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Negative gravity and space

  1. Nov 22, 2015 #1
    These questions shows my lack of understanding of the expanding universe specifically the Friedmann equation. Here is a quote from Brian Greene (The Fabric of the Cosmos, page 273): “The early universe provided an arena in which gravity exerted its repulsive side with a vengeance, driving every region of space away from every other with unrelenting ferocity.”
    Relativity proves that gravity curves spacetime but isn’t space expanding different from space curvature? In other words, how does negative gravity exert a force on space?
    If the universe stopped expanding, like in a “bounce” theory, would the expanse of space (containing the Higgs field and cosmological constant) contract as the density of mass and energy increased?
  2. jcsd
  3. Nov 22, 2015 #2


    Staff: Mentor

    Yes, spacetime, not space.

    It can be--space can be expanding but still be flat--but it is not different from spacetime curvature--if space is expanding, spacetime is curved.

    Space expanding due to "negative gravity" or "repulsive gravity" is not a "force on space". (This is one reason I don't like Greene's terminology here, since it invites that very misunderstanding.) It's just how the geometry of spacetime works out when you have a positive cosmological constant. There is no "force" since all the objects whose motion tells about the expansion of space are in free fall, feeling no force.
  4. Nov 22, 2015 #3
    Thanks for the specific answer. A follow-up question, in the inflationary theory, the inflaton field was similar to a positive cosmological constant only much stronger. Right?
  5. Nov 22, 2015 #4


    User Avatar
    Science Advisor

    Yes. Of course it wasn't truly a constant, but certainly the energy density changed very slowly during the bulk of the inflationary expansion.
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