# Why did the universe continue to expand from inertia after inflation stopped, if galaxies have no motion but space between them expands?

I’ve read many answers on this site about space expansion and I will write from what I understand so please correct me if I’m wrong.

• The space between galaxies expands but the galaxies themselves are stationary and have no real motion due to the expansion.
• After inflation stopped, the universe continued to expand because of inertia. (decelerating expansion)

What I’m asking is, why didn’t the universe started to contract immediately after inflation stopped?
How can there still be inertia to drive the expansion after inflation stopped, since its the space itself that expands, not galaxies moving in space?

The space between galaxies expands but the galaxies themselves are stationary and have no real motion due to the expansion.

Not true. General relativity doesn't define any notion of the velocity of an object relative to another, distant object. You can say the galaxies are moving relative to one another, or you can say that they're not. Neither statement corresponds to any mathematical fact about the theory.

After inflation stopped, the universe continued to expand because of inertia. (decelerating expansion)

Describing this in terms of inertia, as in Newtonian physics, is just an analogy.

The basic problem here is that you're trying to use English words and Newtonian analogies to understand cosmology. That just doesn't work.

The reason that cosmological solutions to the Einstein field equations behave as they do is that they are mathematical solutions to the equations.

• We should add, unless OP question was somehow historical, that expansion is even not decelerating. I know this point is not necessary in this answer but avoid possible confusion in readers. Jan 21, 2018 at 16:06
• @alchimista I meant the decelerating era of the expansion Jan 21, 2018 at 16:36
• @parker. Ok, but it requires some detailed knowledge Jan 21, 2018 at 16:59
• @parker this is what I wonder about, too. Answer is probably in GR as for defining the frame in which K energy is calculated might be local or somehow difficult. I posed a question that I can't find . It was "if I rope myself to a very distant galaxy, will expansion lift me?" Obviously yes. What does the work? Sometimes I don't ask because I can't concisely formulate.... Jan 26, 2018 at 12:16