system
Given the following post, respond as if you're commenting on Reddit:
human
Is the rate of acceleration of the expansion of the universe changing? i.e., does the universe expansion have a non-zero "jerk"?

This question occurred to me because I often think about how science tells us that based on the most recent evidence, we think the expansion of the universe is accelerating. I just find it fascinating to imagine the heat death of the universe and such. But if the acceleration was constant or changing would have a significant impact on if we think a big "Crunch" is ever possible, or if the fate of the universe might be more complicated then a long slow heat death.
gpt
Within the measurement accuracy the dark energy density is constant, this contribution leads to a constant acceleration.  The density of matter decreases as the universe expands, however, so the (negative) acceleration component from matter decreases. This means the time derivative of the acceleration is positive. Experimentally we know it was positive for most of the age of the universe at least.