Why did expansion slow down after the big bang?

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Why did expansion slow down after the big bang?

In: Physics

The universe has slowed down and speeded up, not just once, but 7 times in the last 13.8 billion years, on average emulating dark matter in the process. It is a mystery. We have named an actor or actors to be formulated in General Relativity, to provide a mathematically similar effect: the Cosmological Constant and/or Dark Energy.

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The truth is, we have no idea. When scientists say “dark matter” or “dark energy,” what they really mean is “we see this force but don’t know what is causing it.” Both of those terms are effectively placeholder terms until we find out or better understand what is really causing the large-scale behavior of the universe.

We know the universe expanded and accelerated and slowed and is now accelerating, and we know the amount of energy to accomplish this is massive, but we don’t know where this force (if it even is a force) is coming from or where this energy is coming from.

At the beginning, the universe was very dense. At this stage, all the stuff in the universe wants to move away from eachother. This is just a regular gravitational effect (although perhaps an unintuitive one). But if the only stuff in the universe was regular matter, the expansion speed would eventually slow down to a halt. However, there is some other stuff in the universe (dark energy) which helps speed up the expansion. The dark energy only becomes effective when the density of the universe is small enough though.

So a couple billion years ago the universe’s expansion was slowing down because the universe was still too dense and the dark energy wasn’t strong enough yet. Now, the universe is much less dense and dark energy has taken over so that the universe’s expansion is speeding up again.