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$\begingroup$ What is your definition of "very large"? within a few hundred million light years expansion is much faster than at billions of light years and that is still not constant. Dark energy is expansion, without either gravity would take over. I dont understand why you consider them different phenomenon. The rate of expansion is not uniform, recent papers have revealed that, whether it is a mistake in ours ways of measuring is another issue. $\endgroup$
– Nathan O'Haire
Commented Nov 29, 2020 at 20:37
$\begingroup$ @Nathan Like I said, "where galaxies are treated like dust (or fluid) particles", so billions of light years. The equations of GR allow for expansion (or contraction) without requiring dark energy. In fact, it's rather unlikely to have a static universe that isn't either expanding or contracting. However, the fact that the expansion is accelerating doesn't naturally fall out of the equations, which is why we had to postulate dark energy. There's some info about it here: physics.stackexchange.com/a/261234/123208 and I guess Pela has posted about it on this site too. $\endgroup$
– PM 2Ring
Commented Nov 29, 2020 at 21:01
$\begingroup$ Also see the classic paper by Davis & Lineweaver: Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe &/or the simplified version (with more diagrams) they wrote for Scientific American, here. $\endgroup$
– PM 2Ring
Commented Nov 29, 2020 at 21:06
$\begingroup$ I understand how expansion works over further distances and it only seems like things are expanding faster close to us because the light that reaches us is more recent but I still dont understand how expansion is different from dark energy and I dont mean expansion due to clusters drawing matter which does make expansion faster on opposite sides of voids. $\endgroup$
– Nathan O'Haire
Commented Nov 29, 2020 at 22:02

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