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Need Help Understanding Expanding Universe

  1. Jun 9, 2012 #1

    I am hoping someone can help me to understand that the universe is expanding and other questions that brings up. If it is expanding, does that mean that there is an edge or a place that the universe has not reached yet but will reach at some point? What would be in the space that it has not reached yet? Are we able to see the entirety of the universe in all directions? Meaning that we can know everything that is out there or are there places that we will never be able to see?
    I realize that my questions are probably a little childlike. I already feel like a kindergartner amongst Ph.D's so please try not to be to hard on me. I am just trying to understand.

  2. jcsd
  3. Jun 9, 2012 #2


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    The usual model universe that cosmologists work with is the simplest possible that makes a good fit with the data.

    For simplicity, it has no outside. Space is all of space. So it is a mistake to try to picture it from the outside. You can't get outside and see it expand because there is no outside. There is no edge or boundary--that would add a lot of needless unwanted complication to the model.

    So think about how the creatures inside something that is expanding experience the expansion process from the inside. For them a uniform pattern of expansion is not like ordinary motion, because nobody gets anywhere by it. Everybody just gets farther apart from everybody, without moving (in the ordinary sense.)
    That's how they experience expansion.

    It helps some people to watch this simple computer simulation:
    It is expansion of a reduced dimension world. All existence, all creatures are in the infinitely thin surface of the balloon. Imagine that there is only this surface, no inside of the balloon, no outside of the balloon. their experience is all within the infinitely thin surface.
    Watch carefully. You will see galaxies sitting still and wiggles of light moving among the galaxies. each galaxy sees the other galaxies getting farther and farther away, but nobody moves, no galaxy gets anywhere. Each galaxy stays at the same latitude longitude position in their world.

    That's just an analogy, a toy model. Real space is 3D, not an infinitely thin curved 2D surface.
    Real space may be infinite volume, or (if it somehow curves around and rejoins itself) finite volume. That's hard to imagine but it is mathematically possible.
    Last edited: Jun 9, 2012
  4. Jun 9, 2012 #3
    THANK YOU!! I really do understand exactly what you mean. One more question, if you don't mind. Are we able to see the entire universe? I guess I am asking if all of space has been mapped out. Are there still things to be discovered?
  5. Jun 10, 2012 #4


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    Courtney we *definitely* do not see the whole thing!
    According to the best most recent estimates I've seen, we don't even come close.
    I'm not an cosmology expert, just a retired mathematician who loves cosmology. I try to follow other people's research.
    If you (or anybody) wants some numbers about how much farther it extends (at least) beyond the most distant matter we can see, then just ask.
  6. Jun 10, 2012 #5


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    I agree with marcus, with one caveat - we can 'see' the entire observable universe. The cosmic microwave background, currently at z ~ 1100, will forever be the limit of the observable universe in EM wavelengths.
  7. Jun 15, 2012 #6
  8. Jun 16, 2012 #7
    One modification to the balloon analogy that I like is to picture the radial dimension of the sphere as time. The BB happens at T=0, the center of the balloon. inside the balloon is the past, outside the balloon is the future. We see the 3D surface at T=now as space. This helps explain why the universe expands, it is simply the nature of spheres that the surface area gets larger if the radius gets larger. Similarly it is simply the nature of hyperspheres that the surface volume is larger now, with a radius of 13.7 billion years then it was with a radius of 5 billion years.

    Please note that while this may be a convenient way to construct a mental image it cannot be an accurate model of the cosmos. Once you start crunching the numbers with it you find that it predicts a universe that is either much smaller or much older then we know the universe to be.
  9. Jun 17, 2012 #8
    If the cosmos is finite then it must contain a finite amount of space, mass and energy. Each unit has a defined location, physical domain and configuration. Even if the cosmos is expanding, for any time=T simple summation should be able to determine where some mythical cosmic wall exists.

    Theorists have come up with an interesting hypothetical twist in which space curves in upon itself in such a way that for every given point 'A' there is a point 'B' within a finite distance at which motion in any direction will not increase the distance between the two; in fact, if a traveler who could instantaneously traverse a sufficient distance encountered such a point, he would begin to return to his point of origin. The concept describes a "finite but unbounded" Universe and implies a cosmos enveloped in a spherical spacecage. There is no evidence, no principle of logic, science or mathematics and no law of nature that implies the existence of any point, however distant, at which progress becomes regress.

    There are two ways things in nature increase in size: inflation (adding more material) and expansion (increasing volume and decreasing density). The first implies the cosmos is somehow conjuring up new materials and/or new locations to occupy (magical thinking). There is no evidence for the second.

    If there ever was a Big Bang (and the jury is certainly still out), the only logical version would be that within our local neighborhood (45 billion light years or so with its theoretical expansion factor) of that infinitely populated expanse we call the cosmos, an immensely large volume of mass somehow collapsed into a hyper-critical black hole which then regurgitated. I could almost (but not quite) lend credence to that scenario, but it certainly didn't create the Universe. At most it was a colossal rearrangement of elements which already existed.
    Last edited: Jun 17, 2012
  10. Jun 17, 2012 #9
  11. Jun 17, 2012 #10
    Unless Big Bang is a local event, Universal expansion would imply a finite cosmos. Expansion - in whatever form you wish to cast it - means increase and only that which is measurable or limited can be increased. If the cosmos is, indeed, infinite, then there is no farthest and there is no 'all'...there is always more.
  12. Jun 18, 2012 #11
    There is actually a great deal of evidence for the second. I suggest you google cosmological redshift and supernova surveys.

    This theory was postulated some years ago, but does not seem to fit the data we now have. Particularly the observation that cosmological expansion is accelerating. If this theory were correct the expansion would be slowing down due to gravity.
  13. Jun 18, 2012 #12
    This is incorrect. Expansion does not imply finiteness. Expansion is a verifiable fact. The jury is still out on finiteness, though in my opinion infinite is more likely because only an infinite and expanding universe can have zero total energy. I find the zero energy universe idea appealing because it can spontaneously exist without violating conservation of energy.
  14. Jun 18, 2012 #13
    Let's break this down.

    Exactly what I plan to disprove.


    And that's where we're going wrong. There's no reason to think this. Why should something being not limited mean it can't increase? At no two points in an infinite Universe is the distance between them infinite, and we can easily increase a real number. Similarly, the distance between any two points is measurable, as it is not infinite.

    Anyway, even something being infinite doesn't mean it isn't measurable or it can't be increased. For instance, there's a fairly basic proof that the number of real numbers is "more" than the number of real integers.
  15. Jun 19, 2012 #14
    Google is not as good as SLAC, CERN, FERMI and other more credible sources. Been there, done that.

    Not if the density of mass to space were higher OUTSIDE our sphere of observation.
  16. Jun 19, 2012 #15
    In order to detect in increase one must have a measurement. If condition A is larger than B then there is an increase. Any value you assign to A or B is a defined (finite/limited) quantity.
    Yes it does. Infinity is the LACK of a quantifiable definition.
    Duhhhh... The set of integers is a subset of the set of real numbers.
  17. Jun 20, 2012 #16
    Neither of these is the standard cosmology modeled by the FLRW universe. For example, inflationary expansion retained a constant energy density. Nature is more complex than either of those statements implies.

    Again, this conflicts with standard cosmology. We observe universal expansion, the FLRW model depicts it, and no one knows if the universeis finite or infinite. That statement is
    pure speculation without supporting theoretical or observational evidence.
  18. Jun 20, 2012 #17
    "Infinite" quantities can also be defined. And, again, assuming the Universe is infinite, even though there is no limit to the distance between two points, it is always going to be finite.

    Quite incorrect. http://en.wikipedia.org/wiki/Infinity#Set_theory

    Oh. I had a completely different proof in mind. :rofl:
  19. Jun 20, 2012 #18
    Yes, IF the standard model was correct. A flat earth used to be the standard model, also.

    You observe red shift....and that observation only pertains to extremely distant objects. We do not detect any expansion of the space between atoms/molecules locally that would lead us to believe the phenomenon is universal.

    And a finite universe is simply illogical on its face. It would contain a finite number of units of mass/energy/space and each would occupy a defined volume and have a defined configuration. Simple summation would predict where some mythical cosmic wall might exist.

    You can torturously contort all the equations you wish to cast the shape of the cosmos into a 'finite but unbounded' spherical spacecage, a hyperbolic, flat or pretzel shaped doodad, but no matter how vast the number of items you wish to consider, their size and shape could be summarized and a boundary would be determined.

    I know of no such boundary. Is it made of green cheese?
  20. Jun 20, 2012 #19
    Also incorrect. The common "torus" model (more of a 3-torus) would introduce no such boundary. The surface, not the interior of the 3-torus and the 4-dimensional "space" surrounding it, is considered to be the Universe, so there is no boundary whatsoever.
  21. Jun 20, 2012 #20
    This is such a profoundly ridiculous view of basic set theory that I refuse to believe that you possess even the slightest education in mathematics; it then follows that you lack any appreciable knowledge of physics. Everyone, stop feeding the crank.
    Last edited: Jun 20, 2012
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