Hubble observed that not only is the universe expanding but that it is accelerating. With the model of the Big Bang as I understand it, the universe began as a big explosion. That would mean that it expanded at a rate exceeding the speed of light or at least close to it. If that is correct, that means the universe over the course of 15+ Billion years had expanded at a high rate of speed, slowed down and is currently speeding up it's expansion again. Is this a correct way to describe it or has this been explained? If so, how can that be?