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Expanding space versus gravity.

  1. Apr 29, 2009 #1
    The first reply in the thread "Why expanding space?", says that it was thought that gravity would slow the rate of expansion. Why? If it is the expansion of space that is moving matter apart, then presumably 'twas ever thus. And if it was able to overcome the gravity of the early universe, when all was within close proximity, the question of gravitational slowing would never arise.
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  3. Apr 29, 2009 #2


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    Is the answer?: It is mass that creates the gravity. Early universe was very smooth, the mass spread about very evenly, and so the gravity fields created by mass also close to a flat balance. No sharp dips that would make for strongly oriented contractions.

    Later, mass clumped and gravity potentials became strong localised deviations. But clumps of mass were also far enough apart that the same general balancing act persisted.

    So it is wrong to think that the gravity strength was greater because things were once closer. The overall balance of matter/gravity has not changed.

    There was a phase change from a radiation dominated universe to a matter based one. And expansion rate adjusted accordingly.

    The really interesting question to me is how to account for freely expanding space once you get rid of the secondary matter/gravity aspect. If there were no clumps of matter or even energy of any kind, making the energy density and thus gravitational potential completely flat, what causes a steady expansion.

    I've read so many accounts of the Big Bang which say the universe was born with a certain heat, a certain amount of kinetic energy. And this trapped kinetics sort of blows things apart. Spacetime has an inertial urge just to stretch apart. But then consideration of things seems to stop there.

    For example, the free creation of "more cold void" would seem an entropic cost. It could be accounted for by saying perhaps the initial hot scale of the big bang was like a bunch of rucked up fabric. Then this fabric is spreading out to create a cool void. So every point of spacetime that will exist at the heat death, existed back then confined at the bang. Total information thus conserved.

    Dark energy acceleration is then more clearly seen as an entropy issue. The universe must be modelled as an open system - a dissipative structure.

    So anyway, the matter/gravity issue is something that rather seems to obscure the deeper modelling. It is a secondary story.

    Though of course the clumping of matter did have the effect of cooling the universe more abruptly than would otherwise have occured - sucking heat out of the void and locating it as congealed lumps of mass.
  4. Apr 30, 2009 #3
    Consider space as a huge ball, when you throw that ball, it will slow down and eventually hit the ground after a while.

    Space is the same thing.

    When the Big Bang occured, space expanded at a certain rate, and it is only logical to assume that the rate of expansion will degrade after a long period of time.

    If a ball had an infinite amount of speed, it would never fall to the ground.
  5. May 1, 2009 #4


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    It's not that simple. If you throw a ball fast enough, for instance, in the direction of the Sun, it will slow down as it leaves the Earth, but then as the gravitational attraction of the Sun becomes stronger, it will speed up again.

    With the universe, the rate of change of the expansion of the universe with time depends upon the energy contents of the universe. Specifically, it depends upon the relationship between the energy density and the pressure of the various sorts of matter/energy that make up the universe. If you have a form of matter out there with enough negative pressure, then it will cause the universe to accelerate its expansion. This is what appears to be happening right now.

    Now, earlier on, when the universe was dominated by normal matter, and even earlier when it was dominated by radiation, the expansion rate of the universe slowed down, just as you would expect from our normal experience with gravity. Since our normal experience with gravity involves normal matter, and since radiation only gravitates more strongly than normal matter (due to its positive pressure), it makes sense that our normal experience with gravity would still hold here. It doesn't hold if there's some negative pressure stuff out there (like a cosmological constant), which appears to be the case.
  6. May 1, 2009 #5


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    News to me that any kind of matter - located stuff - would have a negative pressure.

    Dark energy would instead be some effect related to the void itself, the non-zero point energy of the unlocated whole, so to speak.
  7. May 2, 2009 #6


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    Scalar fields can have negative pressure.
  8. May 2, 2009 #7


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    Yeah but you were talking about "a form of matter". It is all pretty hypothetical as to what the dark energy is. But calling it already a form of matter rather than a hypothetical field of same strength everywhere seem two different things to me.
  9. May 2, 2009 #8


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    A scalar field would be a form of matter. A "field" is just a quantum-mechanical description of matter. Most of the normal matter we know and love, for instance, is made out of spin 1/2 fields. A scalar field is just a spin 0 field.

    And the reason why there would be the same amount of this scalar stuff everywhere would be for the same reason that it drives the expansion of the universe: because of how it gravitates.
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