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Universe Expansion

  1. Jun 25, 2009 #1
    If the universe is expanding like great minds predict, wouldn't that mean that our very solar system (along with our planet) would have to expand at the same rate? After 4.6 billion years, one might expect that our planet should rip apart. Why doesn't this occur? You'd also expect an increase of radial distance between the planets and the Sun (well I would). Why has this not occurred? I'm not saying that universe expansion does not occur, I just want to know why said phenomena doesn't happen.
  2. jcsd
  3. Jun 25, 2009 #2


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    This is a very interesting question.

    My first thought is :

    The universe started out as an infinitely dense point-mass which started expanding right after the big-bang. according to what you are suggesting, the entire universe would be filled with uniformly scattered "powder". Since this is not the situation, and mass is concentrated in lumps, one possibility is that the expansion of the universe is not homogeneous.

    To the best of my knowledge the expansion is in space-time, being warped by mass-energy, so I think within relatively large concetrations of mass-energy (like our galaxy) the expansion may not be larger than our existing measurement errors.

    But this is only my view...:shy:
  4. Jun 25, 2009 #3
    Ah, a non-uniform expansion would be an ideal conclusion. Regardless, when something is expanding, matter within the expanding substance also experiences expansion, no?
  5. Jun 25, 2009 #4
    Ah, must have something to do with "our" state. A liquid, and a gas would also expand. However, a solid is not affected by expansion.
  6. Jun 25, 2009 #5


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    The expansion of gas is something different - the only thing changing is the distance between atoms (or molecules), if I understand you correctly, the effect you want to see, would make the atoms themselves increase in size proportionally to the universal expansion, right ?

    I think (without having too much time to consider) that if everything was expanding on the particle level - we would never know it, because then the particles that we (and our measurement equipment) are made of would also expand proportionally, and in that case i doubt we would be able to measure the universal expansion....

    So.... The fact that we are able to measure universal expansion - IMO is on its own a proof that matter does not expand along with space-time......

    If some of this reasoning is wrong please point it out as this is just a quick thought !
  7. Jun 25, 2009 #6
    I remember from university that the space between galaxies expands, the galaxies (and their contents) do not
  8. Jun 25, 2009 #7
    This has to do with the force of gravity being stronger within solar systems, galaxies, and clusters of galaxies, than whatever force is causing the expansion.
  9. Jun 25, 2009 #8
    Yeah that's right. That answers the OP's question doesn't it?
  10. Jun 25, 2009 #9
    Your answers are useful, but they don't necessarily answer my query entirely. AND I hate to be a "Doubting Thomas," but, wouldn't; Fg = (Gm1m2)/r^2 = ma, defeat the argument that the force of such gravities may be stronger than the the pull of universal expansion?

    I just picture that if the universe is expanding, wouldn't (at least) the radial distance between celestial bodies increase as well? Could there also be a repulsion factor (mutual, or otherwise) between celestial bodies that plays a role?
  11. Jun 26, 2009 #10


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    Theoretically - the distance between the earth and the moon should also increase with universal expansion, only the rate of expansion is not uniform (as already established) and obviously is warped by mass / energy. IMO it is simply too small for us to measure with existing equipment.

    This means that also the distance between subatomic particles is changing - and possibly "constants" as we consider them - do not exist - they are only approximations for the sake of our limited understanding.

    *** I would like to add that the universal expansion is not only galaxies moving away from one another - it is space-time itself expanding.

    So IMO with universal expansion the speed of light changes, Planck's "constant", the rate of time-passage, operational ranges of the 4 universal forces and much more.....

    What is the force driving the universal expansion ? My information may no longer be up to date (boss requires too much of my time) but I thought once measurement showed the expansion rate was accelerating - this implies a high probability of the driving force to still be active - not only the blast of the big-bang (and whatever force caused that...).

    And to the original poster - without the "doubters" asking all the difficult questions - we would have no science at all.

    Last edited: Jun 26, 2009
  12. Jun 26, 2009 #11


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    Here's the best explanation to your query:

    First, I would like to point out that I am talking only about expansion due to the initial inflationary event. i.e I am ignoring any effects of dark energy on the expansion.

    As we know the universe is expanding. What this means is that the distance between any two points in space is increasing. Now, this expansion is NOT the result of a force, but is rather a product of inertia. Simply put, the initial expansion event (likely inflation) that occurred in the early epochs of the universe started the expansion. Objects, following newton's first law, simply continued to expand and go about their business in the absence of any external force.

    If you must think of the big bang as an explosion of sorts, all the pieces are simply flying away from each other, governed by inertia, NOT some force compelling them to fly away.

    So, now that we have established that expansion is merely the natural tendency for bodies to move under their own inertia, it becomes obvious why gravitationally bound systems are not expanding. Once gravity comes together strong enough to effectively nullify the initial "push" of the inflationary event, the system is no longer in an expanding state. Once the expansion has been stopped, since there is no "expansionary force", there is nothing to cause it to expand so the system evolves under its own gravity.

    This is why dense clumps of matter (galaxies, solar systems, and the like) do not expand.

    Now, my initial assumption was no dark energy, because I believe that is the model being discussed here anyways. With the inclusion of dark energy, there is some force causing an expansion, so you can modify the above logic accordingly.

    I hope this all makes sense, this was my best attempt at wording a sometimes unwordy concept.

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