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I Gravitation and expansion

  1. Dec 28, 2017 #1
    In the universe, there are several celestial bodies, all of which attract each other. Then why is it so that they do not collapse towards each other. Is it a consequence of expansion. If so, will a change in acceleration of expansion have any effect on it.And can then this be considered a concrete evidence of an expanding universe ?
     
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  3. Dec 28, 2017 #2

    CWatters

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    We already have evidence the universe is expanding and that it's accelerating.
     
  4. Dec 28, 2017 #3

    Orodruin

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    Even if you just consider a Newtonian universe, things would not just collapse spontaneously. Even if things attract each other, this does not mean that things will fall straight to each other. This is because stars, planets, black holes, and whatnot are generally in relative motion that is not directed straight at each other. Two bodies that interact gravitationally according to Newton's law of gravitation will generally have a separation that behaves as an ellipse or a hyperbola, depending on whether or not they are gravitationally bound to each other.

    Of course, it does happen that celestial objects collide.
     
  5. Dec 28, 2017 #4
    I know we have such evidences. However, I wanted to know if this phenomenon can be considered a proof.
     
  6. Dec 28, 2017 #5
    I understand your point. So, do you mean expansion is not playing any role in this observation ?
     
  7. Dec 28, 2017 #6

    Orodruin

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    Yes, at the scales where collisions are relevant, the effects of expansion are negligible (also, expansion does not occur within gravitationally bound systems).
     
  8. Dec 28, 2017 #7

    Drakkith

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    There is no such thing as proof in science. Theories are supported by evidence, but they can never be proven.
     
  9. Dec 28, 2017 #8

    CWatters

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    Every star is an example of matter/dust collapsing together.
     
  10. Dec 28, 2017 #9

    Orodruin

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    Yes, but this is generally not purely due to gravity. In order to contract into a star, a lot of energy has to be radiated away through other processes for the dust cloud to collapse.
     
  11. Dec 30, 2017 #10
    Gravitation provides acceleration, which means that for example, objects pulled together by mutual gravity are accelerated - with just 2 of them, can attract directly - but with n-body at varying distances, they will all be accelerated giving them different velocities - this velocity may be sufficient to then slingshot away or somesuch. Electromagnetic attractions and repulsions are also necessary to help ensure the particles that get close enough together to actually stay close too. This EM is generally balanced at larger scales, but without it, at short scales, the gravity alone would not be enough to necessarily form large bodies.

    Though DE or universal expansion however you call it may overcome gravity on the largest scales, galaxies remain intact and even galactic clusters seem to preserve structure.

    (Edit- sorry, this was kinda what was answered in the first couple of replies already.)
     
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