This diagram shows how the universe slowed down and then revved up since the Big Bang. The concentric red circles denote that galaxies are migrating apart at a slower rate during the first half of the cosmos. Than a mysterious dark force overcame gravity and began pushing galaxies apart at an ever-faster rate, signified by the green circles. Astronomers found evidence of the universe's deceleration when they observed the farthest supernova ever seen, which detonated so long ago that the universe was still slowing down.
Credit: Ann Feild (STScI)