Submitted by Tank_AT t3_11r2ae9 in askscience
Aseyhe t1_jc7kya8 wrote
Reply to comment by mesouschrist in Does space expansion occur uniformly in all directions and dimensions? by Tank_AT
Within the framework of general relativity, dark energy induces gravitational repulsion, which essentially accelerates everything away from everything else. That means it accelerates cosmic expansion.
While this repulsion is sometimes framed as a consequence of accelerating expansion, it doesn't really make sense to put the causation in that direction. For example, in Einstein's static universe, there is repulsion, even though the universe is static (because the repulsion balances matter's attractive gravity). It's really just gravitational repulsion arising from the energy density of dark energy (or the cosmological constant).
araujoms t1_jcahgbm wrote
Which implies that your top-level comment is incorrect. There is a force acting at small scales, due to the cosmological constant, there is a tendency to expand that you need to counteract via gravity for things to become static.
Note that the Weinberg interview you quote is from 1993. This was years before the discovery of the accelerated expansion of the universe. At the time it was thought that the expansion of the universe was purely inertial, but we know better now, and you should stop spreading obsolete information.
Aseyhe t1_jcaitww wrote
The repulsive force acting at small scales is not linked to the cosmic expansion rate, as detailed in other replies. It is highly misleading to attribute it to the "expansion of space". It's just gravitational repulsion arising from the (stress-)energy density of dark energy.
araujoms t1_jcaj9kq wrote
Of course it is linked to the cosmic expansion rate, it is the very thing that is making the expansion rate accelerate!
Aseyhe t1_jcb1zf4 wrote
Dark energy's gravity accelerates cosmic expansion. Dark energy also induces small-scale gravitational repulsion. However, we cannot say that cosmic expansion induces small-scale gravitational repulsion, because the two are not directly linked.
That is the claim in the top-level comment: that cosmic expansion does not have a local dynamical influence. This claim remains correct.
araujoms t1_jcb3ioi wrote
Of course cosmic expansion does not induce small-scale gravitational repulsion, it's the other way around. It's incomprehensible how can you say that they are not directly linked.
I think you are focussing too much on correcting the misconception that the inertial expansion of the universe is somehow a force pulling things apart, so much that you are ignoring the fact that the cosmological constant is a force pulling things apart that in fact causes the cosmic expansion to accelerate.
Aseyhe t1_jcb4c5l wrote
I'm addressing expansion of space because that's the topic of the question. I am confused as to why you think I should answer a different question.
(Regardless, it would seem to me that the appropriate course for you would be to make a separate answer, rather than to falsely claim that the existing answer is incorrect!)
araujoms t1_jcb5ryj wrote
Because you are so emphatic in your answer that expansion is not a force that one gets the impression that no such force exists. For example, you quote > You shouldn’t think of galaxies as being pulled apart by some kind of expanding space
and > there is no local effect on particle dynamics from the global expansion of the universe: the tendency to separate is a kinematic initial condition, and once this is removed, all memory of the expansion is lost.
While both sentences are technically correct, a lay person will incorrectly conclude that no local repulsive force exists. I don't think this is good science communication.
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