivalent to only the surface of the higher dimension object. You are used to a normal sphere having a surface area, in this case the "glome" as it is sometimes called has a surface volume. The region inside the glome's surface would be four dimensional and doesn't correspond to anything meaningful.
  15. Nov 25, 2016 #14
    Thanks for the responses. But what is exactly the difference between unoccupied space that is devoid of all objects (including elementary particles like quarks, etc.) and absolute nothing?
  16. Nov 25, 2016 #15
    There is no difference: space is the region where energy and matter are present.
  17. Nov 25, 2016 #16


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    Neither of these terms have a well-defined meaning in physics, so this question is unanswerable as you state it. But see below.

    This is not a bad heuristic, but it's not a precise definition either.

    The precise definition of "space" in this context is a 3-dimensional spacelike slice of a 4-dimensional spacetime. Since quantum fields are present at every event in spacetime, and since their presence contributes to determining the geometry of that spacetime, it is meaningless to talk about "unoccupied space"; there is nothing in physics corresponding to a 3-dimensional space (or a 4-dimensional spacetime) all by itself, with no quantum fields present. It is also meaningless to talk about "space outside the universe", since "space" only has meaning as part of spacetime, i.e., of the universe.
  18. Nov 25, 2016 #17
    So, these terms are the same regardless of what context we're talking about. Am I correct?

    Without dwelling into mathematics, I think space without any objects (including elementary particles, quantum fields, etc.) is no different from absolute nothing.

    But if we try to draw or illustrate a finite but unbounded Universe that become an example of the Balloon Analogy, such as the illustration that I had posted earlier above in my previous comment, the "space" within the Universe turns out to be the volume of the finite but unbounded Universe. Expanding is a dynamic process, and dynamic processes necessarily require a background of nothingness. For people who define space as the background of all events, objects, etc., space is indeed infinite.

    finite but unbounded universe.png
  19. Nov 26, 2016 #18


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    No. The terms are meaningless. That doesn't make them "the same regardless of context". It means you can't say anything meaningful whatever using them.

    This is your personal opinion, not physics. Please review the PF rules on personal theories; they are out of bounds for discussion on PF.

    This is all right provided you bear in mind that this "space" is coordinate dependent.

    This is your personal opinion, not physics.

    This is your personal opinion, not physics.

    This thread is now closed. Please do not post your personal opinions in future threads; if you do, you will receive a warning.
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