So what I did to fill my time was two things. First, I talked about different ways the universe could have existed before the Big Bang, classifying models into four possibilities (see Slide 7):
1.Bouncing (the universe collapses to a Big Crunch, then re-expands with a Big Bang)
2. Cyclic (a series of bounces and crunches, extending forever)
3. Hibernating (a universe that sits quiescently for a long time, before the Bang begins)
4. Reproducing (a background empty universe that spits off babies, each of which begins with a Bang)
I don’t claim this is a logically exhaustive set of possibilities, but most semi-popular models I know fit into one of the above categories. Given my own way of thinking about the problem, I emphasized that any decent cosmological model should try to explain why the early universe had a low entropy, and suggested that the Reproducing models did the best job.