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Stretching Space-Time

  1. Nov 26, 2014 #1
    Hello anyone who can answer. I have a question concerning dark energy (Please realize that my knowledge is limited on this subject)

    It is known that the universe is expanding at an accelerating rate, scientists say this is due to the mysterious dark energy. It seems to go against our understanding of gravity, shouldn't the universe be expanding at a decelerating rate?

    What if at a certain amount of density (say black holes!) of matter space time is stretched (we know it can be warped, we think there can be waves, so why can't it be stretched?)

    This might explain why the universe is accelerating, its just space getting bigger. It might also explain why its only expanding on a large scale (in addition to the other forces, such as electromagnetism, that are greater than it at a small scale). It might just be the relationship between black holes.

    Maybe dark energy is just a strange aspect of general relativity. Instead the warping of space-time, it is the stretching of space time due to large, possibly infinite, amounts of density.
     
  2. jcsd
  3. Nov 26, 2014 #2
    That's exactly the problem that dark energy was invented to solve.

    You say space is getting bigger. That's just another way of saying space is expanding, and is just begging the question.

    Dark energy is included in general relativity as the cosmological constant term.
     
  4. Nov 26, 2014 #3
    Interesting. In what way do you imply it is stretched?

    It is also possible that at the rate space expands (in its early stages, faster than light. and constantly increasing speed) makes it immune to gravity. Not to say the universe's expansion breaks the laws of physics, only that at any speed faster than light, the force of expansion is greater than the sum of all gravitational pulls in the universe. And because of that, it can constantly increase acceleration (assuming that is the nature of things that move FTL; there is more acceleration for every less gravitation)
     
  5. Nov 26, 2014 #4
    Just to be sure I am understanding you right, could you give an example? (such as the rubber sheet example)
     
  6. Nov 26, 2014 #5
    Thanks you for the response! This makes more since.
     
  7. Nov 28, 2014 #6

    PeterDonis

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    I suppose this is "possible" in principle, but it's not what happens. (Also, the universe's expansion is accelerating now, so whatever we use to explain it can't be something that only applies in the "early stages".)

    This is misstated in two ways. First of all, the "accelerated" expansion, which we think is explained by dark energy, does not require "faster than light" expansion; it works just fine at any expansion rate. Second, there is no "force of expansion"; all the objects in the universe whose relative motion is described as "accelerating expansion" are in free fall, feeling no force.

    With those misstatements corrected, it is true that the accelerating expansion can be viewed as the effect of dark energy (which makes expansion accelerate) being larger than the effect of ordinary matter (which makes the expansion decelerate). Up until a few billion years ago, it was the other way around: the effect of ordinary matter was larger than the effect of dark energy, so the expansion was decelerating. But as the universe expands, the density of ordinary matter decreases, so its effect on the expansion rate also decreases, while the density of dark energy remains constant. By a few billion years ago, the density of ordinary matter had decreased to the point where its effect became smaller than that of dark energy. When that happened, the expansion stopped decelerating and began to accelerate.
     
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