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Expansion force

  1. May 6, 2007 #1
    Ok this may sound silly but humor me .

    If there was an edge to our universe a sor of big expanding bubble ,is it possible instead of an internal dark energy force causing expansion from within could their be an external force attracting or sort of sucking our universe in.

    Maybe like our space is a giant vacuum with one polarity or energy level (if that makes sense )and outside our universe sits a void another vacuum with a different polarity or energy which ours is getting pulled into.
    Last edited: May 6, 2007
  2. jcsd
  3. May 6, 2007 #2


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    this is about the universe, so belongs in Cosmology forum
    (astrophysics is about the physics of stars and things more on that level, not the expanding universe as a whole, so this should probably be moved from Astrophysics to Cosmology)

    the way you begin "if there was an edge" is such an unrealistic assumption that it makes it hard to reply. Nobody I know assumes there is an edge to the universe---if you start by assuming that then your speculation is not likely to make sense whatever you say afterwards.

    There are various kinds of horizons, for example there is an horizon bounding the part of it we can see, but that is not what you are talking about-----things beyond the horizon are assumed to be on average similar to what we can see and over time more of that becomes visible as its light reaches us. but the limit of what we have gotten light of so far is not the kind of mechanical edge you are talking about. Also in the highly speculative "eternal inflation" scenario there are bubble universes---but in that picture there is intrinsic expansion without forces needing to be applied on the domain wall.

    as far as anyone knows space is not a mechanical entity that can be pushed or sucked

    so even if it had an "edge" your idea would seemingly not make sense.


    you say: "instead of an internal dark energy force causing expansion"
    but dark energy is not NEEDED to cause expansion

    even with dark energy ZERO the universe would still be expanding

    expansion was taken for granted (with zero dark energy) for 70 years---until finally in 1998 accelerating the expansion was discovered.

    the role of dark energy is to slightly ACCELERATE the expansion, but the expansion doesnt depend on there being dark energy, it is just a standard feature of the way space behaves----the plain vanilla solutions of the einstein equation either have it expand or contract
    Last edited: May 6, 2007
  4. May 6, 2007 #3


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    if you were talking about something different------not why space expands but why the expansion appears to ACCELERATE ----then I could make sense of your picture (minus the edge)

    Some cosmologists have considered the possibility that the acceleration discovered in 1998 might be the effect of concentrations of mass out beyond our present horizon. That idea was proposed, IIRC, and got discussed some in the past few years.

    I think David Wiltshire may have put it out on the table as a possible way of dispensing with dark energy. But his latest paper is about something else (a new different way of getting rid of dark energy) so he may have given up on the earlier one.

    We are not talking about the expansion which happens normally with space and doesnt require any force or energy to drive it. Wiltshire was not proposing an alternative explanation of expansion per se, as I recall it.

    IIRC he was proposing an alternative explanation for the observations that indicated a slight ACCELERATION of the normal expansion.

    If that were your idea then it would make sense to the extent that some experts have already written articles about it and discussed it----although it may have gotten dropped if it didn't work out right.

    (and it didn't involve any "edge" to space----that part was unnecessary)
    Last edited: May 6, 2007
  5. May 6, 2007 #4
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  6. May 7, 2007 #5


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    The cosmological assumption is that the universe is isotropic.

    It turns out that with this assumption of gravity, that the isotropic material outside the universe won't cause any net effect. This is one of the ways of describing Birkhoff's theorem.

    So one would have to abandon either the cosmological principle of the isotropy of the universe, or one would have to modify general relativity to get rid of Birkhoff's theorem, to make this sort of approach work.

    The standard approach assumes that GR works (which implies Birkhoff's theorem works), and also assumes the cosmological principle that the universe is homogeneous and isotorpic throughout.
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