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B Gravity and expansion

  1. Apr 11, 2017 #1
    If the Universe is expanding, wouldn't it be common sense to think everything inside it is expanding too?
  2. jcsd
  3. Apr 11, 2017 #2
    No. the space in between objects is expanding but not the objects like earth themselves... This is what you asked, correct?
  4. Apr 11, 2017 #3
    And to add on, this is pretty much just happening at the intergalactic level.
  5. Apr 11, 2017 #4

    David Lewis

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    If spacetime is expanding then new heres and nows are being created.
  6. Apr 12, 2017 #5


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    Staff: Mentor

    The expansion of the universe is about the distance between objects increasing over time. Note my choice of words there. The distance between objects. Objects bound together by strong forces, such as the atoms in molecules or planets to their stars, are not expanding as far as we know. But, given a large enough distance, these forces drop off far enough to allow expansion to take place and the objects get further away from each other over time.
  7. Apr 12, 2017 #6


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    Staff: Mentor

    This is not correct. Spacetime is a 4-dimensional manifold; it already contains all "heres" and "nows". "Expanding" just labels a particular feature of the geometry of this 4-dimensional manifold.

    Also, since as far as we can tell spacetime is a continuum, even if we adopt a particular coordinate chart (such as standard FRW coordinates), expansion does not create new "space" in these coordinates. Each "point in space" can be labeled in FRW coordinates and the labels are the same at every instant of time.
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