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Confused about space expansion? Me too.

  1. Jul 29, 2008 #1
    Confused about space expansion?? Me too.

    Space expansion is occurring from what we can observe.

    In another thread about gravity i read that gravity is space time essentially, as if you introduced an object into earth's atmosphere it would instantly be subjected to earth's gravity as it would instantly be in space time, and that gravity waves would travel along space time should the earth be split in 2 thus the object not instantly feeling the change in gravity, but would be affected once it felt 'waves'.

    Now, what are these waves?? Is it a ripple of space time, or a ripple of gravity?? Or is gravity in fact space time, and any change in gravity causes a ripple of space time??

    Now i need to understand this as a precursor to my next questions about space expansion.

    Can space time expand of it's own doing, or does it need gravitational force of seperate bodies of mass moving away from each other, in order to be expanded??

    I understand on a basic level how gravity can cause distortion to space time, by the movement of mass. But i can't understand the reversal of this process where space time can cause the movement of mass. Yet essentially if space expansion is the 'cause' of distant recessional bodies that are observed, it is not the mass of the bodies moving causing expansion, but the expansion moving the bodies, so what is the actual force causing this mass to move??

    Also, has space expansion been observed on anything of a smaller scale, whereby distance has increased between 2 objects because the space expanded?? why does not everything get bigger??

    On a tangent to this topic, recessional bodies that were >c can now be observed because they are no longer travelling >c. this would indicate reduction in speed. what is the cause of this reduction?

    Now if space time is gravity, and can expand, then gravity is not related to mass, as space time(gravity) has expanded yet the mass has remaind the same.

    Please you demons of knowledge HELP!!!
  2. jcsd
  3. Jul 29, 2008 #2


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    Re: Confused about space expansion?? Me too.

    Mass causes 'ripples' in space-time, which is gravity according to Einsteins theory of gravity.

    Now, are you really asking what gravity waves is and whats causing them?
  4. Jul 29, 2008 #3
    Re: Confused about space expansion?? Me too.

    "Gravitation" is how humans call the curvature of spacetime (which is composed of curved space and delated time), and any change in curvature of spacetime causes a ripple of spacetime that travels with speed of light (c) causing gravitational effects wherever they get.

    According to Einstein the mechanism it this: when space gets a little expanded by the presence of mass M in its vicinity, and the time gets a little dilated, it makes the coordinate speed of light (one seen by any observer, [tex]C(x)=\sqrt{\frac{d\tau(x)}{dt}}c[/tex]) a little slower outside the mass M since [tex]\frac{d\tau(x)}{dt}<1[/tex] Now, the energy of any physical object is its [tex]mC^2(x)[/tex] and this [tex]C(x)[/tex] drops along x towards mass M so the energy of this object also drops towards mass M. The result is that there is a force [tex]F = - \frac{dE}{dx}[/tex] which humans used to call "gravitational attracton". Which obviously is no attraction, just looks like one since this force is directed towards mass M causing the whole mess I just mentioned. And it also is equal (approximately only) to [tex]\frac{G M m}{r^2}[/tex] that was by one of them humans "discovered" as the "law of universal gravitational attraction". It was hundreds years ago when humans were not able to measure timespace precisely enough to notice the changes around masses that Einstein noticed, having rather a keen mind. If not for this the humans might think many more years (as some still do) that "mass attract each other".

    I just explained this point but as you may see the gravitational forces can't push anything appart since the gravitational mechanism can emulate the attraction only. And therefore gravitation can't be responsible for pushing bodies away from each other. At least not according to Einstein. So we need another theory of gravitation, which as far as I know isn't ready yet. We have just the observations that I can't explain since I know only Einstein's gravitation.

    Last edited: Jul 29, 2008
  5. Jul 29, 2008 #4
    Re: Confused about space expansion?? Me too.

    I'm still a bit puzzled by it all, but i doubt i'll ever fully comprehend it. assuming that space time was expanding >c then would the gravity in this space time be travelling >c also?? as i thought it impossible for gravity to >c.
  6. Jul 29, 2008 #5
    Re: Confused about space expansion?? Me too.

    Could gravity beyond where the recessions are moving exist, and be the cause of the recession, and why the reduction in recession?? I thought the space time was expanding at an accelerating rate not a slowing one.
  7. Jul 30, 2008 #6
    Re: Confused about space expansion?? Me too.

    The >c stuff hangs only on an assumption that the distancies between bodies are increasing. This assumption my be wrong because it hangs also ona an assumption that also may be wrong that the observed redshifts of galaxis are caused by increase of the distances from us to them. IMO it is safer to assume that it is the time at those galxies that is running slower due to Einstein's relativity (it may be shown that such assumption follows Einstein's physics). Then you don't have any >c situations.
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