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B Dark energy and the expansion of spacetime

  1. May 28, 2017 #1
    PLEASE BE ADVISED: I'm a complete ignorant with regards to the details of what I'm about to say and I've never looked at the mathematics of the subject, let alone understand it. So most of what I am about to say comes from pop science. Excuse me in advance if I sound stupid or rhetorical.


    My question: how is space being accelerated in it's expansion? I think:

    1. It happens everywhere (inside galaxies etc.)
    2. However, within galaxies, G > than the force of dark energy thus we don't see it
    3. It must act on the whole scale of the universe where it becomes visible between galaxies (that's how we measured it -- as per supernova's)
    4. Therefore I've some discomfort with this situation, namely:

    a) How come is the force of gravity bigger than dark energy at those scales? How do we explain it? What model do we use and what data? Is there any link between dark matter and dark energy? Could dark matter override dark energy in galaxies/galaxy clusters?
    Physicists tend to emphasize how tiny is the force of gravity compared to the other forces (i.e. strong nuclear force) but nobody seems to have a word of how even tinier the force of dark energy is - why is that?

    b) Does it mean that "empty space" is acted upon by dark energy but when (dark) matter is present it tells space how to curve and also how to expand or how to avoid expanding in that region/area? Would this even make sense? Probably not: it would just overcome the force as we see it in atoms: we don't see gravity effect on such scale (i.e. weak nuclear force is much much bigger than gravity yet we don't disregard gravity, we simply state it's insignificant at that scale, but at larger scales obviously, gravity does play a role we can observe -- so it's scales between dark gravity <-> dark energy? or could there be a relationship in terms of interacting fields?).

    c) How can this play into our model of quantum gravity (for further explorations) and just ordinary current models?


    [Moderator's note: second part moved to separate thread.]
    Last edited by a moderator: May 28, 2017
  2. jcsd
  3. May 28, 2017 #2


    Staff: Mentor

    This should be a red flag that you should take a look at a source that is not pop science. You can't learn actual science from pop science sources.

    Do you mean just the expansion itself, or the effects of dark energy? They're not the same thing.

    The expansion itself exerts no "force" at all; it's just inertia. Things are moving apart and inertia keeps them moving apart.

    Dark energy does exert a "force", in the sense that if you pick any two free-floating objects in a spacetime with dark energy, the dark energy will cause them to accelerate away from each other. But the magnitude of the acceleration varies with the distance between the two objects. That's why the effects of dark energy are completely negligible on scales such as our solar system or our galaxy or even clusters of galaxies. You have to get up to separations of hundreds of millions to billions of light-years before the acceleration caused by dark energy becomes significant.

    Notice that I said all that above without once using the term "space" (much less "empty space"). "Space" is not something physical; it's an artifact of how you choose your coordinates. Spacetime is the physical thing. The "acceleration" caused by dark energy can be thought of as an effect of the way dark energy curves spacetime.

    This belongs in a separate thread, and will be moved to one shortly.
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