system
Given the following post, respond as if you're commenting on Reddit:
human
After the big bang, the universe underwent hyperinflation. Currently, our universe is expanding at an increasing rate. Can someone explain how the expansion of the universe was fastest immediately after the big bang, slowed down (?), and is now increasing at an increasing rate?
gpt
There are different mechanisms at play.

Inflation in the super early universe arises when the universe is in an initial state with the fields having some value, but the vacuum can reduce its energy by a field shifting to a different value.  The process is somewhat like happens when a ball rolls down an inclined plane to get to a stable minimum.  This produces the exponential expansion of inflation.

After that, we have a competition between two mechanisms.  The universe is expanding outward, but the gravitational attraction among things tends to slow that expansion down.  However, there is an additional effect -- consistent with what's called a cosmological constant term, and generally just called dark energy -- that causes a negative pressure term, which promotes the expansion of the universe.  As the universe expands, the matter is denser at the beginning and then gets more spread out, so the relative size of the attractive term compared to the cosmological constant/dark energy term decreases over time.  As a result, there was a period when the gravitational attraction dominated and slowed down the expansion, followed by the epoch we are now in, in which the cosmological constant/dark energy term dominates.