Sean Carroll

So to the rest of you, these three questions don't sound like they're pointing in the same direction, but they all have to do with this paper that we just came out with.

This is myself, Nadia Diachenko, who is a grad student here at Hopkins, and Sakshi Dulani, who is a postdoc here at Hopkins.

And the title of the paper, I'm going to try to remember the title of my own paper.

It's something like, Toward a Phenomenologically Acceptable

Quantum Cyclic Universe.

So what does that mean?

Let me first mention that there's this idea out there called cyclic universes, right?

What if the universe is currently expanding, but what if it stops expanding, starts recontracting, and goes toward a big crunch?

And what if, besides that, the big crunch is not the end of the universe, but it just goes into a bounce?

So after the big crunch, you could bounce back into a big bang, you could expand, have an ordinary expanding universe, and then you could again re-collapse and contract to a crunch and then bounce, and you could do this an infinite number of times.

That's the idea of a cyclic universe.

And so there's sort of two pieces of magic that are going on there.

One is turning the Big Bang into a big bounce.

So saying that there is a prehistory to the Big Bang.

And, of course, I've said similar things and other people have said similar things.

This is an old idea, but it's still not exactly clear the best way to make it work.

So you don't just have an end of space-time at a bang or a crunch.

You have a bounce from one phase to the other.

And then the other piece of magic is you need the current expansion of the universe to stop and turn around and go toward collapse.

And there's various philosophical questions here about a universe that repeats.