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Great voids made from jello?

  1. Oct 18, 2012 #1
    I know the title sounds stupid, but I have a serious question.
    If you look at the simulations for the massive structures in the universe, there are great voids and large areas of matter in between them.
    The matter parts of the universe contain gravitation objects with large masses, and from the simulations it looks like these areas have sucked all of the matter out of the voids, which is why they are voids, technically low density vacuums.
    So the question is two fold,
    1) at what pressure/temperature can a vacuum exist at before the matter in it is destroyed/changed to something else?
    2) If an area within the 'void' moves, the vacuum pressure where it moves away from increases, if there is a maximum vacuum pressure (at which vacuum become void) then the surrounding space would attempt to fill the void, which it couldn't do because this would create more void. So could we consider these great 'voids' to be inelastic in nature?

    Could the 'voids' be considered as a single (soft/hard) body, which increases in elasticity nearer the edges?
    What does that do to the dark matter problem if we consider the 'voids' to be immense solid bodies that slowly rotate?
     
  2. jcsd
  3. Oct 19, 2012 #2

    Drakkith

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    A void is simply a large area where the density of matter is much less than the average. It is still a vacuum. Matter is never destroyed or changed into something else inside the voids. Space is not a physical object and cannot move to fill anything. Only matter can do this.
     
  4. Oct 19, 2012 #3
    and yet it expands in time...and its rippled in some areas. i think..eigther we use the wrong words to define space or we should consider it a physical object.
     
  5. Oct 19, 2012 #4

    Drakkith

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    The metric is a mathematical construct that describes spacetime in General Relativity. The metric is what is expanding and is what gravity waves are "in". Whether this applies literally to "actual" spacetime or not is unknown. All we know is that it accurately describes the observed behavior of objects within spacetime, including the fact that they are receding away from each other and that they can lose energy in the form of gravitational waves. (That we haven't been able to detect yet)

    The argument of whether spacetime should be regarded as "something" or just a framework is one that is mostly pointless in my opinion. How would we even know?
     
  6. Oct 19, 2012 #5

    marcus

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    It helps to watch the whole movie and not merely look at a few stills.
    There are computer simulations that show an almost uniform cloud of particles condensing (under ordinary Newton laws of motion) to form wispy cobwebby structures with some lower-density pockets. They fall together naturally, by their own gravity, to form the observed structures.

    I think the only special or modern thing about those simulations is that the effect of uniform xyz expansion is put in by SLOWING the motion down very gradually. I think that slowing effect is put in. Otherwise it is just a computer movie of a cloud of particles attracting each other by standard Newton gravity pull and moving in a standard rectilinear xyz frame.

    So the patterns that form do not need any explanation. The computer is explaining how the cobwebs form---by applying the simplest classical laws of motion to a cloud of particles in the simplest possible framework.

    What you see happening, when you watch the movie, is happening completely naturally according to the most basic understanding. So we don't need to make up a "jello theory" or any other additional explanations.

    Of course we don't ultimately know that the laws of conventional physics are RIGHT, all we can say in this case is that the conventional model reproduces the appearance of reality with surprising accuracy and in a remarkably simple way. The key thing is having a computer that is big and fast enough to handle 100s of thousands of particles each one moving bit by bit in response to the pull of the others. That's what makes the cobwebs with their comparatively empty pockets.

    BTW in reality the "voids" we see are not completely devoid of galaxies, they have matter in them like every place else, just a lot less than average. Or so I'm told anyway--I've not personally studied them. The idea is that the pull from surrounding higher-density regions has drawn out most of their stuff making them comparatively (but not perfectly) empty. Stuff "falls" from lower density into higher density regions, whether it be gasclouds, dust, galaxies, or dark matter.
     
    Last edited: Oct 19, 2012
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