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The acceleration of the expansion of the known universe

  1. Apr 7, 2013 #1
    I am a novice and dabbler, but very interested in the evolving understanding of the laws of nature. Most recently, I was listening to a discussion about the surprising discovery (granted, not a new discovery) that the universe is expanding at an increasing rate rather than at a decreasing rate.

    Here's my understanding of the now-outdated theory: The expansion of matter must be decreasing because of gravity. Just as a ball, when thrown straight up, slows and then begins to fall, so must the matter in the universe be slowing as it distances itself from other matter.

    My question: Obviously, the ball in the above scenario slows then accelerates back toward the ground because of the Earth's gravity. Since we do not know what the matter is expanding toward, has it been determined that there is not a force, possibly even gravity itself, acting on the matter and pulling it outward (for lack of a better word) away from the matter we previously thought would be pulling it back in?

    I hope I have described my question clearly. If not, please feel free to ignore, mock, or deride me as you see fit!
     
  2. jcsd
  3. Apr 7, 2013 #2

    mfb

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    If you throw it fast enough (and ignore the atmosphere), it will never return to earth. The same could happen for the universe, even without accelerated expansion.
    It is not expanding "towards" anything. The universe is not an object in space. It IS space (and time, plus some stuff inside).
     
  4. Apr 7, 2013 #3

    Drakkith

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    Actually this is exactly what was happening until recently. Well, recently in cosmological terms. The rate of expansion was decreasing earlier in the universe because matter was closer together, which meant that the force of gravity between all these galaxies was stronger, which resisted expansion and slowed it. Recently, the increasing distance between galaxies has resulting in less gravitational force between them, and possibly more 'dark energy', which is causing the expansion to accelerate. (We're still a little unsure of what dark energy is)

    The expansion is causing all objects not bound together to recede from each other. This means that while the distance between objects increases over time, no one ever gets closer to anyone else. Gravity simply isn't capable of pulling everything back together as far as we can tell.
     
  5. Apr 7, 2013 #4
    Thanks for the replies!

    mbf: My problem with this is that since the universe is infinite, and we can't detect things that are infinitely far away, do we really know whether the expansion is toward anything? Is it only the known portions of the universe that are expanding, or is it possible there is matter infinitely far away that is behaving differently? Can we ever know the answer to this? True, if you throw the ball far enough and fast enough it will leave the atmosphere and, if it doesn't settle into an orbit, it will likely continue on forever without encountering any other objects, given the vastness of the universe and the sparseness with which it is populated by stuff.

    Drakkith: Interesting that it's now speeding up because it is getting further from the gravitational influence of other stuff. Even more interesting that dark matter may be responsible for the acceleration. One of the things I find refreshing about physics is that even the most brilliant minds in the field will say that there are still things we don't know. It certainly makes me feel better about all the things I don't know!

    I'm a pretty smart guy, but I have resigned myself to the fact that either:
    A) I don't have the mental capacity to understand many of the current theories in physics, or
    B) I don't have the time and dedication to do so.

    It's tough for me to accept things based on the knowledge and intelligence of others, but if a reputable physicist tells me what the current predominant theories are, I accept it is probably the way things work - until we learn it isn't!
     
  6. Apr 7, 2013 #5

    Drakkith

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    We have absolutely no way to know what is happening beyond about 48 billion light years away. This is about the distance that we can see thanks to the finite age of the universe and the finite speed of light. However, we can extrapolate and say that the rules of the universe as we know them should still be the same at far beyond this distance, otherwise we would see interactions between those areas of the universe we can see, and those we cannot.

    Realize that you need to get REALLY far away from stuff before expansion actually starts to separate you from our galaxy. Like, several tens of millions of light years.

    It's B. If you dedicate the time to learn it, it isn't very difficult to understand.
     
  7. Apr 7, 2013 #6
    If you're interested, the early results from Planck said that the acceleration isn't quite as fast as the scientific community thought it was.
     
  8. Apr 8, 2013 #7

    mfb

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    It is unknown whether the universe is finite or infinite. In any way, it is pointless to talk about "directions" "outside" the universe where the universe could expand "into", as the universe (by definition) is the whole space(time) construct.
    Who is mbf?
     
  9. Apr 8, 2013 #8
    If I refer to direction in the context of toward or away from, I'm only speaking in terms of relative to another object.

    I thought it was fairly well accepted that the universe is infinite, but as I said in my original post I'm a novice and dabbler and am just starting to educate myself about such things.

    My fascination was recently rekindled (from my college days in the early 1980's) by a discussion about whether a line or a ray is "longer" given that both are infinitely long. That led to infinite subsets of infinite sets, and other things that make my head spin - but in a good way. The fun part for me was that a line segment is not infinitely long and therefore definitely "shorter" than a ray or a line, but still has an infinite number of points, and therefore each point in a ray or line can be mapped to a point in a line segment.

     
  10. Apr 8, 2013 #9
    I'll have to look into that. One ofmy favorite quotes is "Imagine what we'll know tomorrow." I don't remember who said it, and I'm not going to look it up right now!

     
  11. Apr 8, 2013 #10
    Thanks for the vote of confidence! I have determined to dedicate a portion of my free time to learning more about physics. Of course the pie chart that represents my free time is largely taken up by family (wife and 3 Beagles), work, and writing. And yes, I'm a proponent of the Oxford comma for those of you who familiar with writing style.

     
  12. Apr 8, 2013 #11
    I have actually heard of the universe as being called "infinitely finite", which is kind of confusing, but here's how I think about it:
    The universe is like the surface of a balloon. If you put a bunch of dots on the balloon, then inflate it, the dots move farther apart. The surface area gets larger, but the number of dots remains the same. At any point, the surface of the balloon is a finite area, but since it keeps growing, technically, it is infinite. (oh, and the balloon is unpoppable)
     
  13. Apr 8, 2013 #12
    So would that theory say, basically, that the universe is always finite, but will infinitely continue to grow (or expand, if you prefer)? Would that mean that the "space" into which the universe expands doesn't exist until it is reached by the expansion? Or is the "space" there, but unoccupied by our universe? It seems that if the "space" is there but unoccupied, the universe is infinite but only a finite portion is occupied by matter.

     
  14. Apr 8, 2013 #13
    Yes, or at least, so I've read. Like you, I am also a dabbler.
     
  15. Apr 8, 2013 #14
    Sorry, I think I added to my post while you were responding. Thanks for the input!

     
  16. Apr 8, 2013 #15

    Drakkith

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    A finite, but unbounded universe is what his example is similar too. In such a universe, if you travel far enough you come right back around to where you started. But expansion still shoves everything apart and there still isn't any "pre-existing space" that the universe expands into. It simply expands. By which I mean things get further apart.
     
  17. Apr 9, 2013 #16
    Is that a generally accepted, or at least mainstream, concept?

     
  18. Apr 9, 2013 #17
    Sort of, we do not know if the universe is finite or infinite. If its infinite now it must be infinite in the past. If its finite then the example above is a good one
     
  19. Apr 9, 2013 #18
    It's both because Schrodinger's Cat. (nerd jokes)
     
  20. Apr 9, 2013 #19

    mfb

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    A finite, unbounded universe? It is one of the common concepts of the global spacetime. We do not know if the universe is finite or not - observations fit to both options, there is no way to distinguish them currently.
     
  21. Apr 9, 2013 #20

    bapowell

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    I don't see how a continuously growing balloon is technically infinite. That's not even confusing language -- it's just plain wrong.
     
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