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Universe Expansion = Drop in Gravity ?

  1. Jun 21, 2012 #1
    Universe is expanding but is it due to drop in gravitational forces all over in the universe ?
    Gravitational force holds all the objects together in the universe .....?
    Universe is expanding means one object is moving away from another, Is this due to drop in Gravitational force ? If Yes, then what causes gravitational forces to drop ?
     
  2. jcsd
  3. Jun 21, 2012 #2

    Drakkith

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    Staff: Mentor

    The expansion was initially believe to be slowing due to the density of the matter in the universe decreasing, meaning that things were getting further apart and thus their gravitational attraction to each other was lessening due to increased distance. However the expansion is now believed to be accelerating, which is hypothesized to be due to something called "dark energy" which we still know very little about.
     
  4. Jun 23, 2012 #3
    No, it is not believed that expansion has anything to do with a drop in gravitational forces.
    Gravitational forces are not observed to drop. In fact, locally, gravity counteracts the expansion : the Solar System does not expand.

    The expansion is only observed for objects separated by very large distances, distances at which gravity has no effect.

    I'm not sure we have a good understanding of what exactly is causing the expansion.
     
  5. Jun 26, 2012 #4
    As already stated, the expansion is from dark energy, not the weakening of gravity. If that were true, galaxies, solar systems, stars, and planets would all start slowly falling apart, since the other three forces do not play a role in keeping the together.
     
  6. Jun 26, 2012 #5
    Dark energy is not meant to explain the expansion itself, but rather the acceleration of the expansion.
     
  7. Jun 26, 2012 #6

    DaveC426913

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    Gold Member

    Even further: Dark Energy is not an explanation at all. It is simply the label we have given to whatever it is that is causing the expansion. If you do the bookkeeping right, you can express the acceleration of the expansion as a force, but that does not mean that what is causing it is actually a force or a form of energy.

    Think of something like the Coriolis Force. That is a label made up to describe something in useful (if not accurate) terms. In a rotating FoR, we see things move as if there were a force applied, but that does not mean it is a force causing it. Coriolis Force does not explain why or how the objects move the way they do; it is simply an operation that needs to be applied, an operation that can be expressed in terms of forces.

    It is the same with Dark Energy.
     
    Last edited: Jun 26, 2012
  8. Jun 28, 2012 #7
    It's crazier than that!
    Note that the 'vacuum' of space where dark energy resides has a constant negative pressure....meaning a type of expansive, repulsive gravity. So as 'space expands', a bit of a misnomer, a constant dark energy density means the overall repulsion increases as the volume of space gets bigger!! [yes, an 'infinite' universe could still expand.] So repulsive gravity is increasing with time!! Nobody expected THAT until Edwin Hubble examined the heavens from Mount Wilson observatory and found expansion.

    In the past more dense 'matter' [galaxies, stellar dust, black holes, planets,radiation, etc] was able to keep expansion slower..but as stuff moves apart, dark energy prediominates. So we are now in an energy dominated accelerated xpansion.


    So this 'hypotheitical explanation' is [a] weird and one we know little about.
     
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