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B How does the Universe's expansion relate to Newton's 1st law

  1. Jul 22, 2016 #1
    How does the universe's expansion relate to newton's 1st law.

    In other words, is all the expansion due to an unknown cause at the big bang and the 4 forces now act on it? Or maybe is the expansion not really due to a force (as is gravity) so the 1st law doesn't apply?
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  3. Jul 22, 2016 #2


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    It really doesn't apply. General Relativity and Quantum Mechanics are both important for understanding the behavior of the very early universe, neither of which were conceived of when Newton wrote that law.
  4. Aug 1, 2016 #3


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    Newtons 1st law is in GR replaced by the 'law' that point particles follow geodesics, the straighest possible paths in spacetime. However, this is a statement about stuff in spacetime, not about spacetime itself. Spacetime itself obeys in GR the Einstein equations. The expansion of space is perfectly described by these Einstein equations.

    So, short answer: it does not.
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