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Can gravity slow down the expansion of the universe?

Physics Asked on September 30, 2021

If dark energy drives a repulsive force between galaxy clusters, proportional to their distance which makes space expand, which creates space between them, and gravity between the clusters has the opposite effect, if it tends to shrink, to annihilate space between the clusters, then gravity only might slow down the expansion of space if the clusters would be anchored to the regions of space they occupy, are enveloped in and squeeze, shrink space between them out of existence.

However, if the clusters aren’t lashed to the regions of space they occupy nor feel any friction from it but it is their inertia which tends to keep them where they are, then can gravity slow down the expansion of space?

2 Answers

Gravitationaly bound objects, such as the galaxies in the local group (Milky Way and Andromeda for instance) don't destroy the space between them, rather space doesn't expand between them.

The expansion of the universe was slowed by the gravity of the objects in the universe until 5 billion years ago when it was dissipated enough that dark energy began to have more of an impact.

Answered by Intrafinesse on September 30, 2021

Yes, gravity can slow down the expansion of the universe. That's why if the universe's average density is greater than critical, it is forecast to collapse into a big crunch (see this section of the Wikipedia article on the Friedmann equations).

I don't understand your second paragraph very well, but it seems like a misconception about what actually is expanding. When the universe expands, space itself expands. It's not the case space stays still while galaxies move away from each other. That's why we have comoving and proper distances in cosmology - the proper distance increases (since things are moving further away from each other), but the comoving distance remains the same if two objects aren't moving relative to one another.

Answered by Allure on September 30, 2021

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