Answer: Cosmic Jerk. The densities of ordinary matter and

Chapter 44, Problem 44.49

(choose chapter or problem)

Cosmic Jerk. The densities of ordinary matter and dark matter have decreased as the universe has expanded, since the same amount of mass occupies an ever-increasing volume. Yet observations suggest that the density of dark energy has remained constant over the entire history of the universe. (a) Explain why the expansion of the universe actually slowed down in its early history but is speeding up today. Jerk is the term for a change in acceleration, so the change in cosmic expansion from slowing down to speeding up is called cosmic jerk. (b) Calculations show that the change in acceleration took place when the combined density of matter of all kinds was equal to twice the density of dark energy. Compared to todays value of the scale factor, what was the scale factor at that time? (c) We see the galaxies in Figs. 44.16b and 44.20 as they were 300 million years ago and 13.1 billion years ago. Was the expansion of the universe slowing down or speeding up at these times? (Hint: See the caption for Fig. 44.20.)