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I Could the original size of the Universe be the Planck length

  1. Jul 4, 2018 #1
    If you guys haven’t noticed through my posts so far is that I am interested in the Big Bang.

    I am considering the concept of the Planck length, but I may have some confusion.

    How did Space time start? Did the universe start from nothing and created units of spacetime? Could the original size of the universe be the Planck length? Space and time breaks down the closer you get to the origin, similar to the Planck length.

    Anyone have any facts or clarify some confusion?
  2. jcsd
  3. Jul 4, 2018 #2


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    Science Advisor

    The Planck length is just a unit of length. For various reasons we expect it to be a sensible unit to measure the sizes where some quantum effects become important, but that's all it is.
    We don't know. Our models break down sometime in the very early universe. We hope thar a future theory of quantum gravity will address this.
    We currently model the universe as spatially infinite, in which case it was always infinite in size. Just very dense in the past.
    Our models of spacetime almost certainly break down as we get close to the origin of the universe, yes. Don't confuse that with anything actually breaking down.
  4. Jul 4, 2018 #3
    Planck length is just the smallest length we could hope to measure..
    It's not a physical constant.
  5. Jul 5, 2018 #4


    Staff: Mentor

    We don't know that this is true. It's a common, plausible speculation in quantum gravity, but it's still just a speculation, not an established fact.

    This seems a bit of a quibble since its value is determined completely by the values of other things that are usually considered physical constants (##G##, ##c##, and ##\hbar##).
  6. Jul 10, 2018 #5
    There is an assumption that the universe is and always has been infinite. The concept that the entire universe was once very tiny is a common misconception. The observable universe was once very tiny. We have no reason to believe that beyond the visible universe things aren't exactly the same out forever. So while everything that we can see was once very very tiny, we believe that there were an infinite number of those beyond it.
  7. Jul 10, 2018 #6
    Aye, "as far the eye can see", this is a reasonable assumption.
  8. Jul 13, 2018 #7
    Wouldn't the entire universe have been gravitationally bound at the beginning of the big bang? Intuitively it would make sense for everything to be equally effected by the expansion of the universe, but what I have found that scientists say gravitationally bound objects are not effected by the expansion of the universe. I am not sure if what they mean is that the effect is just not significant or the space is not actually expanding in that part of the universe.
  9. Jul 13, 2018 #8


    Staff: Mentor

    No, because the universe was expanding so rapidly. Your intuitions about "gravitationally bound" are taking into account the universe's density, but implicitly assuming an object that dense which is at rest. An object that dense which is expanding rapidly is not the same.

    That is because "gravitationally bound objects" here means objects much, much smaller than the universe as a whole--objects like Earth, or the solar system, or our galaxy, or a galaxy cluster. All of these objects are more or less at rest--more precisely, all of their parts are on average at rest relative to each other. They are not expanding rapidly; if they were, they wouldn't be gravitationally bound.
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